Profiled Metal Roofing Design

8/9/2019 Profiled Metal Roofing Design

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THE METAL CLADDING & ROOFING MANUFACTURERS ASSOCIATION LIMITED

PROFILED METAL ROOFINGDESIGN GUIDE

MCRMA Technical Paper No. 6

CI/SfB

Nh2(23)

JUNE 2004


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Page1.0 Typical construction and assemblies 1 1.1 Introduction 1 1.2 Single skin system 1 1.3 Double skin system 1 1.4 Secret fix 1 1.5 Site-assembled composite 1 1.6 Factory made composite panels 1 1.7 Under-purlin lining 2

2.0 Components 2 2.1 Profiled sheets 2 2.2 Coatings 2 2.3 Spacer systems 3 2.4 Fasteners 3 2.5 Insulation 5

3.0 Weathertightness 5 3.1 Performance requirements 5 3.2 Roof pitch 5 3.3 End laps 5 3.4 Side laps 6

4.0 Thermal Performance 7 4.1 Regulations 7 4.2 U-values 7 4.3 Thermal bridging 9 4.4 Air leakage 11

5.0 Interstitial condensation 13 5.1 Risks of interstitial condensation 13 5.2 Design to avoid interstitial condensation 14 5.3 Breather membranes 14 5.4 Rooflights 15

6.0 Acoustics 15 6.1 Sound reduction 15 6.2 Sound absorption 15

7.0 Performance in fire 16 7.1 Introduction 16 7.2 Reaction to fire 16 7.3 External surface 17 7.4 Cavity barriers 17 7.5 Building insurance 17

8.0 Structural performance 17 8.1 Design loading 17 8.2 Load span tables 18 8.3 Spacer systems 18 8.4 Lateral restraint 18

Contents


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Page9.0 Durability 19 9.1 Introduction 19 9.2 Materials 19 9.3 Design details 20 9.4 Maintenance 20

10.0 Sustainability 21 10.1 Sustainable construction 21 10.2 Life Cycle Assessment 21 10.3 ISO 14001 22

11.0 Construction details and accessories 23

11.1 Junction details 23 11.2 Rooflights 24 11.3 Roof drainage 24 11.4 Small penetrations 24 11.5 Large penetrations 24 11.6 Flashings 25

12.0 Site work 26 12.1 General 26 12.2 Transport, handling and storage 26 12.3 Site cutting 26 12.4 Health and safety 26

13.0 Inspection and maintenance 27

14.0 References 28

© The Metal Cladding & Roofing Manufacturers Association Limited. June 2004

For up to date information on metal roof and wall cladding, including downloadable constructiondetails, visit www.mcrma.co.uk.


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1.1 IntroductionProfiled steel or aluminium sheets are used invarious roof constructions as shown below. Theguidance in this document applies in principleto all of these constructions, but primarily to thedouble skin system with trapezoidal or secret fixprofiled sheets, which is the most common type ofmetal roof construction used in the UK. Sinusoidalprofiles (for example corrugated iron) are not usedfor modern industrial and commercial roofs andthey are not covered by this guide. Secret fixsystems and composite panels are both describedfully in separate MCRMA technical guides. Theguidance in this document is generally consistentwith that in BS5427 The Code of Practice for theuse of Profiled Sheet for Roof and Wall Claddingon Buildings .

1.2 Single skin systemAn uninsulated profiled metal sheet fixed directly tothe purlins.

Fig 1: Single skin system

1.3 Double skin systemThis common type of construction consistsgenerally of a shallow profiled metal liner, a spacer

system and an outer sheet usually in a deeperprofile. The insulation is typically a mineral fibrequilt.

Fig 2: Double skin system

1.4 Secret fixSecret fix or standing seam systems use specialprofiles for the weather sheet with virtually noexposed through fasteners. This provides greaterreliability and means they can be used on very lowslope roofs. Secret fix sheets can be used as theweather sheet in all the roof constructions shown.

NOTE: For detailed information see MCRMAtechnical paper No 3 Secret fix roofingdesign guide

Fig 3: Secret fix system

1.5 Site-assembled compositeThis is a special site-assembled double skin

construction which uses profiled rigid insulation tocompletely fill the space between liner and weathersheets. The insulation may be polyurethane,polyisocyanurate or mineral fibre. No spacersystems are required.

Figure 4 – Site-assembled composite

1.6 Factory made composite panelsThese panels combine a liner, rigid insulation andweather sheet in a single factory made component.Although it is possible to use various types of rigidinsulation only polyurethane and polyisocyanurateexpand and autohesively bond to the facingsduring manufacture and they are therefore used for

the majority of composite roof panels.

Typical construction andassemblies


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Fasteners can be divided into two distinctcategories:

a) Primary These fasteners are designed to transfer all loadson the cladding system back to the supportingstructure so their strength is particularly important.If the fasteners are exposed they must alsoprovide a weathertight seal. They are normallyused in the valley of the profile. If crown fixingis recommended by the manufacturers, saddlewashers are usually required.

Fig 9: Typical primary fastener positions

The most common fasteners are self-drilling andself-tapping which can be installed in one simpleoperation into a variety of materials. Self-tappingscrews (with no drill point) are also available andmay be preferred in some applications. Mostscrews are now available in plated carbon steel orstainless steel, they come complete with a 16mmor 19mm diameter sealing washer and they canhave integral colour matched plastic heads orseparate push-on plastic caps. The strength ofa fixing depends on the fastener design and thethickness and yield strength of the purlin. Fastenermanufacturers and suppliers can supply strengthdata for their products installed in a range ofmaterials and thicknesses.

b) Secondary These fasteners are used to connect side laps,flashings etc. and they are not normally consideredas structural. However, where the fasteners areproviding lateral restraint or in a stressed skin

design, their strength would have to be consideredin the structural calculations.

Fig 10: Primary fasteners

Fig 11: Secondary fasteners

�

Mini-zed topurlin

through

ferrule

Bracketto purlin

Outer sheetto spacer

Compositeand site-

assembledcomposite

Metalsheet to

hot rolledsteel

Metal sheetto timber

Self drilling and tapping

Self tapping

Self drilling stitcher screws

Rivets

Neoprene grommet for rooflights


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In most cases these fasteners are exposed sothey must provide a weathertight seal. The mostcommon types are self-drilling and self-tappingstitcher screws with sealing washers and plasticheads/caps, or sealed rivets with or without plasticcaps.

When selecting the fasteners for a roof, thestructural performance, environmental conditions,corrosion resistance (including bi-metalliccorrosion), weathertightness and ease ofapplication must all be considered.

2.5 InsulationThe majority of site-assembled double skin roofconstructions use mineral fibre (glass or rock)quilt insulation supplied in rolls. When unrolled onsite the material expands to at least its requiredthickness and normally fills the cavity created bythe spacer system between the liner and weathersheet. The material is normally quite soft anddeforms around the small profiled ribs on the liner,and under and around the spacer system.

Rigid mineral fibre insulation slabs are also usedin some circumstances but this is less deformablethan the quilt and the roofing system has to bedesigned and installed with this in mind so that nogaps are left in the insulation layer.

Site-assembled composite constructions do notuse spacer systems and rely on the rigidity of theinsulation and special fasteners to support theouter sheet. In this case rigid profiled polyurethane,polyisocyanurate, and mineral fibre insulation areused.

3.1 Performance requirementsThe effects of the internal and external conditionson roof constructions are more severe than onwall cladding. The satisfactory performance ofany roof cladding system depends on the correctselection of materials, and the design, detailingand assembly of every aspect of the construction.The position and shape of the building, all types ofloading and the internal and external environmentsmust all be considered.

3.2 Roof pitchOver recent years roof pitches on industrial andcommercial buildings have tended to be as shallowas possible in order to reduce the heating costsof the large empty roof spaces, and for aestheticreasons. Most through fixed trapezoidal metal roofsheets incorporating normal side and end lapsare suitable for roof pitches of 4° or steeper. Notethat the risk of leakage decreases as the pitchincreases. For increased reliability and for pitchesdown to I ° (after all deflections), a secret fix systemmust be used with no exposed through fasteners,

special side laps and preferably no end laps.

NOTE: For detailed guidance refer to MCRMAtechnical paper No 3 Secret fix roofingdesign guide .

3.3 End lapsThe standard end lap arrangement for trapezoidalsteel weather sheets at any roof pitch greater thanor equal to 4° is shown in Fig 12. Typically, twocontinuous runs of 6 × 5 or 6mm diameter butylsealant strips are used. The lower run is the

primary weather seal and should be positionedas close as possible to the edge of the top sheet.The upper seal is to prevent moisture entering theoverlap from inside the cavity.

The fasteners clamp the two sheets directly to thespacer (or rigid insulation) and both runs of sealantare compressed. The frequency of the fastenersdepends on the profile but must be sufficient toconsistently clamp the sheets and seals along thefull length, typically every trough. For optimumdurability the edge of the overlapping sheet must

be painted.

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Weathertightness


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Fig 12: End lap detail for roof pitches greater than 4°

Aluminium weather sheets may require a slightlydifferent end lap arrangement to allow for the

coefficient of linear expansion, which is twice thatof steel. The actual arrangement will depend onthe length and colour of the sheets. The colourdetermines the maximum temperature that thesheets reach and therefore the maximum degreeof expansion.

Fig 13 shows a typical detail, which allows somemovement at the end lap joint without affectingthe fasteners. Special sealants need to be usedto provide a suitable sliding joint, and the overlapmay be increased to 200mm. When aluminium

profiles are being considered the manufacturer’srecommendations should be followed.

Fig 13: End lap detail to allow some movement inthe joint

For increased reliability the number of end lapsshould always be minimised as far as possible.

3.4 Side lapsA typical side lap between trapezoidal sheetsis shown in Fig 14. This arrangement is idealbecause the outer rib of the underlapping sheetis complete and provides the optimum support forthe overlapping sheet so that the side lap can besealed and stitched effectively.

It is common to see specifications recommendingthat side laps should be arranged away from theprevailing wind. This is sensible in principle but isoften not practical. The joint must be weathertightin all circumstances.

Fig 14: Typical side lap detail

Typically, a continuous run of 6 × 5 or 6mmdiameter butyl sealant strip is used on the centre ofthe underlapping rib or on the weather side of thelap on all trapezoidal sheets. All trapezoidal roofsheets should be stitched through the centre ofthe top of the side lap at typically 450mm centresusing sealed stitcher screws or rivets.

1 5 0 m m

1 5 m m

Two runsof sealant

2 0 0 m m

Two runsof sealant

Alternative seal positions


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4.1 RegulationsThe thermal performance of the roof cladding isimportant because it affects the amount of energyrequired to heat the building and will influence therunning costs and the comfort of the occupants.Approved Document L2 to the Building Regulations(2002 Edition) Conservation of Fuel and Power defines the required levels of performance interms of U values, thermal bridging, and airtightness, and explains how the requirements canbe achieved. Site testing of the completed buildingis required in many cases, to ensure the designperformance has been achieved. It means that thebuilding designer and cladding contractor need topay more attention to the thermal performance ofindividual details of the construction, which werenot previously defined.

The Approved Document refers to MCRMAtechnical paper No 14 Guidance for the designof metal roofing and cladding to comply withApproved Document L2:2001 , for further moredetailed information.

4.2 U-values4.2.1 Methods of determining U-values The elemental U-value or thermal transmittancespecified in Approved Document L2 for industrialand commercial roofs is 0.25 W/m 2K.

Approved Document L2 requires that profileshapes, and repeating thermal bridges, such asmetal spacers and other fixings, are taken intoaccount when a U-value is calculated using the‘combined’ method, which is specified in BS EN

ISO 69461 and CIBSE Guide A3

2. However, thismethod is designed to deal with the type of thermal

bridges such as timber studs, which distort theheat flow relatively little, and it specifically excludeselements in which the insulation layer is crossed bythin metal components.

Ultimately, the U-value of this type of elementhas to be calculated by developing a full two- orthree-dimensional thermal model of the structureto calculate the heat flows. This is a complex taskrequiring software, which complies with BS EN

ISO 10211:1996 3, and is usually carried out only

by specialist consultants. A number of systemmanufacturers provide U-values of their systemsfor different insulation thicknesses, fixing spacings,etc that have been calculated by independentbodies.

A range of simplified methods has been developedfor use in specific cases; these can be obtainedfrom the following sources:

- Twin-skin systems with zed spacers arecovered in BRE Information Paper IP 10/02 4.

- Twin skin systems with rail and bracket spacersare covered in SCI Technical Information SheetP312 5.

- A software package KOBRA, which allows thecalculation of the thermal properties of a rangeof wall and roof systems is used by membersof the MCRMA.

- A U-value calculator, which covers all typesof walls, roofs and floors, is available and canbe downloaded from http://projects.bre.co.uk/ uvalues/.

The appropriate methods that should be used tocalculate U-values in all instances are specified inBRE Report BR443 6.

Alternatively the thermal transmittance (U) of aroof cladding construction can be measured usinga calibrated hot box, in accordance with EN ISO8990.

4.2.2 Insulation thickness required

The following table indicates the approximateinsulation thicknesses required to achieve a U-value of 0.25 W/m 2K using a typical type of railand bracket, liner and outer sheet. The effect ofdifferent insulation materials and varying purlin andbracket centres is also indicated.

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Thermal Performance


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Purlin spacing : m

Insulationconductivity : W/mK

Bracketspacing : m

0.9 1.4 1.8

0.035 0.5 175 160 155

1.0 160 150 150

0.04 0.5 210 195 185

1.0 190 180 175

0.045 0.5 220 210 200

1.0 205 200 195

Fig 15: Insulation thickness in mm necessary to achieve a U-value of 0.25 W/m 2 K

(Assuming a liner sheet with 18mm profiles at 200mm centres and an outer sheet with 35mm profiles at167mm centres)

These are the theoretical minimum insulationthicknesses required. In practice, the insulationquilt used is normally selected from a range ofstandard thicknesses such as 80mm, 100mm etc.and a building might contain a variety of purlinspacings so the construction and materials have tobe specified with care.

No matter which insulation is used it is vital that thematerial is installed carefully throughout, ensuringthere are no gaps particularly at apertures, ridge,eaves, corners etc. The continuity of insulation isspecified in the Regulations, and a thermographic

survey might be used to check the consistency ofthe insulation in the completed building. Individualmanufacturers’ literature provides details of theirsystems.

In some cases it may only be necessary for theroofing supplier to demonstrate that the systemU-values are less than or equal to the value of0.25 W/m 2K required in Approved DocumentL2. However a more detailed knowledge ofthe specific U-values of all the elements of thebuilding will be necessary if it is proposed to

trade off areas, assess the contribution of thermal

bridges or trade off heating system efficiency withimproved fabric performance (see below). In thesecases it will be necessary for some member ofthe design team, generally the architect, to takeresponsibility for collating the heat loss from allthe plane areas (U-values), and thermal bridges at

junctions between the plane areas ( -values), anddimensions of the individual components, from anumber of different suppliers.

4.2.3 Trading off U-values and areasWithin certain constraints, Approved Document L2allows trading between the elemental U-values and

the areas of components specified in the ApprovedDocument, provided that the overall heat lossfrom the building is less than that from a notionalbuilding of the same size and shape, with theelemental U-values and areas.

The various constraints are:

a. The U-value of any opaque part of a roof is noworse than 0.35 W/m 2K.

This constraint is designed to reduce the riskof surface condensation. It accommodates

situations, such as the need to include morefixings, which raise the U-value, in areas of a

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roof subject to greater wind uplift. The areaweighted average U-value of the roof shouldmeet the 0.25 W/m 2K limit.

b. If the area of openings in the proposed buildingis less than the values shown in the ApprovedDocument, the area weighted average U-valueof the opaque parts of the roof cannot exceedthe appropriate value in Table 1 of ApprovedDocument L2 by more than 0.02 W/m 2K.

c. No more than half the allowable rooflight areacan be converted into an increased area ofwindows and doors.

The possibilities for trading off and the constraintsmay be illustrated by some specific examples:

A) Table 1 of Approved Document L2:2002 givesthe elemental U-values for a roof with integralinsulation as 0.25 W/m 2K and for rooflights as 2.2W/m2K. Table 2 gives the maximum allowable rooflight area as 20%. This means that the averageU-value for the roof will be 0.8 × 0.25 + 0.2 × 2.2 =0.64 W/m 2K.

If the rooflight area was only 10%, the rooflightU-value could be increased to U rl so long as therelationship: 0.9 × 0.25 + 0.1 × U rl ≤ 0.64 wassatisfied. This means that U rl can be as high as4.1 W/m 2K. It is not, however possible to carry outthe similar calculation: 0.9 × U rf + 0.1 × 2.2 ≤ 0.64to allow a U-value of the opaque parts of the roofup to 0.47 W/m 2K; constraint b), above, limits thisto 0.27 W/m 2K.

B) If, for structural reasons, more fixings are used

near the eaves and ridge, raising the U-value of25% of the opaque area of the roof to 0.35 W/m 2K,the maximum allowed by constraint a), the U-valueof the remaining 75% of the opaque part of the roofhas to be reduced to satisfy:0.75 × U rf + 0.25 × 0.35 ≤ 0.27, i.e. to 0.24 W/m 2K.

4.2.4 Responsibility for calculation of U-values The U-values of the individual components of abuilding should be specified during the designstage. These will often follow the elemental valuesspecified in Table 1 of Approved Document L2,however there may be circumstances in which

lower (better) values may be specified in certainareas to allow trading off with other areas. Theresponsibility for calculating the U-values ofcladding systems usually lies with the supplierof the system, who should be in a position to justify the values quoted by reference to thedocumentation discussed above or by producingresults from calculation carried out to BS EN ISO6946 or BS EN ISO 10211-1 as appropriate. Thevalues for other cladding elements such as smokevents or rooflights should be provided by theappropriate manufacturer.

4.3 Thermal bridging4.3.1 Effects of thermal bridgingThermal bridges are areas of the fabric where thegeometry of the structure and/or the presence ofhigh conductivity materials crossing the insulationlead to heat flows that are locally higher than insurrounding areas. These both add to the totalenergy demand of the building and lower theinternal surface temperatures. Depending on theenvironmental conditions within the building and

the nature of the internal surfaces this can leadto surface condensation or, less commonly inindustrial buildings, mould growth.

Approved Document L2 requires that the buildingfabric should be constructed so that there are nosignificant thermal bridges or gaps in the insulationlayer(s) within the various elements of the fabric,at the joints between elements and at the edgesof elements such as those around window anddoor openings. Thermal bridging within elements,such as at spacers, is taken into account when

calculating U-values, so here it is necessary toconcentrate on the thermal bridging at junctionsand penetrations. One way of demonstratingcompliance is to utilise details and practice thathave been independently demonstrated as beingsatisfactory. This can be done for domestic styleconstructions by following the details given in therobust construction details publication. However,as these often do not apply to industrial buildings,calculations or the use of robust design practicesmust be used to demonstrate compliance. BRE IP17/01 7 and the MCRMA technical paper No 14 8 specify the necessary procedures.

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4.3.2 Design to avoid condensation and mouldgrowth In buildings, especially housing, which haveabsorbent internal surfaces, the lowered surfacetemperatures at thermal bridges can cause mouldgrowth, which is a major cause of respiratory allergiessuch as asthma. Mould growth, which occurs at asurface relative humidity of 80%, is, however, veryrare on the impermeable internal surfaces of metalfaced roofs. Condensation, depositing drops ofwater on the surface, which does not occur until thesurface relative humidity has reached 100%, is muchmore likely in this case. Even when condensationoccurs, often less than 10 grams per square metreaccumulate, seen as a fine mist on the surface,which rapidly disperses as temperatures rise. Atleast 70 g/m 2 must accumulate before it will rundown a sloping surface and 150g/m 2 before drippingwill occur from a horizontal surface.

The concept of surface temperature factor, orf-value, is used to separate the performance ofthe structure from the imposed environmental

conditions; this is defined by:

f =Ts - TeTi - Te

where Ts is the local surface temperature Te is the external air temperature Ti is the internal air temperature.

Surface temperature criteria, which define f min and which are more appropriate to industrialbuildings have been established based on BSEN ISO 13788:2001 9. Different building types fall

into the different classes shown in Fig 16, withthe minimum f-values, which are required to avoidcondensation in the different internal environments.

The f-value for a particular construction can beestablished by computer modelling, as part of theheat loss calculation (see 4.3.4).

4.3.3 Design to reduce heat loss throughthermal bridges Heat loss through linear thermal bridges (at

junctions and openings) is expressed in terms

of the linear thermal transmittance or -value(pronounced ‘psi’). This is the extra heat loss

Fig 16: Internal humidity classes and theminimum temperature factor necessary to preventcondensation

Humidityclass

Building type Minimumf-value

1 Storage areas 0.30

2 Offices, shops 0.50

3 Dwellings with lowoccupancy

0.65

4 Dwellings with highoccupancy, sportshalls, kitchens,canteens; buildingsheated with un-fluedgas heaters

0.80

5 Special buildings,e.g. laundry, brewery,swimming pool

0.90

through the junction, over and above the heat lossthrough the adjoining plane elements.

BRE IP 17/01 contains a table of -values forthermal bridges in typical domestic constructions;if the calculated -values in the building underdesign are less than or equal to these values,then no further action is necessary. If the thermalbridges under investigation do not appear in thistable, which is most likely in the case of industrialbuildings, the total heat loss for the buildingfabric must be calculated, and compared withthe permitted maximum heat loss from a notionalbuilding with the same size and shape.

The permitted heat loss through thermal bridges is10% of the sum of the heat loss through the planeareas of the notional building. So to satisfy theRegulations:

Aac ·Uac + L· < 1.1 × Ael·UelWhere: A ac and U ac are the elemental areas and

U-values of the real building.

L· is the sum of the heat loss through junctions.

Ael and U el are the elemental areas and U-values of a notional building with the samesize and shape as the real building.


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The fact that the heat loss from the real buildingis compared with that from the notional buildingmeans that, provided that the surface temperaturecriteria in Fig 16 are met, it is possible to trade offareas and U-values against thermal bridges. Sothat if it was not possible to design out a severethermal bridge, the criterion could, for example, bemet by reducing the rooflight area from 20%, whileretaining the rooflight U-value at 2.2 W/m 2K. (Seeexample in MCRMA technical bulletin No 11).

The calculation of -values and f-values is not

required by Scottish Technical Standard J.

4.3.4 Sources of -values and f-values Because thermal bridges occur at junctions,complex two dimensional calculations usingsoftware that complies with BS EN ISO 10211-1:1996 are needed to determine the -values andf-values. These can be carried out by specialistcontractors, but are time consuming and expensive.A number of manufacturers have commissionedcalculations of the necessary values and publishthem in catalogues of standard details. These aregenerally accepted by building control authorities.There are however, no generally acceptedconventions for carrying out these calculations,and it is possible that cases of dispute will arise.

While cladding suppliers can generate thenecessary thermal bridge parameters for theirsystems, there may be particular difficultieswhere different systems meet, for example wherea twin skin roof from one manufacturer meetsa composite panel wall from another. In thesecases it will be necessary for the building designer/ architect to arrange for the -values and f-valuesto be calculated for the particular instance.

Calculating the total heat loss for the buildingrequires information on the areas and U-values,and details of junctions of all the variouscomponents of the building, which will have beenprovided by a number of suppliers and trades.This means that realistically it can only be done bythe building designer/architect.

Some typical junction details for twin skin metalroofing are shown in section 11. Many of these

have no additional metal crossing the layerof insulation and the insulation is effectivelycontinuous, and as a result the values are verylow. Any details which incorporate these designprinciples would be expected to have low values,and contribute very little to the total heat loss fromthe building.

4.4 Air leakageBesides the fabric heat loss through the planeareas and thermal bridges, described in sections4.2 and 4.3, air leakage from the interior can add

very significantly to the total energy demand ofa building. Ventilation heat loss can account foras much as 50% of the total heat loss from aleaky building, leading to a significant potentialfor savings if the leakage paths are identifiedand sealed. A further disadvantage of very leakybuildings is that their internal environment isdifficult to predict and control, leading either toplant oversizing or to uncomfortable conditionswithin the building.

Approved Document L2: 2002 lays stress onthe need to achieve a reasonable standard ofairtightness, and makes suggestions as to howthis might be achieved by appropriate sealingmeasures. It also suggests that it may benecessary to demonstrate compliance with a reportfrom a ‘competent person’ that appropriate designdetails and building techniques have been usedor, for buildings larger than 1000 m 2 floor area,to carry out an air leakage test. It is, however,important to remember that the cladding system isonly one of the areas of the building envelope that

contribute to the leakage. Junctions and openingssuch as doors, loading bays, windows, rooflights,smoke vents and service penetrations may bemore important.

4.4.1 Methods for measuring airtightness Techniques for the testing and analysis of the airleakage of buildings are described very fully inCIBSE Guide TM23, which should be consulted forfuller information. An air leakage test is carried outby mounting a fan (or fans) in a suitable aperturewithin the building envelope, usually a doorway,

running the fan to blow air into the building creates

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a pressure difference across the building envelope– see Fig 17 and Fig 18. The fan speed is varied tocreate a range of pressure differences up to about50 Pa (1Pa = 1N/m 2) and the flow rate recorded ateach stage. The flow rate at 50 Pa, Q 50 , normallyin m 3 /h, which is found from a plot of the flowrate against pressure difference, is reported asthe parameter which defines the ‘leakiness’ of thebuilding.

Fig 17: Diagrammatic representation of wholebuilding leakage test

Fig 18: Large fan for testing industrial buildings

A pressure test relies on the assumption thatthe pressure difference is uniform over theentire building envelope. This imposes certainrestrictions on the external climate parametersthat prevail during the test. Ideally the internalto external temperature difference should be lessthan 10°C and the wind speed should be less than

3 m/s. Higher temperature differences lead to

high stack pressure differences that distort the testresults, especially in very tall buildings, and highwind speeds cause random pressure differences,which make accurate readings difficult.

During a test, all internal doors should be wedgedopen and, where appropriate, combustionappliances must be switched off and any openflues and air supply openings temporarily sealed(flues from room-sealed appliances, such asbalanced flues in domestic appliances, do not needto be sealed). External doors and other purpose-

made openings in the building envelope should beclosed and mechanical ventilation systems turnedoff, with the inlet and outlet grilles sealed. Firedampers and ventilation louvres should be closed.Drainage traps should contain water.

The cladding or roofing system is only one of thepossible leakage routes from a building and asimple fan pressurisation test quantifies the totalair leakage of a building, but does not directlyidentify the leakage paths. A number of methodscan be used to provide more information, includingreleasing smoke from smoke pencils or puffers tovisualise specific flow paths; an internal infra-redsurvey of a depressurised building in cold weatherwill rapidly reveal areas that are being chilled byincoming air; repeated testing of a building assuccessive specific components are sealed, andisolating and testing specific components.

4.4.2 Required air tightness The parameter that is specified in ApprovedDocument L2 to quantify the air leakagerate through the building envelope is the airpermeability. This is measured by the fanpressurisation technique and is expressed in termsof the volume flow of air per hour (m 3 /h) suppliedto the space, per square metre (m 2) of buildingenvelope (including the ground floor area) for aspecified inside to outside pressure difference of50 Pa.

Section 2.4 of Approved Document L2 requiresthat the air permeability should be less than10 m 3 /h/m 2 at 50 Pa.

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4.4.3 Important leakage routes To minimise air leakage through twin skin metalroofing, the liner side of the construction must besealed as effectively as possible. This involvessealing:

- Liner end laps – typically with butyl sealant stripand additional fasteners

- Liner side laps and side joint at perimeter– typically with a wide butyl faced tape

- Fasteners – with normal washers- Perimeter or ends of sheets – with profiled liner

fillers and additional fasteners- Any penetrations – such as pipes

With a good standard of construction and goodattention to detail it is easy to achieve a twin skinmetal roof structure that is tight enough to pass theApproved Document L2 criterion.

However, as noted above, the cladding system isonly one of the areas of the building envelope thatcontribute to air leakage. Junctions and openingssuch as doors, loading bays, windows, rooflights,

smoke vents and service penetrations, may bemore important.

The typical junction details shown in section 11indicate where air seals have to be fitted, butreference should be made to manufacturers’information for details of their particular systems.In every case, satisfactory sealing will only beachieved by good workmanship on site.

Interstitial condensation means condensationthat occurs inside a construction, usually hiddenfrom view. In normal metal clad industrial buildingapplications it is not an important issue, but insome more specialist applications it can be acritical issue.

5.1 Risks of interstitial condensationInterstitial condensation may occur if watervapour, generated within the building, is able topenetrate the liner and reach cold areas outsidethe insulation, where it may condense, usually

on the outer sheet. If sufficient condensationoccurred, it could potentially cause problems suchas corrosion of metal components and wettingof thermal insulation, and the condensate couldrun into the building staining internal finishes ordripping onto equipment or processes. Because ofthe inherent impermeability of metal liners, theseproblems do not occur in normal applications.Small amounts of condensate, 0 – 10 g/m 2, afine mist, will commonly occur on most nights,even in the summer, however these will usually

evaporate rapidly during the day, without leading toany long term accumulation. Problems will occurwhen the rate of condensation is high enough toallow accumulations sufficient to cause running ordripping of the condensed water.

The risks of interstitial condensation depend onthe design and construction of the roof, and theinternal and external climate of the building. Riskswill be higher in colder external climates and inbuildings such as swimming pools, laundriesor some food processing plant, that operate at

high internal humidities. Buildings, which areoperated at an over pressure, to reduce theingress of contaminants, are at especial risk asthe over pressure can drive warm moist air into thestructure.

It should be remembered that, while interstitialcondensation is usually caused in the UK by coldexternal climates, severe problems can occur inair conditioned buildings in warm humid climates,when the vapour drive is from outside to in. Itis essential that designs are appropriate to the

climate in which the building is situated.

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Interstitial condensation


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General guidance for the design of structuresto minimise the risks of interstitial condensationis given in BS5250:2002 and a procedure forcalculating the accumulation of condensate in anyclimate is given in BS EN ISO 13788:2002.

5.2 Design to avoid interstitialcondensationThe moisture content of the internal environmentshould be assessed and, if appropriate, controlledby providing the correct levels of ventilation orby air conditioning. This would normally be theresponsibility of the building services engineer.However, it should be recognised that there willbe some degree of moisture load in almost allbuilding; the design of the roof structure shouldtherefore:

• prevent water vapour reaching the coldareas of the roof structure, by including animpermeable vapour control layer on the warmside of the insulation; and

• provide means for any water vapour that does

penetrate the structure to escape.

5.2.1 Vapour control layer To minimise the potential for interstitialcondensation in double metal skin constructionthe most critical part of the construction is thevapour control layer. This is used to minimise theamount of moisture vapour which can enter theconstruction by diffusion and air leakage. It mustbe positioned on the warm side of the insulation.The vapour control layer can be made by carefullysealing the profiled metal liner or by providing aseparate polythene membrane on top of the liner.In either case it is essential that the vapour controllayer is continuous throughout the roof and alllaps are sealed, including at abutments, rooflights,penetrations, gutters, ridge etc. Note thatconstructing the liner to minimise air leakage (seesection 4.4) will automatically provide the vapourcontrol layer.

The same principles apply to all the other typesof metal roof construction shown in section 1. Allsystems require attention to detail in their designand construction on site.

5.2.2. Ventilation No matter how well the vapour control layer isconstructed, some moisture vapour may get intothe construction and there must be provision in thedesign to deal with it. The smaller the roof voidsand the insulation cavity the less moisture there islikely to be present to condense. The roof voidsshould therefore be the minimum required and theweathersheet should be in contact with the insulation.

In site-assembled double skin constructions withstandard trapezoidal sheets ventilated fillers

must be used at the ridge and eaves to allowsome breathing through the ribs in the profiledweathersheet. This helps to disperse any moisturevapour which is in the construction.

Other types of profiled sheets, such as secret fix,use different methods of ventilation. See individualmanufacturers’ details.

5.3 Breather membranesA breather membrane is sometimes fitted on topof the insulation in site-assembled double skin

constructions. This is designed to allow moisturevapour to breathe out of the construction and toprevent any condensation, which might form onthe ribs of the outer sheet from dripping onto theinsulation. It should be detailed to allow any wateron its surface to drain down to the gutter.

Experience has shown that a breather membraneis not required in most circumstances. Itsrequirement depends on the building’s internalenvironment and the application. In normalapplications, provided the vapour control layerhas been properly installed, the amount ofcondensation is likely to be small and no breathermembrane is required. A breather membraneonly needs to be considered when the buildinghas a high humidity internal environment, andthen it should only be used in conjunction with aneffective vapour control layer and eaves to ridgeventilation through the profile ribs.

Site-assembled composite systems and factory-madecomposite panels with rigid foam insulation have novoids above the insulation so there is no requirementfor rib ventilation or breather membranes.


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5.4 RooflightsEnsuring continuity of the vapour control layer andventilation through profiled ribs normally meansthat site-assembled rooflights should be usedwith site-assembled double skin cladding systems.Factory-assembled rooflights are used withcomposite panels.

Compared to the surrounding insulated cladding,rooflights have a significantly worse thermalperformance, there is therefore an increasedpotential for condensation at rooflights. Misting

of all factory or site-assembled rooflights canbe anticipated from time to time, but it generallydissipates without harmful effects. However, ifcondensation is unacceptable rooflights should beavoided completely.

NOTE: For detailed guidance refer to MCRMAtechnical paper No 1 Recommendedgood practice for daylighting in metal cladbuildings .

The acoustic performance of a building canbe an important aspect of the design and theroof construction can play a major role in theperformance. For detailed guidance see MCRMAtechnical paper No 8 Acoustic design guide formetal roof and wall cladding . There are twopotential acoustic requirements:

6.1 Sound reductionIn this case the objective is to provide soundreduction through the structure in order to reducethe airborne transmission of noise inside a

building to the outside, or vice versa. The level ofperformance is referred to as the Sound ReductionIndex (SRI), measured in decibels (dB). Increasingthe SRI by 3dB will approximately halve thetransmitted sound energy.

While all metal skin roofing will provide somedegree of sound reduction the best performance isachieved with double skin constructions using softmineral fibre insulation. Additional layers may beadded within the construction, to further improvethe performance. Generally the heavier thecladding materials the better the Sound ReductionIndex. The actual performance of each claddingsystem also depends on the weathersheet andliner profiles and the method of assembly.

As a guide a typical site-assembled double skinsteel roof cladding might achieve a weightedSound Reduction Index (Rw) of approximately38 dB whereas the value for a factory-madecomposite roof panel (rigid insulation) might be 26dB. Individual manufacturers can provide advice ontheir own systems.

6.2 Sound absorptionSound absorption is used to reduce noise levelsand improve the acoustics inside a building byreducing unwanted multiple reflections from hardinternal surfaces (reverberation). This can beachieved with site-assembled double skin systemsby using a perforated liner and mineral fibreinsulation – see Fig 19. The perforations allow thenoise to enter the insulation where it is absorbed.

A typical system would include a polythene vapourcontrol layer over a relatively thin layer of acoustic

Acoustics


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insulation, followed by the main layer of thermalinsulation. The acoustic insulation should befaced with a glass fibre tissue or similar, to reducethe risk of dust falling through the perforatedliner. The exact acoustic performance of a roofcladding cannot be assumed from data for a similarconstruction and must be determined by test.

Fig 19: Twin skinned system with sound absorbingliner

7.1 IntroductionThe manner in which all elements of buildingconstruction perform in the event of a fire is ofprime concern to the designer, the occupant,the building owner and the building insurancecompany. Profiled metal cladding constructionsmust therefore conform to specific requirementswhich are defined in the Building RegulationsApproved Document B. They may also haveto comply with other requirements defined bybuilding insurance organisations, such as the LossPrevention Council (LPC).

The purpose of the Building Regulations is toensure the health and safety of people in or aboutthe building. The main requirements are:

a) To provide a safe means of escape for thebuilding occupants by preventing internal firespread.

b) To prevent the spread of fire to neighbouringproperty.

c) To prevent an external fire from setting fire tothe building.

d) To provide access for the Fire Brigade.

Generally metal roof cladding has to limit thespread of fire on its internal liner face, prevent thespread of fire through any cavity, and resist thespread and penetration of fire on its weathersheetside. Roofs do not usually need to provide anyperiod of fire resistance.

European Standard test methods have nowbeen published, and it is now possible to claimcompliance with Approved Document B usingeither the original BS 476 tests or the newEuropean tests.

7.2 Reaction to fireThe Surface Spread of Flame performance of theinternal surfaces of cladding sheets is determinedby testing to BS476 Part 7, giving classificationsof Class 1, 2, 3 & 4. Profiled liner sheets generallyachieve Class 1, the best rating. The sheets willalso achieve a rating of I


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of these two results means that liner sheets canbe designated Class O according to the BuildingRegulations, and their use is unrestricted.

Alternatively, the reaction to fire classification canbe determined using the Single Burning Item (SBI)test BS EN 13823 and the classification to BSEN 13501-1. Profiled steel liner with 25 micronpolyester coating achieves a classification A1,which is the best classification, and means use ofthe liners is unrestricted.

7.3 External surfaceExternal surfaces of roof cladding mustdemonstrate adequate resistance to the spreadand penetration of fire from outside. This istested according to BS476 Part 3 and givenclassifications of AA, AB etc through to DD. Thefirst letter denotes the rate of penetration throughthe construction, A being the best. The secondletter refers to the spread of flame, and again Ais the best. The classification is accompanied byEXT for external, S for sloping or F for flat. Profiledcladding generally achieves the top rating EXT.SAAor EXT.FAA.

There is no equivalent European classification forthe external fire performance of roofs in ApprovedDocument B (2003)

7.4 Cavity barriersThe regulations require that the size of all hiddencavities is restricted by cavity barriers to preventthe unseen spread of fire. However, accordingto B3 10.11g, cavity barriers are not requiredin double profiled metal skin roof constructionsprovided the surfaces of the insulation havea surface spread of flame of Class O or 1, orEuropean Class C-s3, d2 or better, and makecontact with the inner and outer sheets.

7.5 Building insuranceTypical twin metal skin roof constructions, whichare insulated with mineral fibre (glass or rock), areconsidered to be non-combustible by insurancecompanies, and do not create any particularhazard in relation to fire. No type approvals, such

as LPC, are normally required.

8.1 Design loadingRoof cladding systems and their fixings must bestrong enough to withstand the worst combinationsof wind, snow, imposed and dead loads calculatedin accordance with BS6399 – see Fig 20.

Fig 20: Load factors

There are two other special load cases for roofs:

• Foot traffic:

For many roof constructions point loading dueto foot traffic is one of the critical load cases.The minimum thicknesses specified in section2 are generally required to enable the roof towithstand foot traffic during assembly, but eventhese are not immune from damage, if care isnot taken. Metal roof cladding is normally onlydesigned for cleaning and maintenance loading.If regular access is required special walkwaysshould be provided. The appearance of someroofs, particularly if made of aluminium, may beimpaired by excessive foot traffic.

Structural performance

Load case (1) Load factors (2)

Dead + imposedDead + snow driftDead + windDead − wind

1.4 Wd + 1.6 Wi1.4 Wd + 1.05 Wsnow1.4 Wd + 1.4 Wind1.0Wd – 1.4 Wind

Attachment (3) 1.0Wd – 2.0 Wind

Notes:1. Wd = deadload

Wi = imposed load (BS6399:Part 3)Wsnow = snowdrift load (BS6399:Part 3)Wind = wind load (BS6399:Part 2)

2. These load factors are mostly from BS5950:Part 5 and are also adopted in BS5427. Theyapply to the metal profile.3. Attachment refers to the fixing assembly

(fasteners or clips). A minimum factor of 2should be used and may be increased for timberand other non-steel substances.


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500mm centres using 5.5mm diameter fastenersgenerally provide sufficient restraint to light gaugepurlins. Some 0.7mm thick aluminium liner profilesof certain alloy grades can give lateral restraint andthe manufacturers’ guidance should be sought.

Perforated liner sheets used in acoustic absorptionconstructions may not be strong enough to providesufficient restraint, depending on metal thickness,profile and the perforation pattern. Each caseshould be considered carefully.

It is the responsibility of the cladding contractor toensure that the correct fastener system is usedas recommended by the cladding or fastenermanufacturer.

9.1 IntroductionDurability is the ability of a building and its partsto perform its required function over a periodof time (see BS7543). Virtually all materials willchange physically when subjected to UV radiation,moisture and atmospheric pollution, and this willaffect their performance. The designer musttherefore ensure not only that the materials anddetails used are suitable initially but also that theywill have a satisfactory life, given the necessarymaintenance.

9.2 MaterialsThe materials selected for the roof can have asignificant effect on its durability and the amount ofmaintenance that will be necessary during its life.The components which are exposed to the weatherare particularly important.

The type of sheeting material, coating and colourmust all be considered. The performance mightalso depend on the shape and orientation of thebuilding and the environment. Generally, lightcoloured coatings are preferable because they donot absorb as much sunlight as dark colours, andthey are therefore cooler. This means they tend tohave the best life and they optimise the thermalperformance of the roof.

The table in Fig 21 gives an indication of the lifeof some coated sheets on a typical 5° roof slope,in an inland, non-polluted environment. Thelife-in-years shown concern the appearanceand the condition of the coating and it can beextended by repainting. The data has been taken

from manufacturers’ literature and is for generalguidance only.

Fig 21: Expected life of sheets with variouscoatings

Durability

Material Coating Life-in-years

Steel Plastisol - lightPlastisol - dark

Pvf2Multicoat

23101520

Aluminium Pvf2ARS

2015


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Note that failure to repaint aluminium will notsignificantly affect the ultimate life of the material.

More detailed information can be obtained from theindividual manufacturers.

Rooflights are also subject to gradual deterioration,which will cause fading and embrittlement. PVCis particularly susceptible. These changes can beminimised by careful attention to detail and regularcleaning and discussion should take place with themanufacturer regarding life span. Replacement

may be anticipated during the life of the building.

Both plated carbon steel and stainless steelfasteners are available. In most situations involvingcoated steel cladding, plated steel fastenersprovide acceptable performance as long as theirheads are protected from the weather. Integralplastic heads are more reliable than push oncaps and are preferable. Stainless steel fastenerscan be used for improved durability and must beused when fixing aluminium sheets, to prevent bi-metallic corrosion.

The durability of sealants and fillers is also animportant issue, for the long term performance ofthe building. Various grades of these materials areavailable, and some are more durable than others.They should be specified carefully to ensure theirperformance will be consistent with the othercomponents in the roof, as most are built into theconstruction and are very difficult to replace.

9.3 Design detailsThe materials chosen for the roof must be

assembled correctly in order to achieve themaximum durability. Avoiding water and dirt trapsby ensuring satisfactory slopes and end laps areparticularly important. For maximum durability,all exposed cut edges of steel outer sheets andflashings should be painted immediately afterinstallation.

The life expectancy of materials inside theconstruction such as spacers, insulation, fasteners,sealants and the reverse side of cladding and linersheets must also be considered. Provided the

building does not have an internal environment

with high humidity, and the roof cladding has beenconstructed correctly with an effective vapourcontrol layer and ventilation (where appropriate),the standard materials and coatings will besatisfactory.

9.4 MaintenanceThe coating life shown above, and therefore thedurability of the roof, is always dependent onregular maintenance. This will involve inspection,removal of debris, cleaning and repair of anydamage found. Gutters are likely to require the

most frequent attention because debris tends tocollect in them and can restrict their capacity, whilstincreasing the potential for corrosion.


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advantage of using a comparatively low massroofing system with less material resource inputthan other commercial systems.

Generally, metal cladding systems have relativelygood sustainability credentials as a result ofthe opportunities for recycling. Although metalsheeting systems would not be routinely reusedat the end of a roof systems life, the materialitself can be processed and recycled for otherapplications. Similarly, the materials used forthe manufacture of the metallic components can

contain some recycled material. Both of thesefactors reduce the environmental impact of metalcladding systems, and the growing recyclingindustry will help to continue to reduce this in future.

Correctly specified metal cladding and roofingsystems competently installed should have goodthermal performance and achieve a consistentlygood air tightness standard. This will ensurethat the heat loss through the structure will beconsistent with that set by the designer, rather thanbeing reliant on the performance of site workerswhich can often let masonry construction down.

10.3 ISO 14001Increasingly organisations are adopting ISO 14001in their management procedures which specifiesthe actual requirements for an environmentalmanagement system. It applies to thoseenvironmental aspects which the organisationhas control over and this extends to the design,procurement and maintenance of new buildings.As a result, such organisations are requiring thatthe design team demonstrates and justifies theperformance of a proposed development, andwhere appropriate establish an EnvironmentalPlan to ensure that the impacts of the constructionprocess are minimized. ISO 14001 is applicable toany organisation that wishes to:

• implement, maintain and improve anenvironmental management system;

• assure itself of its conformance with its ownstated environmental policy (those policycommitments of course must be made);

• ensure compliance with environmental lawsand regulations;

• seek certification of its environmentalmanagement system by an external third partyorganisation

Awareness of the issues in these documentscan help clients meet the increasingly complexdemands in a world where environmental concernsare growing in importance.


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11.1 Junction detailsThe typical details shown in this sectionillustrate the general arrangement at various

junctions. In each case, the value and f valueassociated with thermal bridging are shown.(See section 4). Proprietary systems may haveparticular requirements and the manufacturers’recommendations should always be followed.

All other junctions and details for a particularconstruction should be designed adopting thesame principles, ensuring continuity of the

weatherproofing, vapour/air control layer, andthermal insulation.

11.1.1 Ridge Provided that the insulation is continued over theridge to provide continuous coverage, as shown inFig 22, this will be a minimal thermal bridge.

Fig 22: Roof typically f min = 0.91 = 0.01 W/mK

11.1.2 Eaves The thermal bridge can be minimised by taking theroof liner back to the wall liner, so that it does notcross the wall insulation, and ensuring that anyvoid between the wall and roof insulation is fullyfilled with insulation as shown in Fig 23.

Fig 23: Eaves typically f min = 0.95 = 0.01 W/mK

11.1.3 Verge The thermal bridge can be minimised by taking theroof liner back to the wall liner, so that it does notcross the wall insulation, and ensuring that anyvoid between the wall and roof insulation is fullyfilled with insulation as shown in Fig 24.

Fig 24: Verge typically f min = 0.95 = 0.02 W/mK

11.1.4. Valley gutterThis detail can cause a very severe thermal bridgeif the metal outer layer of the gutter top and roofliner cut across the gutter insulation.

Making the gutter liner more robust and replacingpart or all of the gutter outer with a lowerconductivity material such as plastic where it cutsacross the insulation will reduce the thermal bridge;

if the roof liner is stopped short as well so that the

Construction details andaccessories

�

�

�

�

�


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Fig 27: Large penetration

Site applied GRP (or similar liquid applied system)can provide a good solution for all sizes andshapes of penetration. This must be carried out byspecialist sub contractors to ensure satisfactoryadhesion to the profiled weather sheeting. ContactMCRMA members for further details.

Whenever a penetration is fitted to the roof it isessential that the vapour control layer, insulationand breather membrane (if fitted) are correctlydetailed and constructed.

11.6 FlashingsFlashings are normally manufactured in the samematerial and thickness as the cladding, at least0.7mm for steel and 0.9mm for aluminium. Wherethe leg length of a flashing exceeds approximately200mm, it will be difficult to maintain a flatappearance and it is advisable to introduce abend or stiffener to straighten and strengthen thematerial. See examples in section 11.1)

Exposed edges should be stiffened using one ofthe details shown in Fig 28.

Fig 28

Most flashings used on roofs are ‘open’ and canbe lapped, typically including stitching fastenersat a maximum spacing of 100mm, and sealant onthe weatherside of the fasteners. ‘Closed’ sections,such as some eaves gutters, use butt strapsbehind the joint. These should be at least 150mmlong. For detailed guidance see MCRMA technicalpaper No. 11 Flashings for metal roof and wallcladding: design detailing and installation guide .

�

Lock roll

Welt

Stiffened edge

Further typical details are available on theMCRMA web site at www.mcrma.co.uk


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12.1 GeneralMetal roof cladding should be fully detailed oncladding drawings for each individual building.The drawings should include all the necessarydimensions, components, fasteners, seals etc.to enable the sheeters to install the cladding inaccordance with the designer’s requirements,in order to achieve the specified performance.Responsible supervision and regular inspection isessential to ensure structural integrity, satisfactoryperformance, acceptable appearance and qualityin general.

Deviations in steel frame construction, raftersand purlins etc. can often be greater than thoseacceptable for the cladding material, especially atcritical bearing positions such as end laps, or at

junctions.

The steel framework should be surveyed prior tohandover to the roofing contractor. Any deviationsin line, level and plumb must be acknowledged byall parties and the necessary adjustments made tosuit the cladding requirements before starting theinstallation.

12.2 Transport, handling and storageLoading and off-loading packs by crane or fork-liftshould be carried out with care to avoid damage tothe outermost sheets or panels in the pack. Neveroff-load with chains, use only wide soft slings forlifting. Use lifting beams, if recommended by themanufacturer

Fig 29: Slings used for lifting

Stacks should be carefully positioned and storedon site to prevent damage or deterioration.Particular attention should be paid to the followingpoints:

a) Position away from vehicle and pedestrianroutes

b) Site on bearers on firm flat groundc) Cover and ventilated) Ensure labelling is intact

Some sheets and panels are supplied with a

protective plastic film on the weatherface to helpprevent minor damage to the coating. This mustbe removed as soon as possible after the claddinghas been installed because if it is left in place forlong periods the film will become very difficult toremove. Individual manufacturers’ instructionsshould always be followed.

Fig 30: Storage of materials

12.3 Site cuttingSite cutting is generally avoided by the use ofsheets cut to length in the factory. However,where site cutting is unavoidable nibblers andreciprocating saws (jig saws) should be used.Abrasive wheel cutters must not be used becausethey generate heat and will damage the coating.For optimum durability site cut edges, eaves andend laps of steel sheets should be painted.

12.4 Health and safetyAs in all building work, good safety standardsare essential to prevent accidents. In accordancewith the Health and Safety at Work Act and theConstruction (Design and Management) or CDMRegulations, the building should now be designedwith safety in mind, not only for the construction

Site work


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period itself but also throughout the normal life ofthe building. This must include considering thesafety of people involved in maintenance, repairand even demolition. It might mean providingpermanent access to the roof via a fixed ladderand hatch, walkways and parapets, for example.

Construction of the roof is one of the mosthazardous operations because of the potential forfalls or material dropping onto people below. Thecontractors must plan and document a safe systemof work before starting construction. This would

take the fragility of the cladding materials intoaccount, but initially, safety nets are normally used.Whilst fully fixed metal roof sheeting is regarded asnon fragile, rooflights and profiled metal liners mustbe treated with more care. (See section 8.1)

In addition to the basic safe system of workingthe following specific precautions should be takenwhen using metal cladding:

a. Take care when handling sheets to avoid cutsfrom the edges of the sheets. Wear gloves to

protect hands.

b. Take normal precautions when lifting heavyawkward objects to avoid lifting injuries,in accordance with the Manual HandlingOperations Regulations.

c. When cutting wear goggles and dust masks.

The strength and visibility of some rooflights candeteriorate during the normal life of a buildingand present a hazard to maintenance workers,particularly if they have not been trained. Therooflights are usually fixed with fasteners with redplastic heads, to highlight their positions.

This sort of information must now be detailed ina safety file prepared by the planning supervisor(using information provided by the designer) andpassed on to the client at handover.

If in doubt about safety issues, guidance can beobtained from the construction section of the localHealth and Safety Executive.

Metal roof cladding is designed and manufacturedto give many years of reliable service, but toachieve this, a regular inspection and maintenanceprogramme is required.

Roof cladding and gutters should be inspectedat regular intervals and any deposits such asleaves, soil or litter must be removed. Any areasof corrosion or damage should be repaired inaccordance with the manufacturer’s maintenancemanual.

Roof traffic should be kept to a minimum and mustbe restricted to authorised trained personnel only,using appropriate safety measures.

Inspection andmaintenance


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MCRMA technical papers

1. BS EN ISO 6946 : 1997, Building componentsand building elements – Thermal resistanceand thermal transmittance – Calculationmethod .

2. CIBSE Guide A3, Thermal properties ofbuilding structures, CIBSE 1999.

3. BS EN ISO 10211-1: 1996, Thermal bridges inbuilding construction - Heat flows and surfacetemperatures, Part 1. General calculationmethods .

4. BRE Information Paper IP 10/02, Metalcladding : assessing the thermal performanceof built-up systems which use ‘Z’ spacers .

5. SCI Technical Information Sheet, P312 2002,Assessing thermal performance of built-upmetal roof and cladding systems using rail andbracket spacers .

6. BRE Report BR443, Conventions forcalculating U-values , BRE 2002.

7. BRE IP 17/01, Assessing the effect of thermalbridging at junctions and around openings ,BRE August 2001.

8. MCRMA technical paper No. 14, Guidance

for the design of metal roofing and cladding tocomply with Approved Document L2 : 2001 .

9. BS EN ISO 13788:2001, Hygrothermalperformance of building componentsand building elements - Internal surfacetemperature to avoid critical surface humidityand interstitial condensation – Calculationmethods .

LiabilityWhilst the information contained in this design guide is believedto be correct at the time of going to press, the Metal Cladding andRoofing Manufacturers Association and its member companiescannot be held responsible for any errors or inaccuracies and, inparticular, the specification of any application must be checkedwith the individual manufacturer concerned for a given installation.

The diagrams of typical constructions in this publication areillustrative only.

References

No 1 Recommended good practice for daylightingin metal clad buildings

No 2 Curved sheeting manualNo 3 Secret fix roofing design guideNo 4 Fire and external steel-clad walls: guidance

notes to the revised Building Regulations,1992 (out of print)

No 5 Metal wall cladding design guideNo 6 Profiled metal roofing design guideNo 7 Fire design of steel sheet clad external

walls for building: construction performancestandards and design

No 8 Acoustic design guide for metal roof andwall cladding

No 9 Composite roof and wall cladding designguide

No 10 Profiled metal cladding for roofs andwalls: guidance notes on revised BuildingRegulations 1995 parts L and F (out ofprint)

No 11 Flashings for metal roof and wall cladding:design, detailing and installation guide

No 12 Fasteners for metal roof and wall cladding:

design, detailing and installation guideNo 13 Composite slabs and beams using steel

decking: best practice for design andconstruction

No 14 Guidance for the design of metal roofingand cladding to comply with ApprovedDocument L2: 2001

No 15 New Applications: composite constructionNo 16 Guidance for the effective sealing of end lap

details in metal roofing constructions

Other publicationsThe Complete Package CDROM

Manufacturing tolerances for profiled metal roofand wall cladding

CladSafe latent defects insurance scheme: basicguide

Composite flooring systems: sustainableconstruction solutions

Please note: Publications can be downloadedfrom the MCRMA web site at www.mcrma.co.uk


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THE METAL CLADDING & ROOFING MANUFACTURERS ASSOCIATION LIMITED

18 MERE FARM ROAD

PRENTON. WIRRALCHESHIRE CH43 9TT

TELEPHONE: 0151 652 3846FACSIMILE: 0151 653 [email protected]

www.mcrma.co.uk

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