Cavity trays are installed in a cavity wall above reveals (such as windows or doors) to ensure that any moisture in the cavity is expelled. They can be made from a variety of materials including plastic, roofing felt, lead, tile, slate amongst other materials.
When they fail, damp problems will start to occur around these openings.
If your property has a cavity wall, the outside skin of the wall should be able to let in a reasonable amount of moisture into the cavity without causing dampness on the inside. If moisture does enter the wall, it should run down the inside of the outside skin and drain away below DPC (damp proof course) level. If there are reveals (openings for windows / doors etc.) on the way down, a suitable cavity tray should be installed during the original build to ensure that any moisture is expelled from the cavity via weep holes / vents.
Cavity walls should be free of debris to avoid moisture tracking across to the inside.
Case study - Failure of cavity tray system.
A typical case is that of the owner who noticed some eroded mortar joints above their conservatory and had contracted a builder to re point the whole wall. It certainly was worn and it would have looked a lot better after re pointing too. The builder was only alerted to the real reason for the customers request for re pointing when they made a throw away comment “At last! After you have finished, I'll have a dry conservatory!” Luckily they said this just in time and a cavity tray system was purchased and installed and some of the worst areas were carefully blended and patched.
The problem was when trying to locate the cause of moisture ingress around the entrance to their conservatory, their natural (and quite logical) response was to associate the deterioration of mortar with the ingress of moisture. To a certain extent, this was true but the real reason was that no cavity tray system had been installed when the conservatory was added. Because there was no tray system, the wall was very sensitive to any slight deterioration in the mortar above. Thankfully, the owners throw away comment saved them a lot of time, trouble and money
Install a new cavity tray system. If a lintel is being replaced at the same time then a variety of systems can be used. If just the tray itself is the sole cause of the problem, consider a modular cavity tray system that can be installed b y removing a few bricks or blocks at a time, inserting the first tray and making good, this process is then repeated along the length required and does not require support.
An excellent use of this type of tray is above conservatories. Often when conservatories are installed, a cavity tray system is not used. Often there is no problem for a few years then water starts to find its way in. It is almost impossible to support a wall like over several metres, without removing the roof. It is therefore going to be impractical to install a lead or dpc based system. “Type e” (modular) cavity trays require only 3 bricks to be removed at a time during installation, avoiding the inconvenience and expense of the other systems.