Cavity Tray Calculator — Length, Weep Holes & Stop Ends

A cavity tray should extend at least 150mm beyond each side of the opening. So tray length = opening width + 300mm (150mm each side). This ensures water collected in the tray drains via the weep holes and doesn't bypass the tray at the edges. Stop ends prevent water running off the tray ends into the cavity.

Weep holes are required at maximum 450mm centres along the base of the cavity tray. The minimum is two weep holes per opening (one near each end), with extra intermediate weeps added so the spacing never exceeds 450mm centres (NHBC 6.1.17). Weep holes can be formed using open perpendicular joints (leaving the mortar out of one vertical joint per weep, roughly a full 65mm × 10mm perpend) or proprietary weep vents that allow water out but prevent pest entry.

Stop ends (or cavity tray closers) are placed at each end of the cavity tray to prevent water running off the end of the tray and down into the cavity below. They turn up and seal against the inner leaf. Every cavity tray installation needs two stop ends — one at each end. They are typically sold separately from the tray.

Cavity trays are required above all openings (windows, doors) in cavity walls, at abutments (where a roof meets a wall), at sills, and wherever a DPC is stepped within the cavity. They redirect any moisture that enters the cavity back to the outer leaf for drainage via weep holes. This is a Building Regulations requirement (Part C). See our DPC Calculator for damp-proof course quantities.

Cavity trays are grouped by where they are used. Horizontal (short) trays sit above lintels, ring beams and at flat-roof or horizontal abutments. Stepped trays follow the brick courses up a sloping roof abutment, intercepting water at every course. Preformed remedial/retrofit trays are inserted into an existing cavity wall a course at a time, and many lintels come with an integral DPC tray built in. Among proprietary ranges, Cavity Trays Ltd label their stepped sloping-abutment tray "Type X", their general-purpose/horizontal-abutment tray "Type G" and their remedial retrofit tray "Type E". Trays are made in self-adhesive membrane, rigid plastic/preformed and, for conservation work, lead or code-compliant alternatives.