How high should a damp-proof course be above the ground?

At least 150mm above the level of the adjoining external ground, under Approved Document C. That gap exists so rain splashing off a hard surface cannot reach the brickwork above the barrier, and so a raised path or a new patio cannot bridge it. It is the single easiest defect to spot on a house, because you can see it from the garden without any equipment at all.

What are weep holes for?

They drain the cavity. Water that gets through the outer leaf runs down the inside face and collects on whatever tray or barrier is beneath it, and the weep holes are the only route out. Approved Document C requires them every 900mm along a cavity tray, and where the tray stops — above every window and door — the tray needs stop ends and at least two weep holes. Block them or omit them and the water backs up and finds its way inwards.

Is rising damp real, or is it always something else?

Genuine rising damp exists but is far less common than it is diagnosed. Most damp at low level in London houses turns out to be a bridged damp-proof course, a raised external ground level, a blocked or missing cavity tray, a failed gutter or downpipe, or condensation. The useful discipline is to eliminate those first, because every one of them is cheaper to fix than an injected chemical course and rather more likely to be the actual cause.

Can I raise my garden level against the house?

Not without checking where the damp-proof course sits. Raising a patio, a path or a border against an external wall is one of the most common ways a sound damp-proof course gets bridged, because the new surface brings soil or paving above the line of the barrier and moisture simply travels around it. If the level has to rise, the wall needs a drainage channel or a tanked retaining detail so the 150mm relationship is preserved.

When do weep holes and cavity trays get installed?

Before the wall closes, and never afterwards. Trays are built into the masonry as it rises, weep holes are formed in the bed joint above them, and both disappear behind finishes within days. This is why they belong on the list of things to photograph at first fix — once the scaffold is down and the render is on, the only way to confirm a cavity tray exists is to open the wall up again.

Extensions

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

Damp is rarely bad luck. Two numbers keep a wall dry: the DPC sits 150mm above outside ground, and weep holes appear every 900mm. Check both from the garden.

By Vladimir Castravet·17 August 2026

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

| The rule | What it requires | Source | |---|---|---| | DPC height | The damp-proof course should be at least 150mm above the level of the adjoining ground. | AD C §5.5(b) | | DPC material | Bituminous material, polyethylene, engineering bricks or slates in cement mortar — continuous with any damp-proof membrane in the floor. | AD C §5.5(a) | | Cavity depth below DPC | The cavity should be taken down at least 225mm below the level of the lowest damp-proof course, or a damp-proof tray provided. | AD C §5.5(c) | | Weep holes | Provide weep holes every 900mm above a cavity tray. Where the tray does not run the full length of the wall — above every opening — provide stop ends and at least two weep holes. | AD C §5.5(c), Diagram 9b | | Minimum cavity width | At least 50mm. Where partially filled, the residual cavity should not be less than 50mm. | AD C §5.11, §5.14 | | Suspended timber floor | Ventilated air space at least 150mm clear from the ground to the underside of the floor or insulation. | AD C §4.11 | | Sub-floor ventilation | Openings in two opposing external walls, not less than 1500mm²/m run of wall or 500mm²/m² of floor area, whichever is greater. | AD C §4.11, §4.13 | | Solid wall, rain resistance | Brick or stone at least 328mm thick, dense blockwork 250mm, lightweight or AAC blockwork 215mm — plus render in two coats totalling at least 20mm. | AD C §5.8 | | Groundwater | Where the water table can rise to within 0.25m of the lowest floor, drain the ground by gravity or safeguard the building by other means. | AD C §3.2 |

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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