Rain enters a damaged rendered wall beside a sound wall with its water-repellent protective surface highlighted in blue.

How to stop penetrating damp through an external wall (UK guide)

November 11, 2026

Penetrating damp is water finding its way in from outside — and unlike condensation, it won't be solved by better ventilation. The patch tends to appear or darken after rain, and it can show up at any height on the wall. The fix is a two-step job: find and repair the entry point, then protect the external surface so water can't soak through again.

Not sure penetrating damp is your problem? Check first with our guide to telling rising damp, penetrating damp and condensation apart.

The signature of penetrating damp: a patch that gets worse during and after rain, often on the most weather-exposed wall, and frequently traceable to a specific defect — a crack, a gap or a leak.

Step 1: Find the way in

Walk the outside of the wall and look for the usual suspects:

Cracked or blown render — hairline cracks and hollow patches let water sit and soak in.
Failed pointing — eroded or missing mortar joints in brick or stone.
Leaking gutters and downpipes — a blocked or split gutter soaks one spot repeatedly.
Defective sills and copings — missing drips or cracked window sills channel water into the wall.
A saturated solid wall — older solid (non-cavity) walls can simply soak through in driving rain, with no single defect.

Fix the obvious defects first: clear and repair guttering, re-point open joints, and patch cracked render. Often that alone stops the ingress.

Step 2: Let the wall dry out

A soaked wall holds a lot of water. Once you've stopped new water getting in, give the wall time to dry before sealing internally — otherwise you trap moisture inside. Internal re-plastering and decorating should wait until the wall is properly dry.

Step 3: Waterproof the external face

For walls that keep soaking through — exposed elevations, porous brick, or solid walls — the lasting answer is a breathable external waterproof coating. The key word is breathable: you want to stop liquid rain getting in while still letting the wall release moisture outward, so you don't trap damp inside.

Our Poseidon Waterproof Membrane Paint forms a flexible, breathable waterproof barrier over masonry and render — it sheds rain while letting the wall breathe. For surfaces that need a tougher, more flexible coating (including areas that take a beating from weather or movement), The GOAT Flexible Waterproof Coating gives a high-build, crack-bridging finish.

Step 4: Deal with the inside

Once the outside is watertight and the wall has dried, make good internally. If the wall has been badly affected, a waterproofing / tanking treatment on the inside face can provide belt-and-braces protection before re-plastering — particularly below ground or on persistently exposed walls.

What not to do

Don't seal the wall with a non-breathable coating. Trapping moisture behind a plastic film causes new problems.
Don't paint over it internally and hope. The water is still coming in; you'll be repainting in months.
Don't skip the dry-out. Sealing a wet wall locks the moisture in.

Frequently asked questions

How do I know it's penetrating damp and not rising damp?

Penetrating damp can appear at any height and worsens with rain; rising damp only affects ground-floor walls from the base up and shows a salty tide mark. See our diagnosis guide.

Should I waterproof the outside or the inside?

Stop water at the source — the outside — wherever possible, using a breathable coating. Internal tanking is a secondary defence for walls that stay exposed or are below ground.