What is a cavity tray, and why do so many damp problems come back to one?
An impervious tray sloping across the cavity that catches water and sends it back out. Where they belong, the stepped trays at a roof abutment, and why a missing one produces damp that appears years later.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 5 min
- Safety
- caution
Short answer
A cavity tray is a layer of impervious material laid across the cavity, sloping from the inner leaf down to the outer, with weep holes in the outer leaf above its front edge. There is one at the base of the wall, one over every opening, and a stepped set wherever a roof or a lower structure abuts a wall with a cavity behind it. A missing or damaged tray is the classic hidden cause of internal damp.
Trays are entirely invisible once a wall is built, they cost very little to install correctly at the time, and getting one wrong produces a problem that surfaces months or years later somewhere that looks unrelated. That combination is why they are the single most useful thing to know about when diagnosing damp in a cavity wall.
Safety
Step by step
- Understand what a tray is for.Water running down the cavity face of the outer leaf has to be intercepted before it reaches anything it can cross on, and returned to the outside. The tray is the interception, and the weep holes are the return.
- Learn the three places trays belong.At the base of the cavity, at damp proof course level. Over every opening, sitting above the lintel. And at every abutment — where a lean-to roof, a porch, an extension or a bay meets a wall that has a cavity behind it.
- Understand the stepped tray at an abutment.Where a pitched roof runs up against a wall, the tray has to follow the pitch, so it is built as a stepped series of overlapping trays, each with its own weeps, matching the stepped flashing on the outside. Getting the laps the right way round is the whole job.
- Know why the front edge must be supported and stopped.A tray needs a stop end at each end, or the water it collects simply runs off the end into the cavity again. Missing stop ends are one of the standard defects.
- Recognise the damp signature of a missing tray.Damp along the head of a window or door, damp appearing internally at the line where an extension roof meets the house, and damp that follows prolonged rain rather than cold weather. It is usually a horizontal band rather than a patch.
- Check the outside before assuming the worst.Look for weep holes above every lintel and at the abutment. If there are none, either there is no tray or nobody left it a way out. Both need investigating, and the second is far quicker to remedy.
- Understand what a retro-fit tray involves.Brickwork is removed in short sections above the line, the tray is installed and lapped, weeps are formed, and the brickwork is rebuilt and pinned before the next section is opened. It is disruptive, it works, and it is the accepted remedy where a tray was omitted at an abutment.
Tips
- The most common place a tray is missing altogether is where a later extension or conservatory roof was built against an existing house wall. The original wall never needed one there, and whoever built the extension did not add one.
- Weep holes are the visible evidence a tray exists. Counting them along a wall is a five-minute survey anyone can do from the ground.
- Replacement windows are a common point at which a tray is damaged. If damp appeared at the head of a window in the year after the windows were changed, that sequence is worth putting in writing to the installer.
Common mistakes
- Sealing the abutment on the outside with mastic instead of trying a tray — It hides the route without giving the water anywhere to go. Sealant at an abutment is a temporary measure at best and often makes diagnosis harder by moving where the damp shows.
- Treating tray-related damp with a chemical damp proof course — An injected course addresses rising moisture in a solid wall. It does nothing at all about water arriving from above through the cavity, which is what a failed tray lets happen.
- Plastering over the internal damp and redecorating — The water is still arriving. Renovating plaster hides it for a while and then fails, and the second failure usually damages more than the first.
If it doesn't work
A horizontal damp band inside, level with where a lean-to roof meets the wall
Cause: No stepped cavity tray at the abutment, or one with no stop ends or weeps — Fix: Look outside for weep holes above the flashing line. If there are none, a retro-fit stepped tray is the standard remedy and it is specialist work.
Damp appearing at a window head after the windows were replaced
Cause: The tray was damaged or the weeps blocked during installation — Fix: Check the weeps first — a filled weep hole is quick to reopen. Raise it with the installer, because damage during installation is their responsibility rather than a pre-existing defect.
Water running out of weep holes in quantity long after rain has stopped
Cause: The tray is doing its job on a large volume, or something above is leaking into the cavity — Fix: Weeps discharging during and shortly after rain is normal. Continuous discharge in dry weather points at a plumbing or roof leak entering the cavity, which is worth tracing.
Questions people ask
Does my house have cavity trays?
If it was built with a cavity wall to modern practice, it should have them at the base and over every opening. Whether it has one at every abutment is much less certain, particularly where extensions were added later. Weep holes above the line are the evidence.
Can a cavity tray be added without taking the wall down?
Yes. That is exactly what a retro-fit tray is, and it is done by removing and reinstating short lengths of brickwork in sequence. It is a well-established specialist process.
What is a cavity tray made of?
Traditionally a flexible impervious sheet, and now more often a preformed system in polypropylene or similar with integral stop ends and weep formers, made in shapes for openings, abutments and steps.
Do timber frame walls need them?
The principle is the same but the details differ, because the outer masonry leaf is a rain screen in front of a framed and sheathed structure. The drainage and the breather membrane arrangement are what matter, and they are specific to the system.