Damp Patches on Internal Walls: Finding the Real Cause

Rising damp: low, level and tide-marked

True rising damp sits at the base of a wall, rarely climbs beyond about a metre, and produces a horizontal tide mark with salt deposits along the top edge. It is present year round and worse in wet months.

Before assuming a failed damp-proof course, check the outside: raised patios or flowerbeds above the DPC line, render or a concrete path bridging it, or a blocked sub-floor vent will all produce the same symptom and are far cheaper to correct.

Penetrating damp: localised and weather-linked

Penetrating damp appears as an isolated patch, often part way up the wall, and darkens noticeably a day or two after heavy or driven rain, then fades. Its position on the inside usually lines up with a defect on the outside face.

Overflowing gutter, split downpipe or blocked hopper directly above the patch · Cracked or hollow render, or perished pointing on exposed elevations · Failed sill, missing drip groove or defective window seal · Debris in the cavity or a cavity closer bridging the gap · Failed roof flashing where a lower roof abuts a wall

Condensation: cold surfaces and black mould

Condensation forms where warm, moist air meets a cold surface, so it shows up in corners, behind furniture on external walls, on window reveals and in unheated rooms. The giveaway is black spot mould rather than a tide mark, and a strong seasonal pattern — it appears in autumn and clears in spring.

It is a building physics problem, not a plumbing one. The cure is a combination of ventilation, background heating and, where the wall is genuinely cold, insulation. Replastering alone changes nothing.

Getting the sequence right

Whatever the cause, the order is the same: stop the water, let the wall dry, then make good. Damp masonry can take several weeks to dry after the source is removed, and finishing too early traps moisture behind the new plaster and paint.

Where salts have contaminated the plaster from a long-running problem, the affected coat is removed rather than skimmed over, because the salts are hygroscopic and will keep drawing moisture into a fresh finish.

Frequently asked questions

Does a damp meter prove rising damp?

No. Conductive meters read salts and surface moisture, so they read high on walls affected by condensation or historic contamination too. A high reading tells you moisture is present, not where it came from.

Why does the damp patch keep coming back after replastering?

Almost always because the source was never removed, or because the wall was replastered before it had dried out. Both trap water behind the new finish where it re-emerges as staining and blown plaster.

Can insulation cure a damp wall?

It can cure condensation, because it raises the internal surface temperature above the dew point. It does nothing for rising or penetrating damp, and applied over a wet wall it can make matters worse.

Should black mould be treated before decorating?

Yes — mould has to be killed and removed, not painted over, and the ventilation or heating problem behind it addressed, or it returns through the new paint within a season.