What is saltpetre on walls?
Saltpetre is the popular name for the white, powdery or crystalline salt deposit that appears on damp walls. Chemically, saltpetre is actually potassium nitrate, but the deposit on walls usually consists of a mixture of salts: nitrates, sulphates and chlorides. These salts come from the ground, from the building materials themselves, or from earlier use of the space, such as a stable or storage. Water carries them through the wall; where the water evaporates, the salts are left behind.
Why saltpetre is a problem
- The salt itself attracts moisture. Many of these salts are hygroscopic: they draw moisture from the air. A wall containing salt therefore stays damp, even once the original source of moisture has been dealt with.
- Crystallisation breaks material apart. As the wall dries, crystals form in the pores, causing plaster, paint, joints and soft bricks to crumble.
- Finishes will not stay put. Ordinary paint or gypsum plaster on a salt-laden wall soon flakes off again.
How to recognise it
- White deposits, fluffy crystals or a white tide mark at roughly the same height along the wall.
- Flaking paint and crumbling plaster at the bottom of the wall.
- Often combined with rising damp, leaks or a poorly ventilated cellar or crawl space.
What to do
- Tackle the source of moisture first. Otherwise the salt will always come back. Think of a leaking gutter, ground level that is too high, a damaged joint or rising damp.
- Brush the deposit off dry. Never wash it off with water: that dissolves the salt and draws it back into the wall.
- Remove affected plaster to well above the visible damage.
- Choose a suitable finish, such as a salt-resistant renovation plaster or a vapour-open lime plaster, instead of gypsum or a dense paint.
- Do not leave the removed plaster lying around, but take it away; it is full of salt.
Related terms
- Relative humidity
- Crawl space
- Cellar
- Slaked lime
- Remediation in construction
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.