Easily-overlooked Cavity Wall Design Details

Whilst best known as a phrase used in film making to signify that a particular scene or film has finished, it’s also an important term used on site. Importantly, it’s a term worth remembering - it will prevent a problem that can otherwise support wet ingress.

Where a gable abuts another wall and the gable slope finishes at or beyond the external corner of that wall, a direct water ingress route is created. The masonry forming the corner return provides a direct entry route for wind-driven rain to permeate the structure.

This is best demonstrated in the photograph where preformed Type X Cavitrays are protecting the roof/wall intersection. The individual trays within the cavity wall create a DPC staircase, with connecting treads and connecting risers. The cavity upstand of each tray is hinged, so they take up the cavity width encountered. Externally, the flashings are already shaped and attached to the trays, so not only is the join between the two assured, but once bedded and flush pointed, they masons job is complete – no need to return later to point-in flashings – that task is eliminated.

The bottom tray is an external angled tray – what is called a Wrap Tray. Supplied with flashings attached to both external sides, it wraps and covers masonry that is below the abutment corner -an often-overlooked consideration. The external to internal masonry connection is externally wrapped.

Protection Without Structural Weakening

When cavity wall insulation is terminated partway up a cavity wall, the top of the insulation should be protected. This detail commonly occurs in house gable walls, where insulation is stopped at the adjacent wall plate level. The top of the insulation must be protected according to the cavity insulation BBA certificates and numerous authoritative bodies.

Protection is required against contamination/distortion by mortar droppings, and subsequent wet ingress from wind-driven rain then permeating the insulation– usually via incorrectly installed or choked wall ties.

The usual DPC protection detail shows roll DPC built into the inner skin, transversing the cavity and then projecting through the external leaf. The arrangement shelters the insulation, but it also weakens the external leaf. The entire masonry triangle is sitting on a potential slip plane. Those who have witnessed leeward wind pressure sucking an entire gable top away from the DPC/masonry off which it previously rested will concur the detail is not a structurally encouraging one and is best avoided?

But look at what is really required and that weakness can be eliminated. The entire outside leaf will get wet in wet weather, that cannot be changed. Accordingly building the DPC into the leaf achieves nothing, except it holds the DPC to shape and weakens the external leaf.

In contrast, using a preformed Dropcloak does not weaken the external leaf, but it does protect the cavity insulation.

The external leaf is still wet throughout its height, the difference is the wall is stronger.

Traditional building relied on floppy roll DPC that required support. Preformed Cavitrays are self-supporting – they protect without diminishing structural integrity. Protecting with a Type CD Dropcloak can eliminate potential witness/cracking lines, eliminate the water evacuation consideration, and eliminate the structural interruption.

Type CD is one of over 200 design solutions offered by the original Cavity Tray Company.

Benefit technical advice + free scheduling service + performance warranty from longest-established Cavity Trays Ltd. The UK Company with European Technical Approval.

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