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Buildingdamp-freecavity walls

Itgoes without saying that This Good Building Guidecavity walls should be built shows how to make sureso that the inner leaf stays new cavity walls do notdry. Many building details faulty design or suffer from dampnessare designed with this construction of damp- problems.express purpose and are proofing measures or tolong-established. However, wrong cho ice of material.

dampness s still acommon problem inmodern buildings, due to

There is advice in the G o o d q a i rGu& serieson dealing with rainpenetration and risingdamp in existing buildings. This GoodBuildiagGuide looks at ways of preventing the problemsoccurring in new cavity wall construction.Indriving rain, water leaks through the outer leafof most cavity walls, often in quite large

quantities. Provided the damp-proofingmeasures are correctly designed and installedand the wall itselfis reasonably fiee fiomdefects, the water does not reach the inner leaf- t flows harmlessly down the cavity face ofthe outer leafuntil it reaches the footings or isdirected out of the cavity.

"1The damp patch on this kitchen wall...

d onstructing the future...was caused by rain penetration across displaced anddeformed cavity batts


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Types of rainresistantpointing: (a) buckethandle, (b)weathered, (c) struck

C 1

Defects in this cavity includewall tiessloping the wrong way, drips too near theinner leaf, and mortar snots and droppingsnot cleaned off

af Inner leaf Water leaks onto cavity faceof outer leaf through cracksbetween bricks and mortar,mainly at perpendsCapillary action throughmortar obstruction

Wall tie sloping downwards,drip touching inner leaf

Water runs down inner leaf

Water splashes off extrudedbedjoints onto inner leaf,, Accumulated mortarings in ht in gs above

I

How rain can cross an unfilled cavity

Defects in the outer leafMost ofthe leakage through the outer leafis at the oints between the bridcsand the mortar. Good workmanshipcan help to prevent h i s- t isparticularly important to fillthe pe properly.This s especially true inexposed areas, where dacross the cavity to wet the inner led The typeofpointingalsohasan effect-bucket handle, weathered, or struck pointing have the best resistance todriving rain. Recessed pointing, which allows water to pond on the exposedledge,canresult in severerainpenetration and should only be considered forsheltered locations.

Mortar extrusionson he cavity face ofthe outer leafmust be cleanedOElarge obstructionscanbridge the cavity and shed water onto the inner l e dWater drippingfiom smaller extrusions can splash across the cavityas tstrikes other obstructions ower down.Try o ensure that oist ends orconcrete landing slabs do not protrude into the cavity- hiscanalsolead tobuild-up of mortar droppings.

grain canbe blownthroughwide oints and

Problems caused by wall tiesWall ties provide an easy route for rain to crossthe cavity ifthey are notcorrectly installed. Points to watch are:0 ties must slope downwards fiom inner to outer leaf,0 drips must be positioned in the centre of the cavity, well away fiom the0 ties must be kept fieefiommortar snots- hey canbridge the cavity or

inner leaf,prevent water running back to the outer leaf

Defects linked to cavity fillGood workmanship to prevent rain penetration is particularly important inpartially or hlly filled cavity walls. If there are mortar droppings or extrusionsin the horizontal oints between batts, or in the cavity ofa partially filled wall,rainwater is very likely to cross to the inner leaf. It is also important to fixpartial-fill batts firmly to prevent them fiom falling across the cavity.

The recommended work sequence forpattzaZZyjZZed waZh is as follows.0 Protect previous work fiom mortar droppings with a batten across

insulation and cavity.0 Build inner leafto the height of the insulation boards plus one course.0 Clean off mortar snotsfrom the cavity side, then remove batten.0 Fit insulation boards between wall ties so that boards fit tightly together.0 Secure boards using retaining clips.0 Build outer leaflevel to the top ofthe insulation.0 Repeat until wall is complete.

Ig the cavity clear of mortar


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0

3InfuZ&jZZedwalls, the advice is similar, except for the following points.0 Lead with the outer leafat least for the first 1 8 .0 Carefilly cleanoffmortar extrusions at the oints between batts.0 Never build the cavity first and insert batts afterwards.0 Course horizontal oints in batts with wall ties (see photo, lefi).

Where dpcs and cavity trays are needed0 A dpc should be installed in both leaves of cavity walls to prevent capillary

rise of moisture. In the outer leafit should be at least 150 mm above groundlevel (see drawing (a)).

0 Openings in cavity walls must be protected by vertical and horizontal dpcsto deflect water away from the inner leaf. Vertical dpcs are needed at thejambs ofopenings where the cavity closes, horizontal dpcs at the head tobridge the cavity in the form of a tray (see drawing (b)).units to prevent penetration of water to the wall below.It should be not less than 150mm above the rooffinish to lap over anycover flashing to the roofupstand (see drawing (c)).

0 There should be a dpc beneath all sills and copings formed with jointed0 In a parapet wall the dpc should provide continuity with the roof covering.

MakL __ ..._.--ntal joints in batts arecoursed with wall ties

(a) Positionof dpc above ground evel(b)Dpcs around an opening n a cavity wall(c) Flashing and dpc in a parapet wall(d) Stepped cavity trays at pitched roofabutments


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40 W here a m asonry chimney penetrates a roof structure, a dpc may beneeded to prevent w ater soaking masonry inside the bu ilding (see GoodRepair Guide15).0 Abutm ents to cavity walls need a tray across the cavity to prevent w aterreaching the w all within th e building at a low er level. In p itched roofabutmen ts, this calls for a carefully detailed series ofstepped trays (seedrawing (d)).Installing dpcsA d pc in masonry must be laid on a 111,even bed of fresh mortar. Lay a 111bed of mortar over the d pc and then a firthe r course ofmasonry. Poorbedding of dpcs in masonry oints c an lead to rain penetration, particularly atthe heads of openings, with water entering below the edge of a cav ity tray.Th e dpc must cover the fill width o fth e masonry and project about 5 mmbeyond any external face. Horizontal oints in dpcs should be lapped by aminimum of 100mm and m ust be sealed where they have to resist thedownward m ovem ent ofwater, for instance in parapets.Make sure that the d pc is not bridged by rendering or plastering.A brickwork dpc should be formed with at least two courses ofdpccatego ry bricks, laid in a designa tion 1mortar (seeDzgest36.Z).Perpends mustbe completely filled and staggered in alternate courses.A slate dpc should beformed in the same way, usingtwo ourses, with each slateas ong as possibleto minimise the num ber ofjoints.In vertical dpcs, make sure the upper dpc laps over the lower. This isparticularly important around openings, where th e dp c or tray at the headshould lap over the vertical dpc at the ambs and extend beyond it by at least25 mm.Th e vertical dpc should extend at least 25 mm into the cavity beyond theclosure and seal properly with any fiame set in the opening. Ifdpcs arelappedthe wro ng way, water com ing down the structure can penetrate behind it.W here a dpc links with a flooring membrane o r tanking, the overlap should bea minimum of 50 mm.


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11 - Boxlintel

k2intel1/ 2intel

tray and stop end profiles above

m m

nimum dimensions for cavi tytrays

At least140mm

Horizontal dpcs which bridge a cavity to form a cavity tray should step upnot less than 140 mm fiom the outer to the inner leaf. Provide supportwherever possible beneath the tray across the cavity, particularly at oints inthe tray.

Installing cavity traysCavity trays play a vital part in preventing rain penetration, but BR E hasfound that they are ofien inadequate or even missing. This defect can beprohibitively expensive o put right.

Trays should be provided:0 at all interruptions likely to direct rainwater across the cavity, such as

rectangular ducts, lintels and recessed meter boxes,0 above cavity insulation which is not taken to the top of the wall, unless that

area is protected by impervious cladding,0 above lintels in walls in exposure zones 3or 4, and in exposure zones 1and

2 ifthe lintel is not corrosion resistant and is intended to hnction as ts owncavity tray (see l7zemalimulation:avoidingtirh or information onexposure zones).

A cavity tray above a lintel should extend at least to the ends ofthe lintel,preferably beyond. In walling with frequent openings, separated by shortsections ofmasonry, it may be best to run a cavity tray along the wholeelevation or even round the whole perimeter ofthe building.

Iftrays are not continuous, stop ends must be provided, hl ly sealed to theends of the tray. Without them there is a risk ofwater running offthe end ofthe tray and tracking back to the inner led.

Many of the site problems involved in installing dpcs and trays can beavoided by using preformed sections- or example, at corners or unctionswhere complex details are involved. They are particularly useful at steppedand staggered layouts at pitched abutments.

In terraces where steps or staggers are small and space for cavity trays islimited, an alternative is to clad the exposed wall above.

Cavity trays should rise at least 140 mm from the outer to the inner leaf, andshould be self-supporting or fully supported.Joints must be lapped and sealed.The rise across the cavity should be at least 100 mm.

WeepholesTo drain water from cavity trays and from the concrete cavity infill at groundlevel, weepholes must be installed at not more than 900 mm centres.should be at leasttwo weepholes to drain cavity trays above openings.formed by omitting mortar from perpends.

Ifthe wall is to be cavity filled, this spacing should be reduced. ThereWeepholes are best formed by using proprietary components but can be


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. Further informationMore advice is available in the foltowing BRE and BRECSU (se8 box, left) publications.Thermalnsulation:avoidingrisks. 2nd edition.8R 262. Garston, Watford, CRC Ltd.1994w-362 Buildingmortar380 Dampproof courseseaOdf?q*-5 Diagnosing the causesof dampness6 Treating rising damp in walls8 Treating rain penetration inhouses102 Externalw a l k injectedcavlty insulation103 External walls:MIfill insulation batts104 Externalwalls: pwtial frll insulation

G o o d k 8 C & 3 8 o I l i M b t ( ~ ~ )

PI constructing the futureBRE is theWsbadngc m fexpdseon unklmgandcwtruction,sndtheprwmtbmand controlofhe . Formfofmationon= and ts rcnices.a atachnkd&e. contactER,m.l(rordwD2 .RTO^ 01923664000Fax: 01923661098cmry: S 0 b r a . C O . U kWubstcvmr.bre.co.uk

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