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4.3 Prevention of Moisture Penetration
In cavity wall construction, the location of DPC’s should be based on the assumption that rain
water will penetrate the outer leaf and, more often that not, run down the internal face of the
external leaf. Damp proofing measures are essential in controlling the ingress of rain through
locations where the cavity is bridged and where there is a potential for moisture to track across
these areas to the internal leaf. Careful attention to design and detailing, combined with good
site practice, is essential in the elimination of damp penetration.
The materials for the damp proof courses and cavity trays are varied. However they should all
comply with the requirements of relevant British Standards:
BS 743: 1970 Specification for materials for damp proof courses
BS 8000: Part 3: 2001 Workmanship on building sites – Code of practice for masonry
BS 8215: 1991 Code of practice for the design and installation of damp-proof courses in
masonry construction
BS 6398: 1983 Specification for bitumen damp proof course for masonry
Position of DPC at head with BS 6515: 1984 Specification for polyethylene damp-proof courses for masonry
dual supporting lintel
Position of vertical DPC Alternatively, they should be supported by an Agrément Certificate.
Position of DPC at cill DPC’s should extend through the full 4.3.1 Heads & Lintels
thickness of the wall or leaf, at least
150mm above the external ground. Cavity trays are vital in providing a watertight
It is good practice to overlap all DPC barrier which will channel and discharge
materials, by at least 100mm to prevent the water to the outside face of the masonry.
upward transference of moisture. Where Cavity trays with stop ends should be
downward moisture movements occur, incorporated over all openings in external
the DPC’s should be lapped and sealed. cavity walls and extend a minimum of
Pitch polymers are generally favoured for 150mm on either side of the opening.
masonry construction because they do The part of the tray that bridges the cavity
not exude under load, have good bond should be adequately supported and this is
characteristics and are not easily perforated. particularly important at locations where the
Using the appropriate adhesives, effective tray is to be jointed. The cavity tray should
joints can be formed giving increased be bedded on both sides in fresh mortar.
flexibility of use. The tray should fall a minimum of 150mm
across the cavity. Weepholes, positioned
The risk of moisture penetration may be directly above the tray in the external leaf
reduced by consideration of the following should be located at a maximum of 1m
features. centres. There should be a minimum of 2
weepholes per opening and each weephole
should be at least 75mm high. In locations
where full cavity insulation is anticipated,
the spacing of the weepholes should be
reduced. The use of label moulding will
