Domestic Extensions Project Guide

7/25/2019 Domestic Extensions Project Guide

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DOMESTIC EXTENSIONS(Up to Two Storeys)

PROJECT GUIDE

Construction Products Association

Chapter 1

Before the project starts

1.1General

Unless you have extensive experience in building it is recommended that you obtain

some professional advice/assistance as extensions can be more complex that is oftenfirst envisaged.

The preparation of suitable drawings by a construction professional will greatly smooth

the process of gaining any necessary approvals. Also, appointing an experienced

builder will greatly reduce any issues that may arise during the construction stage.

Alternatively, to greatly simplify the whole project, it is worth engaging the servicers of a

specialist company who can deliver the whole project from preparation of drawing,

obtaining approvals and undertaking the actual construction through to obtaining the

final completion certificate from the local authority Building Control,

1.2

Planning Permission

An extension or any addition to a house is considered to be permitted development, not

requiring an application for planning permission, subject to the following limits and

conditions:

No more than half the area of the land around the original housewould be covered

by additional or other buildings. The term original house means the house as it was

first built or as it stood on 1 July 1948 (if it was built before that date). Although you

may not have built an extension to the house, a previous owner may have done so.

No extension is permitted forward of the principal elevation or side elevation frontinga highway.

No extension can be higher than the highest part of the existing roof.

Single-storey rear extensions must not extend beyond the rear wall of the original

house by more than 3m if an attached house or by 4m if a detached house.

The maximum height of a single storey rear extension should not exceed 4m.

Extensions of more than one storey must not extend beyond the rear wall of the

original house by more than 3m


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The maximum height of the eaves of an extension within 2m of a boundary is limited

to 3m.

The maximum height of the eaves and the ridge of the extension cannot be greater

than the existing house.

Side extensions can only be of a single storey with a maximum height of 4m and

width of no more than half that of the original house.

Two-storey extensions to be no closer than 7m to a rear boundary.

The roof pitch of extensions higher than one storey must match the existing house.

The materials used in the construction must be similar in appearance to the existing

house.

No verandas, balconies or raised platforms are permitted.

Upper-floor, side facing windows must contain obscure-glazing. Any opening to be

1.7m above the floor.

If the area is designated land the permitted development for the rear extension is

limited to one storey. Designated land includes national parks, and the Broads,

Areas of Outstanding Natural Beauty, conservation areas and World Heritage Sites.

No cladding of the exterior is allowed on designated land.

No side extensions are permitted on designated land.

[Note

a) These restrictions apply to houses only. Flats, maisonettes or other buildings have different

rules.

b) If the proposed extension is over 100m2, it may be liable for a change under the Community

Infrastructure Levy.]

If unsure about these restrictions, it is advisable to contact your local planning office who will

be able to advise whether or not planning permission is required. If planning permission isrequired then a one-off fee will be payable when submitting a completed Full (detailed)

Planning Application Form. This can be filled out on paper or be an on-line application at

www.planninghportal.gov.uk .

1.3Building Regulations

Building Regulations set standards for the design and construction of buildings to ensure

the safety and health of people in or about those buildings. They also include

requirements to ensure that fuel and power are conserved and facilities are provided for
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people, including those with disabilities, to access and move around inside the building.

Simply, Building Regulations determine how the proposed work is carried out.

Most extensions of property will require approval under the Building Regulations. To

achieve compliance you are required to use one of two types of Building Control service:

The Local authority Building Control service.

An approved inspectors Building Control service.

Each local authority in England and Wales has a Building Control section whose duty it is

to ensure that the building work complies with the Building Regulations except where it is

formally under the control of an approved inspector. Individual local authorities

coordinate their services regionally and nationally via an organisation known as the Local

Authority Building Control (LABC). LABC has developed an online service for creatingand submitting building control applications. Homeowners can apply to most local

authorities in England, Wales and Northern Ireland using the servicewww.submit-a-

plan.com. Further information is available from the LABC website onwww.labc.uk.com

Approved inspectors are private-sector companies or practitioners and are approved for

the purpose of carrying out Building Control services as an alternative to the local

authority. Approved inspectors can provide a service in connection with work to existing

buildings, including extensions. All approved inspectors are registered with the

Construction Industry Council (CIC) who can provide a list of members. Some approved

inspectors have an online service for creating and submitting Building Control

applications. Further information regarding approved inspectors can be found on theAssociation of Consultant Approved Inspectors website:

www.approvedinspectors.org/home.asp.

The way to obtain approval will depend on whether the homeowner chooses to use the

Building Control service of a local authority or an approved inspector.

1.3.1 Local Authority Building Control

Procedures to follow are set out in the Building Regulations. Some of these relate to

pre- site procedures and others relate to procedures once work is under way on

site. Two types of application for approval can be made:

a) Full Plans: An application deposited under this procedure needs to contain plans and

other information showing all construction details, preferably well in advance of when

the work is scheduled to start on site. The local authority will check the plans and

consult the appropriate authorities e.g. fire and sewerage. They have five weeks in

which to complete the procedure and issue a decision, or, if agreed, a maximum of

two months from the date of deposit.

If the plans comply with the Building Regulations, the homeowner will receive a

notice stating that they have been approved. If, however, they are not satisfied, they

may request amendments to be made or ask for more details to be provided.Alternatively, a conditional approval may be issued. This will specify further
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information which must be deposited on revised plans. The local authority may only

apply conditions if they have been requested to do so or if the homeowner has

consented to them doing so. A request or consent must be made in writing. If the

plans are rejected, the reason will be stated in the notice.

A full plans approval notice is valid for three years from the date of deposit of theplans, after which time the local authority may send a notice to declare the approval

of no effect if the building work has not commenced.

b) Building notice: Plans are not required with this process so it is quicker and less

detailed than the full plans application. It is designed to enable some types of

building work to get underway quickly, although it is perhaps best suited to small

work. If you elect to use this procedure you need to be confident that the work will

comply with the Building Regulations otherwise you will have to correct any work

carried out if the local authority requests this. In this respect the homeowner does

not have the protection from the prosecution process provided by the approval of full

plans. If before the start of work or while work is in progress the local authority

requires further information such as design calculations or plans, then you must

supply the details requested.

A building notice is valid for three years from the date when the notice was given to

the local authority, after which time it will automatically lapse if the building work has

not commenced.

1.3.2 The Building Control service will undertake inspections as the work progresses to

ensure compliance with the Building Regulations and other legislation. Being the

homeowner, you are required to notify the local authority each time the work reachesa particular stage. Then work must stop until the local authority has carried out their

inspection. It is important to understand that if the local authority is not informed of

when relevant stages of work have been reached, they can give written notice for the

work to be open up for inspection. Building Control has a legal obligation to ascertain

whether or not the extension complies with the Building Regulations.

Once the work is completed, the Building Control services will issue a final or

completion certificate provided they consent that the work complies with the Building

Regulations.

1.3.3 If the service of an approved inspector is employed, you will need writtenconfirmation of his terms of reference. Both parties need to independently advise the

local authority that the inspector is carrying out the Building Control function for this

works. The local authority has five days in which to respond to this initial

notification. Once accepted by the local authority, the responsibility forchecking of

plans and undertaking site inspections will be formerly placed on the approved

inspector.


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Chapter 2

Site Preparation and Foundations

2.1

Site Clearance

Before any construction work commences, it is necessary that the ground to be covered bythe extension is reasonably free of any material that might damage or affect the stability thestructure. This includes the removal of vegetable matter (including tree roots), topsoil andany pre-existing foundations.The possible existence of contaminates in the ground on which the extension is to stand and

any land associated with the building which may lead to a health and safety risk for

occupants or construction workers needs to be investigated. This sounds expensive, but it is

a requirement that reasonable precautions be taken to identify any possible issues and what

remedial action to take if found necessary.

If the property to which the extension is to be added is reasonably new, then a paper

investigation at the local council offices should provide the answers, however, if the dwelling

is older, then more laborious measures will have to be taken to provide the answer.

It is important that adequate drainage of the sub-soil be provided to avoid the migration of

ground moisture to the interior of the extension. Damage to the foundations arising from the

transportation of water-borne contaminants must similarly be avoided. Sulphates are

present in the ground in some areas and this attack and weaken mortar, blocks and

foundations, in fact, anything made from concrete. Local building control officers will be able

to advise if the soil or groundwater on your specific site has such a problem and tests can be

carried out. The problem can be overcome by an additive being mixed with the mortar orconcrete and by using resistant blocks below ground.

The word contaminant has a wide meaning and includes any substance which is or may

become harmful to persons or buildings including substances which are corrosive, explosive,

flammable, radioactive or toxic.

Adequate measures must also be taken to protect the extension itself and the people using it

from the harmful effects caused by ground water.

In some areas the naturally occurring radioactive gas radon and other gases produced by

some soils and minerals can be a hazard so foundations need to be designed to protectagainst this.

Your architect and/or builder will be able to provide assistance with any investigative work

needed. Local authorities keep a register of contaminated land and solicitors searches

should reveal any risk of contamination. The extent and level of investigation needs to be

tailored to the particular development being proposed and the previous use of the land.

Desk studies, the digging of trial pits and a soil description are just some of the measures

employed.

The removal of vegetable matter such as turf and roots should be undertaken to a depth that

will prevent later regrowth. Where mature trees are present on sites with shrinkable clay the


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potential damage from ground movement to services and floor slabs and over site concrete

needs to be assessed.

Trees and Hedges: -

Building regulations do not apply to trees and hedges but foundations can be affected by

tree roots and soil moisture. Such matters should be considered when planting/removing

trees or building new structures as certain tree species can affect foundations over 20m

away.

The deeds to your property will show if any of the trees on the land are covered by a

preservation order. These days, many trees are so protected and, therefore, council

consent is required before any pruning or felling can be undertaken. In addition, there are

controls over many other trees in conservation areas. If you are unsure about the status of

any tree(s) you will need to contact your council.

Permission to plant a hedge in your own garden is not required and there are no laws that

state a maximum height for your hedge. Despite this, you retain responsibility for

maintaining any hedges on your property and for making sure they are not a nuisance to

anyone else.

If a hedge does adversely affect the owner /occupier of an adjoining domestic property then

they may be able to take action through the High Hedge complaints system introduced by

the Anti-social Behaviour Act 2003. The complaint system specifies the type of hedge and

the adverse effects that it covers and, if you have concerns about the effect a hedge is

having on your property you should contact your council to see whether the High Hedgescomplaints system is applicable to your particular circumstances.

2.2Sub-soil drainage

General excavation work for foundations and servicers can alter groundwater flows through

the site. If contaminants are present in the ground then it is advisable to install sub-soil

drainage to prevent these from reaching the foundations or possibly entering the extension

or blocking drains and sewers which could lead to a back flow of sewerage into the property

see Fig. 2.1.


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Fig. 2.1 Examples of subsoil drains cut to avoid groundwater affecting foundations

Where the water table can rise to within 250mm of the lowest floor or where surface water

could enter or adversely affect the extension there is a need to provide drainage in the

ground to be covered by the extension or some other form of effective measure needs to be

taken.

If an active sub-soil drain passes under the extension it should be laid in pipes with sealed

joints and have access points positioned outside the building. Alternatively, the drain can be

re-routed around the structure or re-directed into another drain.

Where the presence of groundwater could adversely affect the stability of the ground,

consideration of site drainage or other protective measures should be taken. Advice needs


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to be sought from the architect/builder on means of preventing water/moisture entering the

structure.

In some instances, particularly in older buildings, the foul water drains may also receive

rainwater so localised flooding could occur. The capacity of combined systems carrying both

foul water and rainwater should be able to accommodate peak flow. For normal situations inEngland a rainfall intensity of 0.014 litres/second/m2 may be assumed. In some areas of

England this rate of flow can be as high as 0.016 litres/second/m 2.

2.3Drains and incoming Utility Services

2.3.1 Drains

2.3.1.1General

Two drainage systems need to be considered:

(i) Foul drainage carries used water from toilets, sinks, bath, showers, dishwashers

and washing machines

(ii) Surface water drainage deals with rainwater and melting snow/ice from hard

surfaces.

A drain serves a single property whereas a sewer serves more than one property. Private

sewers are owned by the property they serve. Public sewers are owned by the company

named on your sewerage bill. Building work on or around sewers requires written

permission from the owner of the sewer.

As of 1 October 2011 property owners no longer have responsibility for certain sewer pipes

that connect their home to a public sewer. New legislation has transferred responsibility for

private sewers and lateral drains to the sewerage companies. Private sewers and lateral

drains are the sections of sewer pipe or drain which are shared with another persons

property, or run through another persons land.

In the past many property owners were unaware that they were responsible for these pipes

until they faced a repair bill, causing confusion and leading to disputes between neighbours.

This change in legislation provides clear ownership and therefore, better long-term

maintenance for the sewer network.

Privately owned septic tanks, cesspits and their connecting pipework and sewers that carry

water directly to a watercourse have not been transferred to the sewerage companies. They

remain under the jurisdiction of the property owner.

It is not wise to build over an existing drain or sewer as this could damage the pipes resulting

in a leak or blockage potentially causing a problem with bad odours, health and

environmental issues. The clearance of blockages and future repairs are also made more

difficult.

Your existing property will have an established foul drainage system which it should bepossible to connect, however, depending on the positioning of the extension or the use to


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which the rooms within the extension are to be put, this may have to be extended or re-

routed.

As the roof size of the existing property will be increased, consideration as to the disposal of

the extra volume of rainwater needs to be planned. New guttering and possible new gulleys

will normally be added to the perimeter of the extended roof which will either carry therainwater towards new rainwater pipes or join up with the existing guttering on the main part

of the dwelling to utilise the existing drain water pipes. If the latter, it may be necessary to

increase the size of both the guttering and the rainwater pipes to cater for this extra volume

of rainwater - see section 6.8.1, Table 6.13: Guttering and rainwater outlet sizes.

The provision of new rainwater pipes can either discharge onto the ground, or into new or

existing underground pipe work. If discharging onto the ground, it will be necessary to avoid

possible damage to foundations and to direct the surface flow away from neighbouring

properties. Generally, it is best to keep this increased volume of rainwater on site to avoid

possible flooding issues elsewhere. A common approach is the provision of a soakaway or

another form of infiltration system. Alternatively, it may be stored for use in flushing toilets or

watering the garden see section 6.7.2 Other systems of rainwater harvest. The use of

infiltration is not always practical because of the closeness of foundations, impermeable or

contaminated ground or high groundwater levels. Whatever system is used for the disposal

of surface water, it is not permissible to discharge it into a foul drain or sewer. For more

information see section 2.3.1.3- Surface water drainage.

It is important to clarify the ownership and responsibility of the existing drainage system as

drains, sewers and manholes may be shared with neighbours or be the property of a

water/sewerage company. Failure to confirm these details or to comply with relevant

legislation and current standards could lead to legal action being taken against you. Anyresulting remedial work would be for your account.

For listed buildings, you will need listed building consent for any significant work either

externally or within the property.

It is strongly recommended that professional advice be sought concerning these matters

from an architect, builder or the local building control office.

2.3.1.2Foul drainage

If the extension is to incorporate a bathroom, toilet and/or kitchen/utility room foul drainage

will be necessary. This can either link up to the existing system or a new drain line may have

to be dug to the public sewer provided this is within 30m and that the developer has the right

to construct drainage over the intervening private land.

In properties where there is no access to a public sewer, connection may be made to a

septic tank, waste water treatment system or a cesspool complying with current regulations

(see Approved Document H2 Maintenance of waste water treatment systems and

cesspools). It will be necessary to ensure that the increased volume of foul water can be

handled by the existing system. If not, then a new system will have to be built.


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In whatever system is used, where levels do not permit drainage by gravity, then a pump will

need to be installed.

Special arrangements are applicable to low lying sites and professional advice needs to be

sought.

Materials to be used for underground pipe work and its associated joints must comply with

the appropriate British Standard as given in the Table 2.1 : Material for below ground

drainage.

Table 2.1 : Material for below ground gravity drainage

Material British Standard

Rigid Pipes

Vitrified clayConcreteGrey ironDuctile iron

BS 65, BS EN 295BS 5911BS 437BS EN 598

Flexible Pipes

UPVC

PPStructured walledplastic pipes

BS EN 1401*

BS EN 1852*BS EN 13476

* Application area code UD should normally bespecified

Any of these materials can be used provided they match the listed British Standard. Flexible

joints should be used to reduce any differential shrinkage. Jointing sections must be of the

same material as the pipe work they are to be coupled and should remain watertight. There

should be no projections into the system to cause obstructions. The use of non metallicmaterials between pipes of different metals should be used to prevent electrolytic corrosion.

All connections to existing drains must be made obliquely or in the direction of flow using

prefabricated components. Any holes cut into pipes must be made using a drilling device to

avoid damaging the pipe. Where connections involve the removal of pipes and the insertion

of a junction, repair couplings should be used to ensure a watertight joint. It is important to

pack the junction carefully to avoid differential settlement with adjacent pipe work.

Pipes need to be laid in a straight line (slight curves are permitted so long as these can be

cleared of any blockages and are located close to inspection chambers); must be to an

even gradient to permit an easy flow of effluent and to reduce the likelihood of blockages


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occurring. Should the gradient have to change this needs to be accompanied by an access

point.

The flow in the foul water drainage system will depend on the appliance connected and the

capacity depends on the size and gradient of the pipes. Minimum sizes and gradients are

given in Table 2.2 : Recommended minimum gradients for foul drains.

Table 2.2 : Recommended minimum gradients for foul drains

Peak flow(litres/sec)

Pipe size(mm)

Minimum gradient(1 in ...

Maximum capacity(litres/sec)

1 75100150

1:801:801

1:1502

2.86.315.0

Note: -1 Minimum of 1 no. WC

2 Minimum of 5 no. WCs

For a single dwelling the flow rate should be 2.5 litres/second. Fig. 2.2 gives the discharge

capacities of foul drains running 0.75 proportional depth.

Fig. 2.2 : Discharge capacities of foul drains running 0.75 proportional depth


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Appliances are seldom in use simultaneously and the minimum drain sizes in normal use are

capable of carrying the flow from quite large numbers of appliances. The flow stated for a

single dwelling of 2.5 l/sec is based on a typical household group of 1WC, 1 bath, 1 or 2

washbasins, 1 sink and 1 washing machine.

It is also essential that a drain carrying foul water should have a minimum internal diameterof 75 mm while a drain carry effluent from a WC requires at least a minimum diameter of 100

mm.

Any combined system carrying both foul water and rainwater need to take account of the

combined peak flow.

The pipe sizes quoted here are nominal sizes which are rounded numbers approximately

equal to a manufacturers size.Equivalent pipe sizes for individual pipe standards can be

found in Table 2.1.

Ground settlement can be problematical for underground drains as it threatens to damagepipe joints. The use of gravel or some other form of flexible fill for the trenches is necessary

to help alleviate such issues. This solution can also be provided if the drain has to run

underneath the extension provided a minimum of 100mm of gravel/flexible filling is packed

around the pipe. On sites with excessive subsidence, the use of flexible joints is

recommended. Special consideration needs to be made where pipes are built into the

building, or manholes etc to prevent damage or misalignment. The use of a short piece of

pipe with its joints within 150mm of the face of the wall with either end connected to a

600mm length of pipe with flexible joints is a suitable solution to overcome this problem.

Alternatively, form an opening with a minimum of 50mm clearance all round the pipe with the

opening masked with rigid sheet material to prevent infill spilling into the building area orallowing vermin passage way - see Fig 2.3 : Pipes penetrating walls. To prevent the

ingression of gas a compressible sealant is used.

Fig. 2.3 : Pipes penetrating walls


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Excavation of drain trenches should not be made at a lower level than the bottom of any

nearby building except, where the trench is within 1m of the foundation, then it shall be filled

with concrete up to the lowest level of the foundation, or, if further away than 1m from the

building, the trench is filled with concrete to a level below the lowest level for the building

equal to the distance from the building less 150mm see Fig. 2.4 : Pipes running under

buildings.

Fig. 2.4 : Pipes running under buildings

The depth of cover for the drains will depend on the levels of connection of the pipes to the

system, the gradient at which the pipes need to be laid and the ground levels.

The pipes also need to be protected from damage so if the depth of cover is not sufficient, a

pipe of greater strength and an enhanced pipe bedding classification may be chosen see

BS EN 1295-1 national Annex NA. Alternatively, special protection can be provided by

following the appropriate course of action outlined below:

(i) For rigid pipes use the types of bedding and backfilling outlined in Fig 2.5 : Bedding

for pipes for the pipe materials given in Tables 2.3 and 2.4.


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Fig. 2.5 : Bedding for pipes


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Table 2.3 : Limits of cover for class 120 clay ware pipes in any width of trench

Nominal Size Laid in Fields Laid in Light Roads Laid in Main Roads

100 mm 0.60.8+ m 1.28+ m 1.28m

225 mm 0.65 m 1.25 m 1.24.5 m

400 mm 0.64.5 m 1.24.5 m 1.24 m

600 mm 0.64.5 m 1.24.5 m 1.24 m

Notes:

1All pipes assumed to be Class 120 to BS EN 295; consult manufacturer for other strengths and

sizes of pipes.

2Bedding assumed to be Class B with bedding factor of 1.9; guidance is available on use of

higher bedding factors with clay ware pipes.

3Alternative designs using different strengths and/or bedding may offer more appropriate or

economic options using the procedures set out in BS EN 1295.

4Minimum depth in roads set to 1.2 m irrespective of pipe strength.


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Table 2.4 : Limits of cover for class M concrete pipes in any width of trench

Nominal Size Laid in Field Laid in Light Road Laid in main Road

300 mm 0.63 m 1.23 m 1.2 2.5 m

450 mm 0.63.5 m 1.23.5 m 1.22.5 m

600 mm 0.63.5 m 1.23.5 m 1.23 m

Note:

1All pipes assumed to be Class M to BS 5911; Consult manufacturers for other strength and sizes

of pipes .

2 Bedding assumed to be Class B with bedding factor 1.9.

3 - Alternative designs using different strengths and/or bedding types may offer more appropriate

or economic options using the procedures set out in BS EN 1295.

4Minimum depth in roads set to 1.2 m irrespective of pipe strength.

(ii) Flexible pipes deform under load and therefore require support to limit this

deformation. Table 2.5 gives the minimum and maximum depths of cover and Fig

2.5 gives the method of bedding and back filling.


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Table 2.5 : Limits of cover for thermoplastics (nominal ring stiffness SN4) pipes in any width of trench

Nominal Size Laid in Field Laid in Light Road Laid in Main Road

100300 mm 0.67 m 0.97 m 0.97 m

Notes:

1For drains and sewers less than 1.5 m deep and there is a risk of excavation adjacent to

the drain and depth, special calculation is necessary, see BS EN 1295.

2All pipes assumed to be in accordance with the relevant standard listed in Table 1 with

normal ring stiffness SN4; consult manufacturers for strengths and sizes of pipe available.

3Bedding assumed to be Class S2 with 80% comparison and average soil conditions.

4 Alternative designs using different pipe strengths and/or bedding types may offer more

appropriate or economic options using the procedures set out in BS EMN 1295.

5 Minimum depth is set to 1.5 m irrespective of pipe strength to cover loss of side

support from parallel excavations.

Drains need to have sufficient capacity to accommodate a flow of 2.5 litres/sec for a single

dwelling comprising of 1 toilet, 1 bath, 1 or 2 washbasins, 1 sink and 1 washing machine. Itis highly unlikely that all these will de discharging at the same time. Capacity will depend

upon the diameter of the pipe and its gradientsee Fig. 2.6.

Some systems combine both foul water and rainwater so the capacity needs to take account

of their combined peak flow. Normally a design rainfall intensity of 0.014 litres/second/m2

may be used for normal situations. Alternatively, other discharge capacities for different pipe

diameters at different gradients can be read off Fig. 2.6 : Discharge capacities of rainwater

drains running full.


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Fig. 2.6 : Discharge capacities of rainwater drains running full

Should a pumping installation be required for inclusion within the extension, this should be

designed in accordance with BS EN 12056-4. Guidance on the design of pumping

installations for use outside buildings can be found in BS EN 752-6.

Where foul water drainage from a dwelling is to be pumped, the effluent receiving chamber

should be of sufficient size to contain a 24 hour flow. The minimum daily discharge should

be calculated as 150 litres per head per day.

Suitable access points need to be provided for clearing blockages. Their position, spacing

and type will depend on the particular site conditions into which the system has been built.

Minimum dimensions for access fittings, inspection chambers and manholes can be found in

Approved Document H1, Tables 2.6 and 2.7, while their maximum spacing can be taken

from Table 2.8.


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Table 2.6 : Minimum dimensions for access fittings and inspection chambers

Type

Internal Sizes Cover Sizes

Depth to

invert from

cover level

(m)

Length x

width

(mm x mm)

Circular

(mm)

Length x

width

(mm x mm)

Circular

(mm)

Rodding

eye

As drain

but

min.100

Same

size as

pipework1

Access fitting

Small 150 dia.

150 x 100

Large 225 x 100

0.6 or less,

except

where

situated in a

chamber

150 x 100

225 x 100

150

225

150 x 1001

225 x 1001

Same size

as access

fitting

Inspection chamber

Shallow

Deep

0.6 or less

1.2 or less

>1.2

225 x 100

450 x 450

450 x 450

1902

450

450

-

Min.

430 x 430

Max.

300 x 3003

1901

430

Access

restricted

to max.

3503

Notes:

1The clear opening may be reduced by 20 mm in order to provide support for the cover and frame.

2Drains up to 150 mm.

3A larger opening cover may be used in conjunction with a restricted access. The size is restricted

for health and safety reasons to deter entry.


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Table 2.7 : Minimum dimensions for manhole

Type Size of

largest

pipe (DN)

Min. internal

dimensions

1

Rectangular

length and

width

Circular

Diameter

Min. clear

openingsize1

Rectangular

length and

width

Circular

diameter

Manhole

1.5 m deep to soffit

150

225

300

>300

750 x 6757

1200 x 675

1200 x 750

1800 x

(DN+450)

10007

1200

1200

The

larger of

1800 or

(DN+450)

750 x 6752

1200 x 6752

na3

225

300

375450

>450

1200 x 1000

1200 x 1075

1350 x 1225

1800 x

(DN+450)

1200

1200

1200

The

larger of

1800 or

(DN+775)

600 x 600 600

Manhole shaft4

>3.0 m deep to soffitof pipe

Steps5 1050 x 800 1050 600 x 600 600

Ladder5 1200 x 800 1200

Winch6 900 x 800 900 600 x 600 600

Notes:

1Larger sizes may be required for manholes on bends or where there are junctions.


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2May be reduced to 600 x 600 where required by highway loading considerations, subject to a safe

system of work being specified.

3Not applicable due to working space needed.

4Minimum height of chamber in shafted manhole 2 m from benching to underside of reducing slab.

5Minimum clear space between ladder or steps and the opposite face of the shaft should be

approximately 900 mm.

6 Winch onlyno steps or ladders, permanent or removable.

7 The minimum size of any manhole serving a sewer (i.e. any drain serving more than one

property) should be 1200 mm x 675 mm rectangular.


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Table 2.8 : Maximum spacing of access points in metres

From

To Access Fitting

To

Junction

To

InspectionChamber

To

ManholeSmall Large

Start of external drain1 12 12 - 22 45

Rodding eye 22 22 22 45 45

Access fitting:

small 150 dia and

150 x 100

Large 225 x 100

-

-

-

-

12

22

22

45

22

45

Inspection chamber

shallow

22 45 22 45 45

Manhole and inspection

chamber deep

- - - 45 902

Notes:

1Stack or ground floor appliances.

2May be up to 200 for man-entry size drains and sewers.

2.3.1.2.1 Connecting appliances to the foul drainage system

Connecting appliances to the foul water system requires the provision of a suitable trap to

prevent foul air from entering the extension. This should retain a minimum seal of 25mm of

water. Table 2.9 below gives the minimum trap sizes and relevant seal depths


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Table 2.9 : Minimum trap sizes and seal depths

Appliance Diameter of trap(mm)

Depth of seal(mm of water or equivalent)

Washbasin1

Bidet 32 75

Bath2

Shower2 40 50

Food waste disposal unitUrinal BowlSink

Washing machine2

Dishwashing machine2

40 75

WC panoutlet < 80mmWC panoutlet > 80mm

75100

5050

1 The depth of seal may be reduced to 50mm only with flush grated wastes without plugs onspray tap basins.

2 Where these appliances discharge directly to a gully the depth of seal may be reduced to less than38mm.

3 Traps used on appliances with flat bottom (trailing waste discharge) and discharging to a gully witha grating may have a reduced water seal of not less than 38mm.

Appliances connect to the foul drains via a branch pipe. These in turn should discharge into

another branch pipe or a discharge stack or a gully. There are a series of requirements

which need to be followed concerning connections of branch pipes and the provision of

ventilation which can be found in Approved Document H1- Foul water drainage, Section 1

Sanitary pipe work.

Materials for use in sanitary pipe work are shown in Table 2.10. If materials of different

metals are used these should be separated by non-metallic material to prevent electrolytic

corrosion. It is essential that any earth bonding requirements are not interrupted.


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Table 2.10 : Materials for sanitary pipe work ``

Material British Standard

Pipes

Cast ironCopperGalvanised steelPVC-UPolypropylene (PP)ABSPolyethylene (PE)Styrene copolymer blends

(PVC & SAN)PVC-C

Traps

BS 416, BS EN 877BS EB 1254, BS EN 1057BS 3868BS EN 1329BS EN 1451BS EN 1455BS EN 1519BS EN 1565

BS EN 1566

BS EN 274, BS 3943

When testing for air tightness, traps should maintain a water seal of at least 25mm under a

minimum positive pressure of 38mm water gauge for at least 3 minutes. An alternative

approach can be found in BS EN 12056 Gravity drainage systems inside buildings see

Part 1: General and performance requirements, clauses 3 6; Part 2: Sanitary pipework,

layout and calculation, clauses 3 6 and National Annexes NA to NG; Part 5: Installation

and testing, instructions for operations, maintenance and uses, clauses 46, 8,9 and 11.

2.3.1.3Surface water drainage

This section deals only with drainage from paved areas and surface water drainage.

Information relating to guttering and rainwater pipes is covered under the chapter dealing

with the roof.

Ideally, surface water should be discharged into a sewer. Should this not be possible then a

soak away or other infiltration system can be used. It may be possible to obtain consent

from the Environment Agency to discharge surface water into a watercourse, however, they

may limit the rate of discharge.

Some sewers carry both foul water and surface water in the same pipe - a combined system.

Care must be taken to ascertain if the sewer has enough capacity to cope with the additional

flow. If the combined system has insufficient capacity then a separate system needs to be

run with its own outfall. Where such a system is being utilised, traps on all inlets need to be

provided.

Information about the depth of pipes is the same as that given above in section 2.3.1.2 - Foul

drainage.

The drain needs to have sufficient capacity to carry the flow which, in turn, is dependent on

the diameter of the pipes and the gradient at which they are laid. Drains should have aminimum diameter of 75mm and that for surface water sewers is 100mm. Approved


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Document H3, Diagram 3 gives the discharge capacities of rainwater drains of varying sizes

at different gradients. A gradient of at least 1:100 should be used for rainwater drains of

diameter 75mm and 100mm. Sewers and drains of 150mm diameter require a minimum

gradient of 1:150.

Details on bedding/backfilling of trenches and access for unblocking pipes are the same asthat given for foul drainssee section 2.3.1.2 Foul drains, Figure 2.5 : Bedding for pipes.

For areas up to 100m2 in size, provision of a soak away is an adequate measure for surface

water to discharge into. A soakaway usually comprises of a pit filled with rubble or lined with

dry-jointed masonry or perforated units.

Soakaways should be designed to a return period of once every 10 years. Its size will be

dictated by the duration of the longest storms recorded in that area and this will give its

storage volume. Further information can be found in the CIRIA Report 156 Infiltration

drainageManual of good practice.

For smaller areas covering up to 25m2, a design rate of rainfall of 10mm in 5 minutes would

be a worst case scenario. For larger areas, design should be in accordance with BS EN

752-4: Drain and sewer systems outside buildings.

It will be necessary to undertake percolation test to determine the capacity of the soil to

efficiently disperse the water accumulated in the soakaway. The test method is described in

Approved Document H2 paragraphs 1.34 to 1.38 will enable the calculation of the average

time in seconds (Vp),for the water to drop 1mm. This value is used in the equation

f = 10-3/ 3Vp

where f= the soil infiltration rate.

The storage volume for the soakaway is the difference between the inflow volume and the

outflow volume. The inflow volume is calculated based on 10mm falling in 5 minutes over an

area up to 25m2, and the outflow volume (O) is calculated using the equation

O = aa50 x f x D

Where aa50 = area of the side of the storage volume when filled 50% of its effective depth

D= the duration of the storm in minutes.

For larger areas, soakaways should be designed in accordance with BS EN 752-4 (see

paragraph 3.36) or BRE Digest 365Soakaway design.

Soakaways and other infiltration devises should not be built closer than 5m from a building,

road or in unstable land; where the water table appears in the bottom of the devise at any

time of the year or where the presence of any contaminant in the runoff may lead to pollution

of the groundwater source or resource. They must also be sufficiently far away from other

drainage devices so as not to over soak the ground.


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Other types of filtration system include swales, filter drains and infiltration basins are more

suited for use in rural areas due to their open nature.

2.3.1.4 Separate systems of drainage

The following situations are to minimise the volume of rainwater entering the public foul

sewer system so as to avoid overloading the sewers capacity it is necessary to separate the

foul drains and the surface water drainage.

If a separate sewer system of foul water and surface water is under construction to replace

an existing combined system, then it is permissible to initially connect the separate foul and

rainwater drains from the dwelling to the combined sewer system. Once the construction of

the separate sewer system is complete, the separate drainage system from the dwelling

shall be reconnected to the newly constructed separate sewer system.

Where the building is to be connected to the public sewer with a separate foul and rainwater

system, it will be necessary for the drainage system leaving the dwelling to also have its foul

and rainwater kept separate.

2.3.2 Utilities

It is assumed that the dwelling to which the extension is to be added will already be

connected to an existing supply of water, electricity and optionally gas. This being the case,

the supply of these utilities will be routed through the existing building rather than be carried

underground. The same can be said for any supplies of heating oil.

2.4Foundations

2.4.1 General

It is imperative that adequate foundations are constructed as these are required to transmit

the weight of the extension safely to the ground. Naturally, each project will be unique and,

therefore, its weight and footprint will be different.

The type of soil on which the extension is to sit is equally important as different soil types

have different load bearing capabilities. Changes in the moisture content of the soil can lead

to its expansion and contraction as it absorbs moisture or dries out. Some clay soils are

particularly prone to such movement. Typically, this movement is limited to the first 0.75m of

soil depth, therefore, foundations need to be deeper so as not to be affected by ground

movement. It is essential that the depth at which the ground movement becomes virtually

non-existent is determined and the foundation depth is calculated accordingly.

Trees roots can be problematical as they spread across a site and draw moisture from the

ground causing it to shrink. The degree of shrinkage will be a combination of the type of

soil and the size, type and number of trees involved. All have the potential to damage


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foundations. It is important to remember that removal of tree(s) to benefit your site could be

detrimental to adjoining sites as the ground will tend to increase in moisture content possibly

causing movement (heave).

Foundations spread the load of the extension downwards in an area outside the footprint of

the foundation. This is typically at an angle of 45. Any drains or sewers sited within this 45area could be damaged by the weight of the extension. To get around this problem, it is

important that depth of the foundation excavation be at least to the same depth as the

bottom of the deepest part of the trench in which the drain or sewer is laid. Foundation

depth is also determined by locating undisturbed ground. In landfill areas, a more extensive

system of foundation will be required than merely a trench. Piling is a typical method of

standing a structure on undisturbed ground when such ground exists many metres down.

The width of the foundation will be governed by the thickness of the extensions wall.

Notwithstanding the above, foundations can be constructed as deep-fill (filling most of the

trench) or shallow-fill, where the foundation is only of minimum thickness to transfer theweight of the extension to the soil. Irrespective of this, a proportion of the wall construction

will be below ground level. This is referred to as being the substructure onto which the

portion of the wall standing above ground (the superstructure) will stand.

It is essential that the substructure is robust enough to provide adequate support to the

superstructure. Thus, the bricks or blocks and mortar of the substructure must be resistant

to frost and possible sulphates in the ground.

Freezing of the subsoil can also cause movement of the ground as can issues with land-slip

and subsidence. All have the potential to damage the foundations as well as affecting the

stability of all or part of the structure.

When excavating foundations, account needs to be taken of adjacent buildings to ensure

these are not undermined. It is good practice to excavate to at least the same depth as the

bottom of the foundation of the adjacent structure. If the excavation runs alongside the

existing footings care is required so it is advisable to dig the new excavations and pour the

concrete in short lengths so as to avoid undermining the whole length of the adjacent

structure.

There is a requirement for you to inform the owner of the adjacent building if you plan to

construct your foundations within 3m of the neighbours building, or if you are excavating

within 6m of the adjacent building where the work will cut a line drawn downwards at 45

from the bottom of the neighbours foundations. It is important to note that an adjoining

property may include the next-but-one neighbour if their foundations are within a distance of

6m. If either of these two scenarios is applicable then you are required to give written notice

to your neighbour(s) at least two months in advance of the planned starting date for the

work.

In the case of excavations near a party wall you will have to give notice to the owner(s) at

least one month in advance of the work commencing. The information you will have to

supply the owner(s) of the adjoining building includes:

- your name and address


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- the address of the building to be worked on

- a full description of the work you propose to carry out

- the date you propose starting

The notice should be dated and it is advisable to include a clear statement that it is a notice

under the provisions of the Party Wall Act. Such information will need to supplied to all the

owners of the adjacent property in cases where more than one owner exists. The notice is

only valid for one year, so do not serve the notice too far in advance of the start date.

2.4.2 Foundations of plain concrete

The use of non-engineered fills is not allowed (see BRE Digest 427 OBTAIN COPY &

EXTRACT INFO) nor should there be a wide variation in ground conditions within the area

carrying the load of the extension. The existence of more compressible or weaker ground

at a depth that can adversely affect the stability of the whole extension is not permitted.

The general design provisions given below should be followed: -

(i) The foundation must be situated centrally under the wall

(ii) Where chemically aggressive soil conditions exist, guidance in BS 8500-1 and BRE

Digest 1 should be followed GET COPIES & EXTRACT INFO

(iii) In non-aggressive soils, Portland cement to BS EN 197-1 and -2 (GET COPIES)

should be mixed with fine and coarse aggregates conforming to BS EN 12620 (GET

COPIES) to form the concrete.

(iv) The materials used to form the concrete should comprise of one of the following

mixtures: -

Table 2.11 : Mixtures for concrete for use in foundations

Mixture A Mixture B

Portland cement

Fine aggregate

Coarse aggregate

50 kg

200 kg (0.1m3)

400 kg (0.2m3)

Grade ST2 or Grade GEN I concrete toBS 8500-2


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(v) Minimum foundation thickness

P W P

Foundation width should notbe less than the appropriate

dimension in Table 6 See Table 2.6 to determine distance P

Trench fill foundations may be used as an acceptable alternative to strip foundations

Fig. 2.7 : Foundation dimensions

T

Wall should be central

on the foundation

Minimum thickness of foundation T should be

150mm or equivalent to the distance P, whichever is

the greater.


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Table 2.12 : Minimum width of strip footings

Type of

ground(includingengineered fill)

Condition

ofground

Field test

applicable

Total load of load-bearing walling notmore than

(kN/linear metre)20 30 40 50 60 70Minimum width of strip foundation(mm)

I.Rock Not inferior

tosandstone,limestone orfirm chalk

Requires at least apneumatic or othermechanicallyoperated pick forexcavation

In each case equal to the width of wall

II.Gravel orsand

Mediumdense

Requires pick forexcavation. Woodenpeg 50 x50mm hardto drive beyond150mm

250 300 400 500 600 650

III.ClaySandy clay

StiffStiff

Can be indentedslightly by thumb

250 300 400 500 600 650

IV.ClaySandy clay

FirmFirm

Thumb makesimpression easily

300 350 450 600 750 850

V.SandSilty sandClayeysand

LooseLooseLoose

Can be excavatedwith a spade.Wooden peg50 x 50mm can beeasily driven

400 600

Note: Foundations on soiltypes V & VI do not fallwithin the provisions ofthis section if the total

load exceeds 30kN/m.VI.SiltClaySandy clayClay or silt

SoftSoftSoftSoft

Finger pushed in upto 10mm

450 650

VII.SiltClaySandy clayClay or silt

Very softVery softVery softVery soft

Finger pushed in upto 25mm

Refer to specialist advice


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(vi) Steps in foundations

Foundation should unite at each level.

For trench fill foundations: -

Minimum overlap L = 2 x height of step or 1m, whichever is greater

Fig. 2.8 : Elevation of stepped foundation

(vii) Foundations on piers, buttresses and chimney should project as indicated in diagram

below where X P.

Fig. 2.9 : Design provisions for piers and chimneys

(viii) To avoid frost damage the minimum depth of a strip foundation should be 0.45m totheir underside except where they are laid on rock. In areas which are subject to

long periods of frost or in order to transfer the load onto satisfactory ground, it will be

necessary to increase this depth. In clay soils which are liable to dry out resulting in

ground movement, strip foundations should be taken down to a depth where the

anticipated ground movement will not impair the stability of any part of the extension.

Consideration must also be made of the potential influence of trees and other

vegetation in the ground.

Typically, the depth to the underside of the foundation in clay soils should not be less

than 0.75m, although this depth may have to be increased so as to transfer the

weight of the extension onto satisfactory ground

L

S

T

S T

P

X

X

X

X


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2.4.3 Foundations for timber frame

Foundation tolerances for timber frame buildings have been established for many years as

and they cover both squareness and their level. Wall length should be +/- 10mm; diagonals

should be equal but the acceptable deviation is +/- 5mm for buildings up to 10m. It isnecessary to ensure that the foundation walls or the slab supporting the wall plates are

levelled to +/- 5mm and the perimeter is lined within +/- 10mm.

2.4.4 Foundations for light steel frame

Light steel framing is dimensionally very accurate and so any irregularity in the foundation

level can be more problematical than in brick and blockwork. Typically the tolerance would

be in the range of 0 to -3 mm per panel measuring 2.8m high x 3m wide.

Various techniques are used to maintain the bearing support under the bottom channel

section of the panels. The most common is to use thin strips of galvanised steel or heavyduty synthetic materials. Another popular system is to use injected grout, especially where

there are larger gaps.

In housing, line loads are relatively low, less than 10 kN/m, and so the bearing pressure on

the bottom channel section is low. However, an accurate level survey would be carried out

on the foundations before the light steel framing is installed and the bearing surface is

carefully built up to the required accuracy. No installers of light steel framing would start

work on site until the correct accuracy has been achieved.

2.4.5 Foundations using thin-joint Masonry

SIMILAR TOLERANCES AS TO THAT OF TIMBER FRAME.- CHECK THIS OUT


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Chapter 3

Ground Floor

3.1 Types and issues

In addition to the information provided in chapter one Foundations a more detailed look at

ground floors is given in this section of the publication.

Five common scenarios exist for ground floors each of which has unique issues which need

to be satisfactorily addressed. These cover both suspended and non-suspended floors and

the issues of moisture movement.

(i) Ground supported floors exposed to moisture from the ground

(ii) Suspended timber ground floors exposed moisture from the ground

(iii) Suspended concrete ground floors exposed to moisture from the ground

(iv) The issue of interstitial condensation in ground floors and floors exposed from below

(v) The risk of surface condensation and mould growth on any type of floor.

Where there is the likelihood of issues with ground water pressure, then alternative

measures need to be taken. The problem of ground water pressure normally only arise if the

extension is being built on top of very permeable strata such as chalk, limestone or gravel.

In such a situation, then the following recommendations need to be adhered to Clause 11

of BS CP 102 ; 1973 Protection of buildings against water from the ground or, BS 8102 :1990Code of practice for protection of structures against water from the ground.

GET COPIES & EXTRACT TEXT.

3.2 General considerations

Some very basic requirements need to be met:

(i) Ground floors need to resist the transmission of moisture to its upper surface which

originates from the ground immediately below the extension.

(ii) Any moisture reaching the ground floor material must not damage the floor in any

way

(iii) Any peculation of ground water must similarly not damage the floor material

(iv) Ground floors must resist the passage of hazardous gasses such as radon or

methane which predominate in many areas of the country. In such areas, a gas

resistant barrier needs to be included. This can double as a damp proof membrane if

careful detailing is followed.


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(v) Design of the floor must ensure that its structural and thermal performance is not

adversely affected by interstitial condensation.

(vi) Floor surfaces should not promote condensation or mould growth.

(vii)

Suitable insulation is required to be incorporated in the makeup of the ground floorassembly. For more detailed information on insulation for ground floors , see

Chapter 10.

3.2.1 Hazardous gasses (methane and radon)

Methane occurs naturally and in areas where landfill is prevalent. In practice, where landfill

has occurred a mixture of gases comprising of methane, carbon dioxide and some volatile

organic compounds can be problematical. These gases can enter the extension through

poor detailing and/or fissures in the materials. Apart from the possible toxic effect of some

gases, there is also the problem of bad odours.

If building is to occur in areas associated with:

(i) Landfill or within 250m of the boundary of a landfill site

(ii) On a site subject to the deposition of biodegradable substances

(iii) On a site that has been subject to the use of petrol, oils or solvents

(iv) Areas of naturally occurring hazardous gases.

(v)

Gas control measures consist of a gas resistant barrier placed across the whole siteplaced above a ventilation layer from which the gas can be vented to the

atmosphere.

Radon is a naturally occurring radioactive gas which is both colourless and odourless. Long

term exposure has the potential to cause lung cancer. In areas where radon is an issue, the

incorporation of a radon resistant membrane will be necessary as will appropriate measures

for the heating and ventilation systems.

3.3 Ground supported floors exposed to moisture from the ground.3.3.1 As a general rule, any ground supported floor will meet requirements if the ground is

covered with dense concrete laid on top of a hardcore bed with a suitable damp-proof

membrane included. Suitable insulation may also be included in this solution.

3.3.2 A concrete ground supported floor may be constructed by any of the four

configurations given below in Fig. 3.1.


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BlindingA layer of dry lean concrete about 50mm thick covering the hardcore to seal the underlying

material

Fig. 3.1 : Construction alternatives for a ground supported floor

a) A well compacted hardcore bed composed of clean broken brick or similar inert material

free of materials including water-soluble sulphates which can damage concrete.

b) A minimum concrete thickness of 100mm to mix ST2 (see section 1.4.2 Foundations of

plain concrete, Table 11) as per BS 8500-1 : ConcreteComplimentary British Standard

to BS EN 206-1 - Method of specifying and guidance for the specifier. This may have to

be thicker if the structural design dictates. When embedded reinforcement is

incorporated, then the concrete mix should be ST4 as given in BS 8500-1.

c) Damp-proof membranes may be poisoned above or below the concrete and should becontinuous with the damp-proof course in the walls etc. Where water soluble sulphates

or other harmful matter could contaminate the hardcore, the damp-proof membrane

should be placed at the base of the concrete slab.

The damp-proof membrane placed below the concrete can be formed using polyethylene

sheeting of minimum thickness 300m i.e. 1200 gauge with sealed joints. This is to be

placed on a bed of material that will not damage the polyethylene sheet.

If the membrane is to be laid above the concrete, the same polyethylene sheeting can be

used. Alternatively, three coats of bitumen solution or similar moisture and water vapour

resistant material, can be applied cold to the top surface of the concrete slab. In both cases,

the membrane should be protected by either a screed or a floor finish unless the membrane


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is composed of a pitchmastic or similar material which can also serve as a floor finish.

(Pitchmastic is a jointless floor covering made form pitch with or silica sand aggregate. It is

fluid when hot and is spread to an even thickness of between 16 and 25mm.)

Where the decision is taken to place the insulation beneath the floor slab it is necessary to

ensure that it has sufficient strength to withstand the weight of the concrete slab and anyanticipated floor loadings both after and during construction. In the latter instance, there is

always the likelihood that the floor could become overloaded during the construction stage.

If the insulation is placed below the damp-proof membrane, it must be of a type that has low

water absorption. Where the issue of ground contaminates exists, the insulation should also

be of a type that is resistant to these problems.

Where a timber floor finish is laid directly on to the concrete it may be bedded in a material

which can also serve as a damp-proof membrane. If timber fillets are laid in the concrete as

a fixing for a floor finish, then these will need to be treated with a suitable preservative

especially if these are below the damp-proof membrane. Advice from a timber treatmentcompany should be sought.

3.4 Suspended timber ground floors exposed to moisture from the ground

3.4.1 Any suspended timber floor next to the ground will meet the requirements of the

building regulations if:

a) The ground is covered so as to prevent moisture reaching the timber and to prevent plant

growth, and

b) there is a ventilated air space between the ground and the timber, and

c) Damp proof courses are present between the timber and the material which can carry

moisture from the ground.

3.4.2 A suspended timber ground floor may be built next to the ground if the following

construction method is adhered to: -

a) The ground covering can be composed of unreinforced concrete type mix ST1 to BS

8500 with a minimum thickness of 100mm laid on top of a compacted hardcore bed of clean

broken brick or similar inert material free from any materials that in quantity could damage

the concrete e.g. water soluble sulphates. Alternatively, a layer of concrete, as above, or an

inert fine aggregate at least 50mm thick, laid on a polythene sheet of minimum thickness

300m (1200 gauge). The polythene must have sealed joints and it must be laid on a bed of

material which will not damage the sheet.

To negate the possibility of water collecting on top of this ground covering, either the top of

the covering should be above the highest level of the surrounding ground, or if the site is

sloping, suitable drainage should be installed on the outside of the extension and on the up-

slope side of the structure.


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b) A ventilated air space exists between the top of the ground covering and the underside of

the timber floor, or insulation if present, of at least 150mm. In addition a minimum distance

of 75mm is present between the underside of the wall plates and the top of the ground

covering. The opposing external walls are required to have ventilation openings so

positioned that the ventilating air has an uninterrupted path between opposite walls and to all

parts. The size of these openings should not be less that 1,500mm2/m run of external wall

or 500mm2/m2of floor area, whichever gives the greater opening area. Any ducting to carry

then ventilating air should have a minimum diameter of 100mm. The ventilation openings

should be of a suitable grille type that prevents the entry of vermin to the sub-floor, but does

not unduly resist the flow of air. In situations where the floor level needs to be nearer the

ground to provide a level access, then ventilation of the sub-floor can be effected by means

of offset i.e. periscope ventilators.

c) The damp-proof courses must be of an impervious sheet material, engineering brick or

slates in cement mortar or other material which prevents the passage of moisture. Guidance

on the choice of suitable materials is given in BS 5628-3: 2001 Code of practice for use ofmasonryMaterials and components, design and workmanship.

d) In shrinkable clay soils, which may be subject to an upward movement, heave, the depth

of the air space may have to be increased.

e) If the space is to be used as a kitchen, utility room or bathroom where there is a distinct

likelihood that water will be spilled, any timber or wood-based panel floor covering should be

resistant to moisture.

Fig. 3.2 : Suspended timber ground floor

An alternative solution can be to follow the recommendations of Clause 11 of BS CP 102:1973.


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3.5 Suspended concrete ground floors exposed to moisture from the ground

3.5.1 Any suspended floor composed of in situ or precast concrete next to the ground,

including beam and block floors, will meet the requirements of the approved documents if it

will adequately prevent the passage of moisture to the upper surface of the concrete/block,and the reinforcement is protected against moisture.

3.5.2 One method of fulfilling these requirements is to use: -

a) In situ concrete containing at least 300kg of cement for each m3 of concrete. Its thickness

should be at least 100mm, but thicker if the structural design requires, or

b) Precaste concrete construction with or without infilling slabs, and

c) Reinforcing steel protected by a concrete cover at least 40mm thick if the concrete is in

situ and at least the thickness required for a moderate exposure if the concrete is precast.

d) A damp-proof membrane is included if the ground below the floor is below the lowest level

of the surrounding ground and will not be effectively drained, and

e) A ventilated air space is incorporated as described in 3.4.2. b) above.

3.5.3 Where flooding is likely, consideration needs to be given to including means for

inspecting and cleaning out the sub-floor void. For guidance on this see the ODPM

publication Preparing for floods: interim guidance for improving the flood resistance of

domestic and small business properties. CHECK OUT THE CURRENT NAME FOR THE

ODPM & CHECK THE PUBLICATION AVAILABILITY.

3.6 Ground floors and floors exposed from below to interstitial condensation

3.6.1 Ground floors or floors exposed from below will meet the requirement if it is designed

and constructed in accordance with Clause 8.5 and Appendix D of BS 5250: 2002 Code of

practice for the control of condensation in buildings; BS EN ISO 13788: 2001Hygrothermal

performance of building components and building elementsInternal surface temperature to

avoid critical surface humidity and interstitial condensationCalculation methods; or BRE

Report 262Thermal insulation: avoiding risks, 2002.

3.7 Floors resistant to surface condensation and mould growth

3.7.1 A floor will meet the requirement if:

a) A ground floor is constructed so that the thermal transmittance (U-value) does not exceed

0.7W/m2K; and

b) For all floors, the junction between the elements are designed in accordance with the

recommendations in the DTLR 2001 report Limiting thermal bridging and air leakage: robust


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construction details for dwellings and similar buildings; or BRE Information Paper IP17/01

Assessing the effects of thermal bridging at junctions and around openings, 2001.


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Chapter 4

Walls

4.1Structural External Walls (masonry)

4.1.1 General

The purpose of the walls is to: -

(i) Provide basic stability to the structure

(ii) Support other parts of the structure such as the floors and roof

(iii) Act as a medium into which doors and windows are positioned

(iv) Resist the penetration of moisture

(v) Provide insulation from thermal loss

(vi) Provide resistance to fire

The stability of the structure is a function of both the layout of the external and internal walls,

their robustness, the mechanical connectors for cavity wall construction and the connection

between the walls and the intermediate floor and roof construction. A suitable layout of all

walls must be to form a robust three dimensional box structure with restrictions on the

maximum size of cells (rooms) measured in accordance with the specific guidance for eachform of construction.

On the structural side the wall must be substantial enough to transfer all loads down through

the foundations. These loads include the walls own weight, that of any floor loads and the

weight of the roof and in addition any super imposed loads such as that resulting from snow.

They must also be able to withstand dynamic loads such as wind.

For moisture resistance, the wall must resist moisture penetration from the ground by means

of a damp proof course as well as adequately preventing the penetration of weather from the

outside of the building. By weather we include wind blow sea water for those properties near

the coast. While brickwork generally gives good weather resistance on its own, blockworkwill normally require rendering to the outside to a minimum thickness of 16mm, however, this

will depend on the type of block used.

The walls have thermal resistance which limits the amount of heat the building will lose from

its internal spaces and any thermal gain from the outside environment. The efficiency of this

thermal resistance will depend on the structure of the wall and the extent to which any

insulation has been incorporated.

The requirement for fire protection will depend on the distance the extension is from the

propertys boundary if this is close to an adjacent building. Also, the area of the wall

permitted to have reduced/undetermined fire resistance, such as windows and doors, will


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depend on that walls proximity to the boundary. In addition, if the wall is load-bearing, it will

also need to have fire resistance regardless of its distance from the boundary.

4.1.2 Proportional Limits

For residential buildings the maximum height of the extension, measured from the lowestfinished ground level adjoining the building to the highest point of any wall or roof should not

be greater than 15m subject to the following the maximum allowable building heights given

in Table 4.1 below. Irrespective of this, the height of the extension will be governed by the

height of the existing building.

Table 4.1 : Maximum allowable building height

Factor Country Sites Town Sites

SDistance to the coast Distance to the coast

50km 50km

24252627282930313233

343536373839

1511.5864.53.53

1514.510.58.56.5543.53

1515131086543.53

1515151513.5119876

543

151515151513119.58.57.5

765.54.543

151515151514.512.510.59.58.5

8765.554

40 3

4.1.3 Thickness of walls

4.1.3.1 The thickness of external walls is determined by the materials used for itsconstruction, whether the wall is solid or has a cavity and the function the wall is

expected to perform.

(i) Solid external walls, compartment walls and separating walls in coursed brickwork or

blockwork need to be at least as thick as 1/16 of the storey height and adhere to the

requirements in Table 4.2 below.


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Table 4.2 : Minimum thickness of certain external walls, compartment walls and separating walls in

relation to their height and length

Height of wall Length of wall Minimum thickness of wall

3.5m 12m 190mm for whole of its height

> 3.5m but 9m

9m 190mm for whole of its height

> 9m but 12m 290mm from the base for the height of onestory and 190mm for the rest of its height

>9m but 12m

9m

290mm from the base for the height of one

story and 190mm for the rest of its height

>9m but 12m 290mm from the base for the height of twostoreys

Probably exclude the red section as dimensions too large for a two storey extension

(ii) Solid external walls, compartment walls and separating walls in uncoursed stone,

flints, clunches, bricks or other burnt or vitrified material should not be less than 1.33

times the thickness determined in 4.1.3 1 (i) above.

(iii) Cavity walls in coursed brickwork or blockwork. Here the cavity should have leaves

at least 90mm thick and cavities at least 50mm wide. Wall ties should be spaced

horizontally every 900mm and vertically every 450mm. This is equivalent to 2.5 ties

per square metre. In addition, extra wall ties should be provided within a distance of

225mm from the vertical edges of all openings, movement joints and roof verges.

Here the spacing must not be more than 300mm apart vertically. Table 4.3 below

gives the selection of wall ties for use in a range of cavity widths.


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Table 4.3 : Cavity wall ties

Nominal cavitywidth mm(see note 1)

Permissible type of tie

Tie lengthmm

(see note 2)

Tie shape in accordancewith

BS 12435

BS EN 845-1 tie(see note 4)

50 to 75

76 to 9091 to 100

101 to 125126 to 150151 to 175

176 to 300

200

225225

250275300

(see note 2)

Butterfly, double triangle orvertical twistDouble triangle or vertical twistDouble triangle (note 3) orvertical twistVertical twistVertical twistVertical twist

Vertical twist style

Types 1,2,3 or 4 toDD 140-25and selectedon the basis of the designloading and design cavitywidth.

Notes:1. Where face insulated blocks are used the cavity width should be measured from the face of the

masonry unit2. The embedment depth of the tie should not be less than 50mm in both leaves. For cavities wider

than 180mm calculate the length as the structural cavity width plus 125mm and select the neareststock length.

3. Double triangle ties of this shape having a strength to satisfy Type 2 of DD 140-2 aremanufactured. Specialist tie manufactures should be consulted if 225mm long double triangleformat ties are needed for 91 to 100mm cavities.

4. Where BS EN 845-1 ties are used reference needs to be made additionally to DD 140-2 for theselection of the type (i.e. 1, 2, 3 or 4) relevant to the performance levels given in DD 140-2.

5. Although BS 1243 and DD 140-2 were due to be withdrawn on 1 February 2005, the tie userclasses (types) given in Tables 1 and 3 of the latter document can continue to be used after thisdate.

The selection of wall tie should comply with one of the three standards named in

Table 4.3 above and should be manufactured from material references 1 and 3 in BS

EN 845-1 Table A1 austentic stainless steel.

GET COPY OF DD 140 ON WALL TIES & BS/EN ABOVE

In cavity construction for external walls, compartment walls and separating walls, the

combined thickness of the two leaves plus 10mm should not be less than the

thickness determined by paragraph 4.1.3.1 (i) and Table 4.3 above.

(iv) Internal load-bearing walls in brickwork or blockwork, excluding compartment walls or

separating walls should have a thickness not less than

x (specified thickness from Table 2.8) minus 5mm

(v) The minimum thickness and maximum height of parapet walls should be as give in

Table 4.4 below.


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Fig. 4.1 : Parapet wall height.

(vi) The maximum floor area which can be enclosed on four sides by structural walls in

70m2. This reduces to 36m2 for areas without a structural wall on one side. See Fig.

4.2 below.


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Fig. 4.2 : Maximum floor area enclosed by structural walls

(vii) All the design information in this section on walls is adequate for the imposed loads

given in Table 4.4 below.

Table 4.4 : Imposed loads

Element Distributive Loads

Roof Spans up to 12m1.0kN/m2Spans up to 6m - 1.5kN/m2

Floors 2.0kN/m2

Ceilings 0.25kN/m2together with concentrated load of 0.9kN/m2

(viii) The maximum height of the extension given in Table 4.1 above correlates to various

site exposure conditions and wind speeds. See Fig. 4.3 below.

Check if we really need this as the extension will not be higher than the existing

house


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Fig. 4.3 : Map of wind speeds in England and Wales (V) in m/s

4.1.4 The maximum allowable length and height of a wall is 12m (reduce this to 9m if

excluding last part of Table 4.2) in both cases. See Table 4.2 above.

To measure the height of walls and storeys follow the methodology given in Fig 4.4

below.


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Fig. 4.4 : Measuring storey and wall heights REDRAW WITH ONLY 2 STOREYS


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4.1.5 Materials for constructing wall

4.1.5.1 The walls must be properly bonded and solidly constructed with mortar using the

types of masonry materials given in Table 4.5 below.

Table 4.5 : Wall material types and Standards

Wall Material Standards

Clay bricks or blocks BS 3921:1985, or BS 6649:1985, orBS EN 771-1

Calcium silicate bricks BS 187:1978, or BS 6649:1985, or

BS EN 771-2

Concrete bricks or blocks BS 6073-1;1981, or BS EN 771-3 or -4

Natural stone square dressed to theappropriate requirements in standards

BS EN 771-6, of BS 5628-3:2001

Stone (manufactured) BS 6457:1984, or BS EN 771-5

4.1.5.2 Compressive strength of masonry units

The minimum compressive strength requirements for the different masonry units are given in

Fig. 4.5 below. The indicated end-use applications i.e. Conditions A, B and C, should have

minimum compressive strengths not less than those given in Table 4.6 below. Normalised

compressive strengths for block sized clay and calcium silicate masonry units not complying

with brick dimensional format are given in Table 4.7.


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a. One storey

Fig. 4.5 : Compressive strength of masonry materials


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Table 4.6 : Compressive strengths of masonry units complying with BS EN 771-1 to -5 (N/mm2)

MasonryMaterial

Clay masonryunits to

BS EN 771-1

Calcium silicatemasonry units to

BS EN 771-2

Aggregateconcretemasonryunits to

BS EN 771-3

Autoclavedaerated conc.

Masonryunits to

BS EN 771-4

Manufacturedstone masonryunits to BS EN

771-5

Condition A (see Fig. 4.5)

Any unitcomplying withBS EN 771-5will beacceptable forconditions A, Band C

Brick Group 16.0

Group 29.0

Group 16.0

Group 29.0

6.0 -

Block SeeTable 7

SeeTable 7

SeeTable 7

See Table7

2.9* 2.9

Condition B (See Fig. 4.5)

Brick Group 19.0

Group 213.0

Group 19.0

Group 213.0

9.0 -

Block SeeTable 7

SeeTable 7

SeeTable 7

See Table7

7.3* 7.3

Condition C (See Fig. 4.5)

Brick Group1

18.0

Group 2

25.0

Group 1

18.0

Group 2

25.0

18.0 -

Block SeeTable 7

SeeTable 7

SeeTable 7

SeeTable 7

7.3* 7.3

*These values are dry strengths to BS EN 772-1

Notes:1. This table applies to Group 1and Group 2 materials.2. For the EN 771 series of standards for masonry materials, the values of declared compressive strengths

(N/mm2) given in this table are mean values.

3. Brick: a masonry unit having work sizes not exceeding 337.5mm in length or 112.5mm in height.4. Block: a masonry unit exceeding either of the limiting work sizes of a brick and with a minimum height of

190mm. For blocks with smaller heights, excluding cuts or make up units, the strength requirements are asfor brick except for solid external walls where the blocks should have a compressive strength at least equalto that shown for block for an inner leaf of a cavity wall in the same position.

5. Group 1 masonry units have not more than 25% formed voids (20% for frogged bricks). Group 2 masonryunits have formed voids greater than 25%, but not more than 55%.

Table 4.7 : Normalised compressive strength of masonry units of clay and calcium silicate blockscomplying with BS EN 771-1 and -2 (N/mm

2)

Standard Condition(See Fig. 14)

Group 1masonry units

Group 2Masonry units

Clay masonry units to BS EN 771-1Calcium silicate masonry units toBS EN 771-2

A

B

C

5.0

7.5

15.0

8.0

11.0

21.0Notes:

1. Values are normalised compressive strengths (N/mm2). Compressive strengths of masonry

units should be revived according to EN 772-1.2. The table applies to clay and calcium silicate block masonry units where the work size

exceeds 337.5mm in length or 112.5mm in height.3. Group 1 masonry units have not more than 25% formed voids (20% for frogged bricks).

Group 2 masonry units have formed voids greater 25% but not more than 55%.


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(ii) Differences in the level of the ground or other solid cons

Documents

Domestic Extensions Project Guide