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Last edited 11 Aug 2016
The external masonry walls of modern buildings are generally cavity walls, that is they are formed by an inner leaf and an outer leaf of masonry, tied together, but separated by an air gap or ‘cavity’. The cavity prevents moisture transmitting from the outer leaf to the inner leaf. It can also provide a ventilation space, allowing moisture within the wall construction to vent to the outside, and can provide a space for the installation of cavity wall insulation.
- At an abutment with a roof.
- Above openings such as doors and windows.
- Where extensions are constructed against existing walls.
- Above concrete slabs or beams.
- Above airbricks, ducts and pipes.
- At the bottom of a wall, if the cavity does not extend 225mm below the damp-proof course.
Cavity trays prevent moisture being carried to the inner leaf. Very broadly, cavity trays tend to prevent moisture that is travelling downwards from being carried to the inner leaf, whereas damp-proof courses tend to be used to prevent rising damp.
Approved document C of the Building Regulations, Site preparation and resistance to contaminants and moisture, suggests that a cavity tray (or damp-proof course or closer) 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 and the sill and its joints will 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 pre-formed, with a wide range of shapes allowing for different cavity widths, corners, stop ends, steps, lintel shapes, arch shapes and sometimes incorporating external flashing.
Cavity trays must always be bedded onto fresh mortar.
Weep holes must be provided in the external leaf of the wall to allow moisture to drain to the outside. Weep holes are generally created by omitting mortar from the vertical joint between bricks, typically at 450mm to 900mm centres. They may include plastic weep vents which incorporate a baffle structure to prevent rain from penetrating through the hole and preventing insects from entering the cavity and provide a drip at the front lip to aid drainage.
Care must be taken where there is insulation in the cavity to ensure that both the insulation and the cavity tray continue to function correctly. This can be particularly problematic where blown insulation is retrofitted in to existing cavities.
Where it is necessary to insert cavity trays into existing walls, for example if an extension is being built against an existing wall, this can be done by removing brickwork a section at a time, or by inserting self-supporting cavity trays through slots cut in the wall.
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