Design Difference: A wet/wet wall or a wet/dry wall?

Design and construction habits can obscure logic. Two good examples are frequently witnessed occurring on sites. At the end of this article is a BBC link to a video lasting just 15 seconds – it puts everything into perspective, watch it.

Where cavity insulation terminates partway up an external gable end cavity wall, the top of the insulation requires DPC protection. (PD 6697:2010. + BBA certificates stipulate DPC protection).

Protection is traditionally achieved by building the DPC into the inner skin, dropping and transversing the cavity and emerging through the outer skin. It does the job but the vulnerable gable triangle is separated from the masonry under. It is resting on a slip-plane and the exposed masonry triangle is susceptible to wind pressure/suction.

Similarly, a parapet cavity wall traditionally adopts the same L-shaped DPC protection within. Again, the entire parapet masonry is rising off a slip-plane.

Masonry can adversely move

What is overlooked is in both instances the outer skin is wet above the DPC and wet below the DPC, unlike an opening or intersection where the area under needs to be kept dry.

Building the DPC through the outer skin achieves nothing, except it introduces a structural weakness. Traditionally-used roll DPC needs to be supported so it is built into both skins, but there is no necessity to do so, because it is a wet/wet wall.

Preformed solutions are available to satisfy the DPC requirements without the accompanying weakening of the structure. For protecting cavity insulation partway up a cavity wall, the Type CD Cavity Dropcloak is built into the inner skin only and the structural integrity of the outside skin remains uninterrupted.

The Type P Cavitray for parapet walls is similar in profile, but is extended and has a downward turned external lip on its roof side that protects the mortar bed from which the roof flashing emerges. Both solutions do not build into and thus do not weaken the exterior masonry wall skin.

Technical manual Protecting the Building Envelope, essential reading delivered to your door with compliments of Somerset-based Cavity Trays Ltd.

Watch the BBC video

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