Before painting over it with an anti-damp product or calling anyone in, it is worth knowing where the water is coming from. Damp in a building almost always has one of these three sources, and each one is treated differently: apply the wrong solution and the stain comes back.
The three types of damp at a glance
| Rising damp | Penetrating damp | Condensation | |
|---|---|---|---|
| Where it comes from | From the ground: it rises up the walls | From outside: rain getting in through the roof, terrace or facade | From the air indoors: water vapour condenses on cold surfaces |
| Where it appears | On the lower part of walls, up to 1 m or a little more | On ceilings and walls, sometimes far from where the water gets in | In corners, behind furniture, on windows and on external walls |
| When it gets worse | It is always there, with little variation | After rain | In winter, when cooking, showering or drying clothes indoors |
| What it looks like | A horizontal band, white salt deposits, paint and plaster that bubble | A stain with a tide mark that grows when it rains; dripping | Black spots of mould, droplets on the glass |
A simple test for condensation
Tape a piece of aluminium foil about 30 × 30 cm over the damp area and leave it for 24 to 48 hours. If, when you remove it, there is moisture on the room side, water vapour is condensing on the wall: it is condensation. If the moisture is on the side against the wall, the water is coming from the wall itself: rising damp or penetrating damp.
Don’t forget the pipes
A fourth common cause is a leak from a pipe, a downpipe or a drain. The stain appears near bathrooms, kitchens or downpipes, does not depend on the weather and keeps growing. If you suspect a leak, turn off the stopcock and check whether the water meter is still turning.
How each one is fixed
- Rising damp: the water rising at the base of the wall has to be stopped and the damaged finish made good.
- Penetrating damp: the entry point has to be found and the waterproofing of the roof, terrace or facade repaired.
- Condensation: you need to ventilate, control indoor humidity and treat the thermal bridges where it forms.
Often two causes occur together, or the stain is far from its source. That is why we carry out a diagnosis at the building before we quote. We explain this in damp.
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