Why Walls Get Damp: The Damp-Proof Course and the Weep Holes

Damp is almost never bad luck — it is usually one missing or bridged detail, and two of them you can check from the garden in thirty seconds. The damp-proof course must sit at least 150mm above the outside ground, and weep holes must appear every 900mm along a cavity tray. Our extensions page covers where these details get built, and the estimator prices the work that puts them right.

Both go in before the wall closes. Neither can be added afterwards.

The two barriers, and what each one does

The damp-proof course (DPC) is a horizontal barrier built into the wall near the base, stopping moisture in the ground travelling up through the masonry. It is bituminous material, polyethylene, engineering brick or slate in cement mortar — anything that will not let water pass — and it must be continuous with the damp-proof membrane in the floor. A DPC that stops short of the floor membrane leaves a gap for moisture to enter at exactly the junction you cannot see.

The cavity tray is a sloping barrier inside the cavity, catching water that has penetrated the outer leaf and directing it back outside. Every cavity wall has water in it — brick is porous and driving rain gets through. The cavity exists so that water can run down the inside face of the outer leaf without reaching the inner one, and trays exist wherever that downward path is interrupted: above a window, above a door, where an extension roof abuts, at the base of the wall.

Weep holes are the exit. Without them a tray is a shelf collecting water rather than a channel draining it.

The figures

Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; your building control body has the final word on your specific building.

The thirty-second check from the garden

Walk outside and look at the base of the wall.

Find the DPC. On a brick house it is usually a visible line in the mortar course — a thin dark strip, or a course of engineering brick — a couple of brick courses above the ground.

Measure down to the ground. You are looking for at least 150mm of clear brickwork between that line and whatever the ground surface is: soil, paving, decking, gravel.

If the DPC is at or below ground level, it is bridged, and moisture is travelling round it. This is the most common damp defect in London houses and it is almost always something somebody did on purpose: a patio laid over the old one, a raised border, a driveway relaid at a new level, a bin store built up against the wall.

Then look above the windows and doors. You should be able to see weep holes — open perpend joints, gaps in the vertical mortar between bricks, at roughly 900mm intervals. Above an opening you want at least two. If a wall has been repointed by someone who thought those gaps were sloppy workmanship and filled them all in, the trays behind them are now shelves.

Before you pay for injected damp treatment

Genuine rising damp exists and is far less common than it is sold. Work through the cheaper and more likely causes first:

• A bridged DPC — the 150mm check above.
• A blocked, missing or badly formed cavity tray, particularly above an opening or where an extension roof meets the wall.
• Blocked or filled weep holes, often after repointing or rendering.
• A failed gutter, downpipe or gully. Stand outside during heavy rain and watch. Ten minutes of observation beats any meter.
• External render carried down over the DPC, which bridges it perfectly.
• Condensation, which produces damp patches on cold surfaces, mould in corners and behind furniture, and is a ventilation problem rather than a water-ingress one.
• A leaking waste or supply pipe in a wall or under a floor.

A damp meter reading on a wall tells you there is moisture. It does not tell you where it came from, and the direction of travel is the entire diagnosis. Any survey that reaches “rising damp, chemical injection” without first excluding the list above is not a survey.

On a new extension, these are photograph moments

Trays, DPCs and weep holes all disappear within days of being built. They are also all inspected as part of the notifiable stages — the damp-proof course and oversite are looked at before they are covered.

That makes them exactly the sort of detail worth photographing while it is open: the tray in place with its stop ends, the weep holes formed above it, the DPC continuous with the floor membrane. It costs nothing at the time and it is the only evidence that will ever exist.

If a builder finds that request odd, that is information. It sits alongside fire-stopping as work you are paying for and will never see.

The takeaway

Two numbers keep a wall dry: the damp-proof course sits at least 150mm above the outside ground, and weep holes appear every 900mm — with stop ends and at least two weeps above every opening. Both go in before the wall closes. Neither can be added afterwards.

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