Damp In Basements And Ground Floor Walls

A damp patch on a ground floor wall is one of three things, and the repair for each is different.

Getting the diagnosis right is the whole job, because the treatments do not overlap.

Rising damp

Moisture drawn up from the ground through the masonry by capillary action.

What it looks like. A tide mark at a fairly consistent height above the floor, usually under a metre. Salts on the surface as a white bloom. Paint and render failing in a band. Skirtings decaying at the bottom.

Why it happens. Either there is no damp proof course, which is the case in many older buildings, or the one that exists has been bridged. Bridging is the more common cause in a building that used to be dry: garden beds built up against the wall, paving laid above the damp course level, render carried down past it, or a cavity filled with debris.

The fix. Remove the bridge first, because it is cheap and it is frequently the whole answer. Lower the soil or paving, cut the render back above the damp course, clear the cavity. Where there genuinely is no damp course, a chemical injected course can be installed, and the affected render has to be replaced with a salt resistant system because the old render holds salts that keep drawing moisture.

Penetrating damp

Water coming through the wall from outside, horizontally.

What it looks like. Patches rather than a band, not at a consistent height, and worse after rain. Frequently traceable to a specific defect above or beside it.

Why it happens. A failed gutter or downpipe. Cracked render. Failed pointing. A leaking pipe. A blocked weephole. Ground levels above the floor level with no drainage. Below ground, a failed or absent waterproof membrane on the outside of a basement wall.

The fix. Find and fix the water source. This sounds obvious and is frequently skipped in favour of a coating on the inside, which traps the water in the wall rather than removing it. A blocked downpipe is the cause more often than anything else and it takes ten minutes, which is why it gets checked first, in the same way any [roof and gutter leak](/blog/roof-and-gutter-leaks-that-are-not-plumbing) gets traced before anything is repaired.

Condensation

Moisture from inside the building settling on a cold surface.

What it looks like. Mould rather than salts. Worst in winter, worst in corners, behind furniture, on window reveals and in unventilated rooms. No tide mark.

Why it happens. Moisture produced indoors with nowhere to go, meeting a cold surface. The mechanism described in [where the moisture goes](/blog/clothes-drying-in-apartments-and-where-the-moisture-goes).

The fix. Ventilation and, where the cold surface is the problem, insulation. Not a damp proof course, and not a waterproof coating, both of which are regularly sold for this and achieve nothing.

Telling them apart

The distinguishing features are reliable.

Salts on the surface mean water has travelled through masonry, which points at rising or penetrating damp rather than condensation. Mould without salts points at condensation. A consistent tide mark points at rising damp. Patches that worsen after rain point at penetration. Worse in winter with no rain correlation points at condensation.

A moisture meter reading helps and is easy to misread, because surface salts give a high reading on a dry wall. Which is why the pattern matters more than the instrument.

Basements specifically

A basement wall has ground water pressure against it permanently, so it is a different case.

Water entering a basement is either coming through the wall, through the junction between the wall and the floor, or up through the floor under pressure.

Fixing it properly means either waterproofing the outside of the wall, which requires excavation and is expensive, or managing the water on the inside with a drained cavity system and a pump. The second is the usual answer in an existing building, and it depends on the [sump and drainage system](/blog/basement-carpark-drainage-and-sump-pumps) working.

Applying a coating to the inside face of a basement wall under water pressure does not hold. The water finds the weakest point and the coating debonds.

The order of work

Drainage and ground levels first, because they are cheap and they are the cause more often than anything else.

Then the source of any penetration. Then ventilation for any condensation. Then the damp course, if the evidence actually points there.

Then, last, the finishes. Replastering a wall before the cause is dealt with is a repair with a twelve month life, and it is the most common way money gets spent on damp without fixing it.

Ample diagnoses and repairs damp across apartment buildings, houses and heritage terraces in the eastern suburbs and inner city.