Cavity Trays asks: have you come across regulations and standards that confuse?

An example of a confusing standard can be found within PD 6697:2010 (page 47), which shows a cavity parapet wall with an outward-stepping DPC traversing the cavity. This traditional approach requires the DPC to be built into one skin, then cross the cavity at an angle, before being supported in the skin opposite. Outward stepping is the correct way, although confusingly PD 6697 says on the next page that inward-stepping is acceptable.

It is important the DPC always steps outwardly. The downside is that in so doing and supporting the DPC in both skins, the highest masonry (and the most exposed to bad weather, wind, and temperature extremes) is separated and isolated from the rest of the structure. Effectively, the parapet is resting on a slip plane.

Masonry moves. Extremes of wet and temperature changes result in the masonry expanding and contracting. As it does, cracking occurs around the DPC, introducing discontinuity of bond between upper and lower masonry. Wind-driven rain is eventually able to permeate these cracks and if the DPC steps inwardly, the water can track on the underside of the DPC and permeate the inner leaf of the room below.

There is a purpose-made cavity tray called the Type P, that eliminates this possibility. The Type P requires building into one skin only, steps outwardly, and is self-supporting. It does not weaken the bonding of the external skin that remains uninterrupted. The Type P from Cavity Trays of Yeovil. Preformed for compliancy and consistency + outward-stepping + structural continuity.

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