Signs of damp walls and how to tell which type it is

The common signs

A musty smell is often the first sign, especially in cupboards, behind furniture and in rooms that are not used much. Dark patches or areas that feel cold and slightly wet appear next. Peeling paint, bubbling wallpaper and blown plaster that sounds hollow when tapped show that moisture has been there for a while.

Tide marks, a brownish line where water has evaporated, are typical of water moving through a wall. White, fluffy or crystalline deposits are usually salts brought to the surface as water evaporates, not mould. Black spots, often in corners, on window reveals and behind furniture, are mould growing on a damp surface.

• Musty smell
• Dark or wet patches
• Peeling paint or bubbling wallpaper
• Blown or crumbling plaster
• Tide marks
• White salt deposits
• Black mould spots

Condensation pattern

Condensation forms where warm, moist indoor air meets a cold surface. It usually shows in the coldest spots: the corners where outside walls meet each other or the ceiling, around and below windows, behind wardrobes and beds against outside walls, and in rooms with lots of moisture such as bathrooms, kitchens and bedrooms. It is worse in winter and in the morning, often with streaming windows, and you will often see black mould spots.

Our explainer on dew point shows why these spots are first, and the dew point calculator lets you check your own room.

Penetrating damp pattern

Penetrating damp comes through the wall from outside. It shows as patches that are larger or wetter after rain, often at any height, and frequently linked to an outside fault: a leaking gutter or downpipe, cracked render, failed pointing, a damaged roof or flashing, or a window sill that throws water onto the wall. Tide marks and salts are common.

Check outside on a wet day, directly opposite the patch. Overflowing gutters are one of the most common causes and one of the cheapest to fix.

Rising damp pattern

Rising damp is water drawn up from the ground into the base of a wall. It is less common than often claimed and is usually seen on ground floor walls, typically as a band of damp, tide marks and salts from the skirting up to roughly a metre, fairly level along the wall, and present all year. It is often linked to a damp proof course that has been bridged by raised ground, paths or render, or to older walls built without one.

Because condensation and penetrating damp can look similar at low level, get a qualified, independent survey before paying for injected treatments. Our guide on what a damp proof course is explains more.

Simple checks you can do

Use a cheap hygrometer to check room humidity. Readings regularly above about 60 to 65 % with damp in cold spots point to condensation. A damp meter used in a grid on the wall shows where readings are highest; our guide on damp meter readings explains how. Tape a square of kitchen foil tightly to the damp area for a day or two: moisture on the room side suggests condensation, moisture underneath suggests water coming through the wall.

Then look outside for gutters, downpipes, pointing and ground levels against the wall.

1. 1

   Check humidity with a hygrometer

2. 2

   Map damp meter readings in a grid

3. 3

   Try the foil test for a day or two

4. 4

   Inspect gutters, downpipes and pointing outside

5. 5

   Check ground levels against the wall

What to do next

For condensation, bring humidity down with ventilation, steady heating and a dehumidifier; see how to treat damp walls. For penetrating damp, fix the outside fault first. For suspected rising damp, get an independent survey. If you rent, report damp to your landlord as described in damp and mould in a rented home.

See dehumidifiers for condensation →

Frequently asked questions

What does damp on a wall look like? Dark or wet patches, tide marks, peeling paint or wallpaper, white salt deposits and black mould spots, often with a musty smell.

How can I tell condensation from rising damp? Condensation appears in cold spots and is worse in winter mornings. Rising damp is a low band on ground floor walls present all year, often with salts.

Why do my walls smell damp? A musty smell usually means moisture and mould somewhere nearby, often behind furniture or in a cupboard on an outside wall.