Cavity trays: where they go and how they fail
A tray without stop ends is a gutter with the ends cut off. The details that decide whether water leaves the cavity or enters the house.
Short answer
A cavity tray collects water in the cavity and discharges it through the outer leaf. It needs a minimum 150mm upstand against the inner leaf, sealed stop ends at each end, laps of at least 100mm, and weep holes in the course above at maximum 900mm centres with at least two per opening.
Key figures
| Requirement | Value | Basis |
|---|---|---|
| Minimum upstand against the inner leaf | 150 mm | Approved Document C |
| Weep hole spacing | 900 mm maximum, minimum two per opening | Approved Document C |
| Minimum lap between tray lengths | 100 mm, sealed | Approved Document C |
Where trays are required
- Over every external opening where the cavity is not fully protected by the lintel.
- Where an abutting roof meets a cavity wall, stepped up the slope.
- Where the cavity is interrupted or closed — at changes of level, over ducts and at cavity fill terminations.
- Above ground floor level where an extension or canopy abuts the main wall.
The three details that fail most
Stop ends: without them water runs off the end of the tray into the cavity below and appears internally, often metres from the actual defect.
Weep holes: a tray with no outlet holds water against the outer leaf until it finds the next weakness. Two per opening is the practical minimum, whatever the width.
Laps: unlapped or unsealed joints in a stepped tray are effectively holes in the middle of the drainage path.
Sequence and inspection
Trays are installed by the bricklayer as the lift goes up and are impossible to verify afterwards. Photograph each tray with stop ends visible, and photograph the weep holes from the outside once the course above is laid.
Common questions
Does a lintel with an integral tray still need a separate cavity tray?
A combined lintel that provides a continuous tray with correctly formed stop ends can serve that function, but the stop ends and weep holes are still required and are the parts most often missed.
How many weep holes over a window?
A minimum of two, and thereafter at no more than 900mm centres across the width of the tray.
Can weep holes be filled to keep insects out?
Only with a proprietary weep vent designed to drain. Mortar or sealant in a weep hole disables the tray.
Basis in regulation
Last reviewed 2026-08-21.