The external masonry of modern buildings, especially those built after the 1930’s are cavity walls. The outer leaf and inner leaf of the walls should have a minimum of 50mm gap, be filled with insulation and tied with a wall tie. Cavity trays are included in cavity wall construction where there are penetrations across the cavity. Such as;
– At an abutment with a roof
– Above openings such as windows and doors
– Where extensions are constructed against existing walls
– Above a concrete slab or beams
– Above airbricks, ducts and pipes
-At the bottom of a wall if the cavity does not extend 225mm below dpc.
Cavity trays prevent moisture, travelling downwards, being carried to the inner leaf. Whereas a dpc tends to protect a build from rising damp.
The diagram above shows a typical cavity tray detail. Cavity trays should be provided to ensure water drains outwards;
– Where the downward flow will be interrupted by an obstruction such as a lintel.
– Under openings, unless there is a sill & the sill and its joints form a complete barrier.
– At abutments between walls and roofs.
Cavity trays can be formed using a pliable material such as lead but more commonly they are preformed and come in a range of different shapes and sizes. Cavity trays must always be bedded onto fresh mortar. Most damp ingress occurs around either window/door openings or roof abutments.
When using roll dpc material, the profile must rise a minimum of 140mm within the cavity and should be shaped to promote the easy removal of mortar droppings. The tray should be positioned directly above the opening and not several courses above. Trays formed this way, using roll dpc material must have stop ends formed by folding the ends into the ‘perp’ joint of the outer leaf. The tray must extend beyond the lintel ends. Weep holes should be provided at 900mm max centres and at each end of the horizontal cavity tray.