Damp Patches on a Wall or Ceiling: What Do You Do First?

Cause first, then the plaster

You can’t just paint away a damp patch on a wall or ceiling. Without removing the cause, the patch simply comes back after a while, often within a few weeks, and by then it’s under a fresh coat of paint too. First you find out where the moisture is coming from. Only then is the plaster up next.

That goes for a yellow ring on the ceiling just as much as for a vague, grey damp patch low down on an exterior wall. The paint isn’t the problem. What’s underneath it is.

The four common causes of a damp patch on a wall or ceiling

Almost every damp patch I come across falls into one of these four categories. Where the patch sits and what shape it has often tells you enough on its own.

A leak from above

A damp patch that starts on or close to the ceiling, with a more or less round edge that keeps growing, usually comes from a leak above you. Think of a leaking roof, a broken pipe or a blocked downpipe. The patch gets worse after rain and then dries out a little again afterwards, which is why you see a kind of ring pattern, a bit like in a tree trunk.

Penetrating damp on an exterior wall

If the stain is on an exterior wall, often on the side that takes the most weather, the moisture is probably coming straight through the wall. That’s called penetrating damp. It happens faster on a wall without a cavity or with damaged pointing, and you usually see it after a long spell of heavy rain and wind.

Rising damp low down

A damp patch low down on the wall, in the bottom strip, that doesn’t get bigger or smaller with the weather, often points to rising damp. Moisture from the ground then travels up through the wall, because there’s no proper damp-proof layer in the wall. You often recognise it by a water line and sometimes by white salt deposits.

Condensation in the bathroom or bedroom

A damp patch in the bathroom, often in a corner up high, or behind a cupboard in the bedroom, usually doesn’t come from a leak but from condensation. Warm, moist air settles against a colder wall or a poorly ventilated corner, and over time that causes a damp patch and often mould too. That happens faster in a room without ventilation or with a cupboard wall against an exterior wall.

Yellow stains on the ceiling

A yellow or brown ring on the ceiling is almost always an old leak. The colour comes from substances carried along by the water, from wood, insulation or the structure above, that bleed through the plaster and later through fresh paint too if you don’t seal them first. If the patch is dry and the leak is gone, you can simply have it treated. If there’s still movement or moisture in it, I’d rather wait until it’s properly dry before I carry on.

Mould on plaster

Black or green spots on plaster are mould, and they’re almost always caused by prolonged damp combined with too little air circulation. You see it most often in a corner, behind furniture against an exterior wall, or in a bathroom or bedroom without good ventilation. Scrubbing off mould and painting over it with ordinary paint only helps for a while. As long as the moisture source and the ventilation problem are still there, the mould simply comes straight back in exactly the same spot.

Who to call about the cause

I plaster and paint, I don’t fix a leak myself. If the problem is with the roof, the gutter or the downpipe, you call a roofer. If it’s a pipe or a tap, that’s a plumber. Not sure whether it’s rising damp, or exactly how big the problem is? A damp specialist can establish that with measuring equipment. Once the cause is gone and the wall or ceiling is dry, I come in for the plaster.

How I approach the plaster once the cause is gone

Only once the wall or ceiling is dry do I start on the plaster. First I let the patch dry out properly, even if it already looks dry on the surface. Then I tap the layer to check it and remove everything that’s loose or damaged by the moisture. What’s left is a clean, solid surface.

On that surface I apply new plaster that blends in with the rest of the wall or ceiling. If there was a yellow ring, I first put a stain-blocking primer over it, so the old stain doesn’t bleed through the new layer and the paint again. Without that step, you’ll simply see the ring come through again after a while, even though the cause has long since gone.

Only once the plaster is finished and properly dry can it be painted. For a hallway ceiling in Geertruidenberg it was exactly that kind of job, after a leak that had already been fixed:

Reviews are in Dutch, from Werkspot; I’ve left them exactly as customers wrote them.

“Waterlekkage gangplafond weer netjes. Snel, netjes en vakkundig geholpen. Alles duidelijk vooraf, tijdens en na de klus. Alles netjes achtergelaten. Nogmaals bedankt!” Source: Werkspot user, Geertruidenberg (Werkspot review)

In a bathroom I choose a different base layer than for an ordinary wall, precisely because damp and condensation occur more often there. Marcel from Gorinchem had his bathroom ceiling done this way:

“Na plaatsing zeer snel contact gehad, communicatie was helder, kon zeer snel langskomen voor een kijkje te nemen en kwam direct met heldere en goede antwoorden op mijn vragen. Voorbeeld: het plafond in de badkamer moest bij ons gedaan worden en de stucadoor kwam direct aanzetten met Knauf Aquastuc ipv gewone stucwerk. Puik werk geleverd.” Source: Marcel, Gorinchem (Werkspot review)

Which cause matches which damp patch

Not sure which of the four it is? This table sets out the key features side by side.

Not sure what’s causing your damp patch, or has the cause already been dealt with and does the plaster still need doing? Send a photo, and I’ll tell you what’s needed. If the wall is dry and the patch is small, I can often repair the patch quickly. If it needs painting afterwards too, I’ll take care of the painting in the same visit.