How long should a cavity tray be?

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.

Where are weep holes needed?

Weep holes are required at maximum 450mm centres along the base of the cavity tray. As a minimum, place one weep hole at each end and one in the centre. Weep holes can be formed using open perpendicular joints (leaving the mortar out of one vertical joint per weep) or proprietary weep vents that allow water out but prevent pest entry.

What are cavity tray stop ends?

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.

When is a cavity tray needed?

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).

What types of cavity tray are there?

Type A trays are stepped (used above openings where the tray follows the brick courses). Type X trays are flat (used at abutments and sills). Type E trays are continuous (used at DPC level and as cavity closers). Self-adhesive and rigid plastic types are both common. Lead cavity trays are used in conservation areas.

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Cavity Tray Calculator — Length, Weep Holes & Stop Ends

Calculate cavity tray length, weep hole positions and stop ends for openings in cavity masonry walls.

Opening Width

Width of window/door opening

Number of Openings

Tray Type

Tray Price

Stop End Price

How We Calculate This

This calculator determines cavity tray lengths, weep hole quantities and stop ends for openings in cavity masonry walls.

Tray length

Tray Length = Opening Width + (Extension x 2)
Standard extension: 150mm each side. Stop ends: 2 per opening.

Weep holes

Weep Holes = ceil(Opening Width / Spacing) + 1
Minimum 2 per opening (one each end). Maximum spacing 450mm.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.