How does a cavity wall actually keep water out?
The outer leaf is expected to let water through. The cavity, the ties, the trays and the weep holes are a drainage system, and nearly every damp problem in a cavity wall is that system being bridged or blocked.
- Difficulty
- intermediate
- Time
- 10 min
- Read
- 5 min
Short answer
A cavity wall is not a waterproof wall. Driving rain soaks through the outer leaf, runs down the inside face of it, is caught by a tray and let back out through weep holes. The cavity works by keeping the two leaves separate so water cannot cross. Anything that bridges the gap — mortar on a tie, debris on a tray, insulation slumped into the void, a filled weep hole — breaks it.
This is the single most useful thing to understand about a modern British house, and almost nobody is told it. People assume the outer brickwork is the barrier and that water getting through it is a defect. It is not. It is the design. What matters is what happens to the water afterwards.
Step by step
- Accept that the outer leaf leaks.Brick and mortar are porous, and in driving rain a single-leaf masonry wall wets right through. That has been known for as long as cavity walls have existed, and it is why they exist.
- See the cavity as a drainage void, not an air gap.Water arriving on the cavity face of the outer leaf runs down it under gravity. The cavity's job is to give it somewhere to go that is not the inner leaf, and to give it a route back out.
- Look at what a wall tie is shaped like.Ties hold the two leaves together against wind, but they are laid to fall slightly towards the outer leaf and have a twist or a drip in the middle. That detail exists solely so water running along a tie falls off before it reaches the inner leaf.
- Find where the trays are.A tray is an impervious layer sloping from the inner leaf down to the outer, collecting water and directing it back outwards. There is one at the base of the wall at the damp proof course, one over every opening, and one at every point where the cavity is interrupted or a roof abuts.
- Recognise the weep holes as the outlet.Open vertical joints in the outer leaf immediately above a tray. They are the only route out for the water the tray has collected. Filling them turns a working tray into a reservoir.
- Understand what bridging means.Anything spanning the cavity that water can travel along. Mortar dropped onto a tie as the wall was built, debris sitting on a tray, an incorrectly installed insulation batt, or a later drill hole packed with mortar. Water crosses on it and appears inside.
- Read internal damp against the diagram.Damp along the head of a window points at a tray. A patch high on a wall points at a bridged tie. Damp at the base points at the damp proof course or at ground level outside. The pattern locates the failure surprisingly reliably.
Tips
- Damp that appears only after prolonged wind-driven rain from one direction, and dries afterwards, is penetrating damp through the wall. Damp that is worst in a cold room in winter and shows on cold spots and in corners is condensation. They are different problems and confusing them wastes a lot of money.
- The elevation that faces the prevailing wind — generally the south-west in Britain — takes far more water than the others, and is where penetrating damp shows first.
- If a cavity has been filled with insulation, everything above still has to work. Retrofit insulation is not a substitute for a tray, and a badly filled cavity can itself become the bridge.
Common mistakes
- Filling weep holes because they look like missing mortar — They are the drain. Filling them holds water on the tray until it finds another way out, which is usually across the cavity and into the room.
- Applying a waterproof coating to the outside of a wet wall — It traps the water already in the masonry and stops the wall drying outwards. On a solid wall it is actively harmful, and on a cavity wall it treats a symptom while the bridge is still there.
- Assuming any damp patch is rising damp — Rising damp is far rarer than the products sold to treat it imply. A bridged cavity, a raised outside ground level, a failed tray or a leaking gutter account for most of what gets diagnosed as it.
If it doesn't work
A damp patch appears on an inside wall only after heavy driven rain
Cause: Penetrating damp, most likely a bridged cavity or a failed tray — Fix: Note the wall's orientation, the weather that triggers it and exactly where the patch sits relative to windows and floor levels. That description is what lets a surveyor find the bridge without opening the whole wall.
Damp along the top of a window opening
Cause: A missing, damaged or wrongly lapped cavity tray over the lintel — Fix: Check the weep holes above the window are present and clear. A tray with no weeps, or no tray at all, is a common defect in both older cavity work and in later window replacements.
Damp low down on an internal wall with dry walls either side
Cause: A bridge at the base — raised ground outside, debris in the cavity at DPC level, or a rendered plinth carrying water over the damp proof course — Fix: Look outside first at the ground level and at any render or path bridging the damp proof course. This is the cheapest thing on the list to find and by far the most common.
Questions people ask
How do I know if my wall is cavity or solid?
The brick bond gives it away from outside — a wall showing only stretchers is usually cavity, while a pattern including headers suggests a solid wall bonded through its thickness. Wall thickness at a reveal is the other check. GuideHQ has a guide dedicated to identifying wall type.
Should the cavity be empty?
In original construction it was. Many walls have since been insulated, either as full fill or as partial fill with a residual clear cavity. Both are legitimate; what matters is that it was done into a suitable wall, properly, and that the drainage details still work.
Can I look inside my cavity?
A borescope through a small hole in a mortar joint is how surveyors do it, and it is the standard way of checking for fill, debris and tray condition. It is a survey technique rather than a casual one, because the hole has to be made and made good properly.
Do solid walls have any of this?
No, and that is the point. A solid wall resists rain by being thick enough that water does not reach the inside before the weather changes and it dries out again. That is why sealing or cement-rendering a solid wall so often causes damp rather than curing it.