Practice Note 1 - Separating Wall - Cavity Masonry

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yIntroductionThe Part E Robust Details scheme was established toprovide an alternative means of compliance withRequirement E1 of the Building Regulations (Englandand Wales), for new build attached houses and flats, toavoid the requirement for pre-completion sound testing (PCT). A fundamental part of the scheme isperformance monitoring of the Part E Robust Details inuse. In this respect Robust Details Ltd (RDL) hasengaged a number of acoustic consultants (the RDLInspectorate) to undertake both visual inspections ofPart E Robust Detail construction in progress andsound tests on completed Part E Robust Details.

The Part E Robust Details Practice Notes are intendedto be a mechanism for feeding back the Inspectorate’sobservations to the industry as a whole. In adopting theuse of Part E Robust Details, organisations shouldensure that all relevant requirements of the RobustDetails Part E Handbook are strictly followed. This firstPractice Note looks at a common issue that theInspectors have identified with the Part E Robust Detailcavity masonry walls – cavities being bridged by mortarand debris.

Choice of wall tie The level of sound insulation between dwellings usingcavity separating walls is highly dependent on theisolation that is provided by the cavity between the wallleaves. Structural connections required for strength andstability will tend to reduce the isolation propertiesbetween the wall leaves. For cavity masonry walls soundwill primarily transmit structurally via the wall ties andfoundations. For this reason cavity masonry separatingwalls should only be constructed using appropriate wallties, such as butterfly ties or Tie Type A - please refer tothe Building Regulations (England and Wales) ApprovedDocument E 2003 (ADE) Section 2: 2.19 and AppendixA of the Robust Details Part E Handbook.

Avoid mortar build up (Figures 1 and 2)In the course of constructing the wall leaves, mortardroppings and debris may fall down to the base of thecavity. Mortar that is able to collect and build up, such that it bridges the wall leaves at and above theground floor structure, will result in excessive bridgingof the separating wall leaves. Installation of radonbarriers, gas membranes, continuous damp proofcourses (dpc) and horizontal cavity stops at floorlevel prevents mortar from falling to the base of thecavity. Bridging effects can lead to significant soundtransmission between attached dwellings, by-passingany useful isolation effect that the cavity provides and may result in a significant reduction in soundinsulation performance.

Dwelling A Dwelling B

Dwelling A Dwelling B

Radon barrier,methane barrier or gas membraneor continuous dpc collects mortar at floor level

Mortar NOTcleaned out andallowed to build

up and bridgewall leaves

Figure 1: OKAY

No abnormal bridging of cavity. Sound insulation performance controlled by what sound transmitsthrough the wall via the cavity or via the wall ties or via the foundation.

Figure 2: WRONG

Excessive BRIDGING of masonry wall leaves by mortarbuild up on barrier or membrane. Sound insulation performance now SIGNIFICANTLY REDUCED as sound mainly transmits through MORTAR bridge.

Issued: 24 April 2006 Page 1/4

Practice Note 1 - Separating Wall - Cavity Masonry

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Issued: 24 April 2006 Page 2/4

Good practice (Figures 3 and 4)On the first page of each of the Part E Robust Detail cavity masonry separating walls is an instruction to keep thecavity free from mortar droppings and debris. It is therefore important that, during the construction of masonry walls,site personnel should ensure that:

■ excess mortar and debris are not permitted to fall down the cavity

■ any mortar or debris that does fall into the cavity should be carefully removed and not allowed to build up in the cavity

■ wall ties are regularly cleaned to avoid mortar build up

■ mortar or debris does not bridge the wall leaves together at any point – the areas most at risk being at or abovethe junctions with floor structures (particularly the ground floor) and wall ties.

Consideration could also be given to the temporary removal of some wall blocks at floor levels (Figure 5) in order toprovide access for cleaning the cavity, although, care will be required to ensure that:

■ radon barriers and gas membranes are not damaged

■ when returning the blocks into the structure, the joints are fully filled whilst not forcing mortar into the previouslycleaned cavity.

DO

■ Keep cavity and wall ties free from mortar droppings and debris

■ Fully fill all blockwork joints with mortar

■ Make sure there is no connection between the two leaves except for wall ties and foundation

■ Keep any chases for services to a minimum andfill well with mortar. Stagger chases on each sideof the wall to avoid them being back to back

■ Ensure that render is applied to the completeface of each leaf (it may be omitted within thefloor joist/beam zone)

■ Refer to Appendix A of the Robust Details Part EHandbook for further guidance

AVOID:

Useful deep cavity beingincorrectly sealed off bycontinuous dpc which will allow mortar to easilybridge between separating wall leaves when constructed.

AVOID:

As construction proceedsmortar droppings collect, if they are not cleaned out,leading to bridging of theleaves of the separating wall.

AVOID:

Mortar may also collect on wall ties, bridging thecavity and effectivelyincreasing the stiffness ofthe wall ties.

Figure 3: Good Practice Figure 4: Text from page 1 of Part E Robust Detailcavity masonry walls

Figure 5: Blocks carefully removed to allow cavityto be cleared of excess mortar and debris

Practice Note 1 - Separating Wall - Cavity Masonry

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Issued: 24 April 2006 Page 3/4

Normal open cavities (Figure 6)The details of each of the Part E Robust Detailcavity masonry separating walls also indicate thatthe ground floor junction should be formed suchthat the cavity, for both separating walls andexternal (flanking) walls, should be maintained for atleast 225mm below the underside of where theground floor structure would or does break thevertical continuity of the wall, the aim being to:

■ mitigate the risk of sound transmission via thefoundation connection

■ to allow for some collection of mortar droppingsbelow the floor level.

Radon barriers, gas membranes orcontinuous dpcsThe installation of radon barriers, gas membranesand continuous dpcs that bridge the cavity atground floor level prevents excess mortar fromfalling to base of the cavity. This may be avoidedthrough careful consideration of detailing andworkmanship on site. The continuous membranecould be dressed down and around the cavity(Figure 7), to maintain a clear cavity of at least225mm below the underside of the ground floorstructure, rather than bridging straight across thecavity. However, particular consideration needs tobe given to ensuring the membrane can beadequately dressed around the cavity, particularly atwall junctions, to ensure that the membrane remainsgas and water tight whilst preventing the membranebulging into the cavity. The use of preformedproprietary products, which help to address theseissues, should therefore be considered.

Alternatively, consideration could be given toinstalling the membrane at a lower level in theconstruction such that it may be laid as a singlehorizontal layer 225mm (min) below the underside ofthe ground floor structure and up to 225mm belowexternal ground level, where appropriate (Figure 8).Typically, this approach would not be suitable forsloping sites or where ground movement ispossible. If this method is adopted undersuspended ground floors it would be necessary touse a reinforced membrane and to protect themembrane, from objects falling during construction,by a layer of sand or lean mix concrete beinginstalled above the membrane.

225mm (min)

Section

Figure 8: Gas membrane installed at lower levelwith layer of concrete over for protection.

Figure 7: Gas membrane dressed around cavity atground floor junction.

Figure 6: Ground floor junction detail from RobustDetails Part E Handbook.

225mm (min)

Section

225mm (min)

Section

Practice Note 1 - Separating Wall - Cavity Masonry

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Issued: 24 April 2006 Page 4/4

Recommendations for improvedsite practice1. Bricklayers should be instructed to build the

separating walls with the aim of avoiding mortar or other debris falling down the cavity.

2. Site operatives should be informed and alertedto avoid build up of mortar or debris within thecavity of masonry walls, particularly at thebase of the separating walls.

3. Cavities should regularly be cleaned to avoidmortar and debris building up, particularattention should be paid to the junctions withfloors (particularly the ground floor) and wall ties.

4. Site managers should conduct regular plotinspections to monitor workmanship andensure that mortar or debris is not allowed tobridge the cavities.

5. Particular care and attention is required whenusing products, such as radon barriers, gasmembranes, continuous damp proof courses(dpcs) and horizontal cavity stops, that will create a bridge between the leaves of cavitywalls that allow mortar and other debris to collect.

6. All relevant requirements of the Robust DetailsPart E Handbook should be strictly followed.

Further detailsFor further details about Part E Robust Detailconstructions or for technical enquiries please contact:

TechnicalRobust Details LtdDavy AvenueKnowlhillMilton KeynesMK5 8NB

RDL Technical DeskTel: 0870 240 8209Fax: 0870 240 8203Email: technical@robustdetails.comWeb: www.robustdetails.com

RD13 04/06