Cavity trays - what's that all about?

On Tue, 27 Nov 2007 13:12:31 GMT, a particular chimpanzee, Lobster randomly hit the keyboard and produced:

Sort of. Certainly any extension that abuts an existing wall, meaning that any water in the cavity can find its way into the extension, should have a cavity tray. If there's a change of use to the building as a whole whereby it contains a dwelling where previously it did not, then Part C2 applies and a cavity tray should be installed. Any other change of use, and only requirement C2(c) applies (condensation).

With a material alteration, it's a bit greyer. The basic requirement is that any work carried out should not make the building any worse after the work has been carried out than before. One could argue both ways: 1) the outbuilding was part of the building before, and therefore has not been affected, or 2) the outbuilding now contains habitable accommodation where before it did not therefore any moisture passing down the cavity is more likely to adversely affect the occupants and the building.

Even if you accept 2) above, the question then comes, is it reasonable to require a cavity tray in an existing wall? I don't know the nature of the construction of your existing building, but some of the things that would determine whether a cavity tray should be insisted upon include: the amount of wall over (is it a large length and height with infrequent windows?), is it rendered?, is it exposed (is it a high wall over, facing out over a hillside, into the prevailing wind, etc)?

These are the legal requirements. Even if you aren't obliged to do so, it may be a good idea to install one where there is the possibility that moisture could affect the building. This is perhaps what the surveyor is pointing out in his arse-covering way to his clients.