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Failed Cavity Tray Damp: Signs, Diagnosis and Repair
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Failed Cavity Tray Damp: Signs, Diagnosis and Repair

14 min read

Failed Cavity Tray Damp: Signs, Diagnosis and Repair

Cavity tray exposed above a window

A failed cavity tray lets rainwater that has crossed the outer leaf of a cavity wall run onto the inner leaf instead of draining out. The damp usually shows as a stain, bubbling paint or crumbling plaster above a window, door or other opening, and it often appears after heavy rain. A cavity tray fault is only one of several causes that look alike, so the source has to be confirmed by inspection before anything is repaired.

This guide explains what a cavity tray does, how it fails, which signs point to it, how a surveyor confirms it and what a proper repair involves.

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Quay Point Plastering investigates damp through visual symptoms and provides documented findings to help define the work needed.
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TL;DR

  • A cavity tray is a barrier in a cavity wall that sends water back out through weep holes. If it is missing, damaged, blocked or badly detailed, water reaches the inside wall.
  • Typical signs are damp patches or plaster damage above windows and doors, worse after rain, sometimes with mould or timber decay in the lintel area.
  • Moisture meters alone cannot identify the cause. A whole-house inspection with visual checks, and often a borescope look into the cavity, is needed.
  • The repair is a continuous tray with stop ends and weep holes, plus making good the plaster afterwards. Gutters, seals and debris should be dealt with first.
  • Book a survey before paying for any damp treatment. Call 01202 287134 and tell us what you can see.

Table of Contents

What a cavity tray does

A cavity wall has an outer leaf of brick or block, an air gap and an inner leaf. Rain will always get into the outer leaf to some degree. The cavity exists so that this water can run down the inside face of the outer leaf and out, without reaching the inner leaf.

That path is interrupted wherever something bridges the cavity: a lintel over a window, a door head, a roof abutment or a change of level. A cavity tray catches the water at those points and directs it outwards. Approved Document C describes the principle: the tray must act as an impervious barrier so that water is drained outwards from the wall.

The same document gives the main installation details for England:

  • The tray should rise at least 140mm from the outer leaf to the inner leaf.
  • Weep holes should be spaced at a maximum of 900mm centres.
  • Where the tray does not run the full length of the exposed wall, as over an opening, it needs stop ends and at least two weep holes.

LABC gives the same 900mm maximum weep hole spacing in its guidance on site-formed trays. If you are in Wales, Scotland or Northern Ireland, check the equivalent regulations for your nation.

How cavity trays fail

A tray can fail in several ways. Most are installation or maintenance problems rather than the product wearing out.

  1. Omitted altogether. Some openings, particularly in older or altered buildings, were never fitted with a tray, or one was removed when a window or door was replaced.
  2. Incorrectly installed. The tray may be too low, level rather than sloping outwards, or missing its stop ends, so water runs off the end and into the cavity.
  3. Blocked. Mortar droppings and debris in the cavity can sit on the tray and bridge it, or clog weep holes so water backs up and tracks across.
  4. Poorly joined. Gaps where trays meet each other or a flashing, especially at roof junctions, let water pass through.
  5. Overwhelmed by another defect. The Property Care Association lists debris in the cavity, defective seals around windows and doors and poorly maintained rainwater goods among the typical external defects behind rainwater penetration. A tray that copes in normal weather may fail when an overflowing gutter soaks the wall above it.

The official guidance puts the result plainly: whether the tray is omitted, incorrectly installed or blocked, water collects at the opening and tracks to the inner wall surface.

Signs that a cavity tray may be failing

No single sign proves a cavity tray fault. Together, the pattern is what matters. These are the observations we ask homeowners to note before a survey.

Where the damp appears

The most telling clue is location. Damp that sits directly above a window, door or patio door, or beside a roof abutment such as an extension meeting the main house, points to something happening at that opening. Rising damp does the opposite: it starts at the base of the wall. If your damp is high on the wall, read our guide to seven signs of rising damp to rule that out.

When it appears

Damp that appears or worsens within a day or two of heavy, wind-driven rain suggests water entry from outside. Damp that is steady all year, or worse on cold, still mornings, is more likely condensation or a leak. Our page on damp appearing after rain goes through that timing in more detail.

What it looks like

  • Brown or yellow staining with a tide mark above the opening.
  • Plaster that is cracking, blowing or soft in the recess or reveal.
  • Paint or wallpaper that bubbles or peels in one patch.
  • Mould growing in that one area, rather than across a cold external corner.
  • Salts forming a white crust on the plaster.
  • Damp or darkened timber, such as the window board, lintel bearing or the end of a joist nearby, which can lead on to wet or dry rot if left.

One caution: cracking and peeling plaster over a window with no actual dampness is a common query from homeowners, and it does not necessarily mean a tray fault. Plaster can crack from movement, a different material at the lintel or shrinkage. A survey will test for moisture before assuming water.

What it is probably not

RICS notes that rising damp is often misdiagnosed and that failure of an original damp-proof course is extremely rare. If a company proposes a chemical damp-proof course for a stain above a window, ask what evidence rules out a tray or rain penetration first.

How the cause is confirmed

Moisture meters tell you that a wall is wet. They do not tell you why. The Property Care Association says damp investigations should take a whole-house approach, using visual inspection supported by moisture measurement, because meters alone cannot separate one source of dampness from another.

A sound investigation of a suspected cavity tray fault usually follows this order:

  1. Record the symptoms. Where the damp is, how big the area is, when it appears, and what changed recently: new windows, an extension, retrofitted insulation, a roof repair.
  2. Inspect outside first. Look at gutters, downpipes, window and door seals, pointing, cracked render, the condition of the brick face and any bridging, such as render or a raised patio, over the wall.
  3. Check for other sources. Plumbing leaks, condensation and failed flashings can all produce the same stain. Our damp testing guide explains how professionals separate them.
  4. Look inside the cavity. A borescope lets a surveyor see the cavity through a small drilled hole, including cavity fill and wall ties, as RICS describes in its inspection guidance. It can show a tray, debris on it, or the absence of one.
  5. Allow for the building. RICS says technology should supplement, not replace, a surveyor’s judgement about the building type, age and materials. A borescope picture is evidence, not a verdict.
  6. Write it down. Findings, photographs and a defined scope of work should be in writing before any repair is agreed.

Cavity insulation changes the picture

If your cavity was filled after the house was built, tell the surveyor. RICS reports that retrofitted cavity-wall insulation is an increasing cause of damp and mould, because some materials can absorb and hold moisture that gets through the outer leaf. Insulation can also bridge a tray or block weep holes. Our article on cavity wall damp covers why these walls can take a long time to dry.

Choosing who inspects

RICS recommends asking for a JPS22-compliant inspection when you instruct a surveyor about damp, and points out that RICS membership does not by itself mean a surveyor is experienced in damp and mould. Ask what damp and building pathology work they have done, and what you will receive in writing. If you want to understand what a good report contains, see what an independent damp survey should put in writing.

How a failed cavity tray is repaired

The right repair depends on what the investigation found. Common routes are below.

Clear and maintain first

If the problem is debris, blocked weep holes, a failed window seal or a leaking gutter, the fix may be as simple as clearing or replacing those. RICS advises homeowners to sort routine maintenance such as clearing gutters and replacing broken seals before spending on expensive damp-proofing that may not suit the problem.

Installing a correct, continuous tray

Where a tray is missing or defective, the repair is to form a proper one. That means a continuous barrier that falls to the outside, with stop ends and correctly spaced weep holes so water drains outwards. LABC’s guidance on site-formed cavity trays sets out these requirements. In an existing building, this usually involves opening up the masonry above the opening, often by removing brickwork in sections, fitting the tray and rebuilding the wall. The scale depends on the opening and the wall construction, which is why a fixed price cannot be given without a survey.

Cavity tray drainage features above an opening

Complex junctions

Pitched roof abutments and similar details need more than a folded sheet. LABC recommends purpose-made preformed stepped trays there, so the installation is durable and works with the flashings. NHBC Standards likewise recommend preformed trays at complicated junctions to avoid complex bending, folding, cutting and sealing on site.

Where other trades are needed

A tray repair can involve lintels, roof flashings or structural masonry. Some of that work needs a roofer, a structural engineer or a building control sign-off. We say so when the scope goes beyond ours and refer you on, rather than stretching our remit. Our masonry repairs page covers the brickwork side.

Drying out and reinstating

Once the water path is fixed, the wall needs time to dry. Damaged plaster, salt-contaminated finishes and any affected timber then need attention. Plaster should not be renewed until the wall is dry enough and the cause is closed off. Depending on the condition, that may mean renovation plaster, skimming and making good. If timber has stayed wet, it should be checked by a timber survey, since wet rot and dry rot can follow persistent moisture.

Drying out and reinstating

Why the damp can come back

Repeat damp after a tray repair usually has one of these causes:

  • The tray was fixed but the gutter above still overflows.
  • Window seals or pointing were left to fail.
  • Debris was left in the cavity, or new debris fell in during the work.
  • Another opening on the same elevation has the same fault.
  • The wall was plastered before it dried.
  • The original cause was something else, such as a leak or condensation, and the tray was never the problem.

To reduce the chance of recurrence, keep gutters and downpipes clear, reseal windows and doors when the seals deteriorate, keep render and garden levels clear of the wall, and ask for photographs of the finished tray detail. If you see the same pattern over other openings, ask for them to be checked while access is available.

Perspective: why we diagnose before we repair

Most callers tell us they have rising damp. Often the stain is above a window, and the cause is somewhere else: a tray, a gutter, a seal or a leak.

That is why we start with a survey. We look at what you can see, inspect the outside and inside, and record what we find with photographs. You then get written findings and a defined scope of work. Nothing is proposed until the cause is known.

After more than 35 years in this trade, we have seen what happens when treatment comes first: money spent, damp still there. A cavity tray fault calls for masonry repair, not injection or a coat of paint. We would rather tell you that and give you a written guarantee on the work we do carry out.

Where a problem sits outside our remit, such as roof structure or engineering, we say so and refer you to the right specialist.

Quay

Get the cause confirmed before you pay for treatment

We are a survey-led damp, mould and timber business working across Dorset, Hampshire and Wiltshire. If you have damp above a window or door, we inspect, document the findings, set out the scope of work and give written guarantees on our workmanship.

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Start with our damp and timber survey, or see everything we cover on our main site. To book an inspection, call 01202 287134 and tell us what you can see. If you are weighing up other types of survey first, our guide to homebuyer damp surveys explains what to expect.

Before the survey: what to note

A few minutes of preparation makes the visit more useful.

  1. Photograph the damp from the inside and the outside wall above it.
  2. Note the dates when it gets worse, and what the weather was.
  3. Write down any recent building work, new windows, insulation or roof repairs.
  4. Check whether gutters above are overflowing during rain.
  5. Keep any earlier damp reports or treatment guarantees.

If you are buying, selling or letting the property, our damp inspection checklist gives six checks worth running through. If the house is a new build, see how owners show damp is a defect, since a missing tray at handover is a construction issue. For more reading, browse our advice and blog.

FAQ

How do I know if damp above my window is a cavity tray problem?

Look at the location and timing. Damp directly above an opening that gets worse after heavy rain is consistent with a tray or lintel fault. Moisture readings alone cannot confirm it, so a visual inspection of the whole house, and often a borescope check of the cavity, is needed to rule out leaks, condensation and other defects.

Can a failed cavity tray be repaired without rebuilding the wall?

It depends on the construction and what is wrong. If the cause is blocked weep holes or debris, no rebuild is needed. If a tray is missing or badly formed, the masonry above the opening generally has to be opened up to fit a continuous tray with stop ends and weep holes. A survey sets out the scope.

Is a missing cavity tray a building regulations breach?

In England, Approved Document C sets requirements for cavity trays, including stop ends, a minimum 140mm rise and weep holes at no more than 900mm centres. Whether a particular installation falls short depends on when it was built and what was approved at the time. For a new build, speak to the builder or warranty provider.

Will injecting a damp-proof course fix damp above a window?

No. A chemical damp-proof course addresses moisture rising from the ground, and RICS says rising damp is often misdiagnosed. Damp high on the wall around an opening needs its water source identified and corrected first.

Sources

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