Rising damp

Brick, stone, mortar and renderRender (plaster)A layer of mortar (sand, cement or lime, and water) spread over brick or concrete to give a wall a straight, sound surface ready for paint or tiles. are full of tiny pores. When a wall stands on damp ground with no damp-proof barrier, water climbs up through those pores, like oil up a lamp wick. That is rising damp, and it is common in ground floors, basements and older buildings.

The signs are typical: a damp band along the bottom of the wall with an uneven edge, peelingPeeling paintWhen paint loses its grip and lifts off in flakes or sheets. It is almost always a sign of a problem underneath, usually damp or poor preparation. paint, render that blows and crumbles, white saltsEfflorescence (salt deposits)A white, powdery or crystalline crust on the surface of a wall. It is salts carried by water through the masonry, left behind when the water evaporates. on the surface and a musty smell. It should not be confused with a leaking pipe or with condensationCondensationMoisture in the air turning to water on cold surfaces such as windows, outside corners and ceilings. It is the most common cause of mould indoors., which need different treatment.

Repainting alone won't stop it; the marks come back. The fix starts with the cause: keeping water away from the foot of the wall (guttersGutter and downpipeThe channel and pipe that collect rainwater from a roof and carry it down to the ground or the drains., falls, soil levels), creating a barrier in the wall, for example by injecting a special product, and using render and paint that let the wall breathe.