The Incline-Decline Rule - It’s Not in the Textbooks

Textbooks overlook a commonly-occurring design consideration that arises where some horizontal and some sloping roofs abut a cavity wall. Consequently, despite construction observing guidelines and regulations, the building suffers wet ingress.

It is accepted the external leaf of a cavity wall will regularly become very wet during rainfall, and when accompanied with wind, rain saturation can be experienced quite rapidly. The wet must be arrested just above roof abutment level, as failure to do so will permit the wetness to continue its gravitational path below roof level, where the skin becomes an internal wall.

Preformed trays can provide the requisite arrestment, the Type X cavity tray being the popular choice for sloping abutments and the only gable abutment tray system in the UK awarded European Technical Approval. Once incorporated, trays create a DPC staircase, with connecting treads and connecting risers forming a protective barrier just above and parallel with the roofline. With flashings already attached, the plumbers’ job is also made easier, dressing-only being required to complete the job.

An often-overlooked design consideration occurs where a sloping roof abuts the innermost skin of a cavity wall, and the photograph shows such an instance. In this situation, both skins of the external cavity wall are exposed to wind-driven rain above roof level. Thus, saturation can occur in both skins, below coping level.

On the roof side, the preformed trays protect against saturating rain continuing its gravitational path below roof level, so no problem arises. On the external (gable end) elevation, the external skin will saturate and function as intended, but in so doing it can feed wet into the building – despite the cavity being maintained.

If one looks at regulations, they refer to outward-sloping DPC’s and trays discharging away from the inner masonry skin, on the logical basis water will flow to falls – outwardly. But with such a gable end parapet, there is no inside skin – both are external and both saturate. Provision must be made to prevent water ‘under-tracking’ the tray cavity upstand rising from the other masonry leaf.

The solution was developed by Cavity Trays Ltd many years ago. Tray cavity upstands are hinged to accommodate the ‘as-built’ cavity width (as opposed to the as-intended) ensuring compatibility. In such a parapet situation, the top of the hinged upstand is supplied with a simple turndown. The tray is still able to protect as intended, but the turndown eliminates the possibility of saturating water under-tracking.

Cavity Trays offers an identification and scheduling service, and will be pleased to draw such considerations and aspects to attention. Approved cavity trays are accompanied with a performance undertaking for the benefit of architect, builder and client. The Yeovil-based Company is the longest-established specialist in its field and is this year celebrating 100 years of expertise.

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