Damp and moisture meters
What are hygroscopic salts, and why can a meter still read them after the wall has dried?
Hygroscopic salts absorb moisture from the air and can keep a patch looking damp, and a meter reading high, after the original water has gone. They are a clue to past wetting, often from the ground or from a chimney, not proof that water is still rising today.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
Groundwater and some chimney deposits leave salts in plaster. Those salts pull humidity out of the room, so the patch can darken in damp weather and lighten when the air is dry. A meter cannot tell salt from free water. That is why a tide mark with fluffy salt is investigated differently from a wet stain under a gutter.
Replacing the plaster without finding where the salts came from can move the problem into the new coat. The Property Care Association’s homeowner advice is to deal with the building defect, not to sell a treatment for a symptom. Government guidance on damp and mould likewise starts with the cause.
What should you ask your surveyor?
- Did you see salt crystals, or only a meter response?
- Is the patch at low level, on a chimney breast, or under a window?
- Would you expect the salts to remain if the water source has already been stopped?
What are the typical next steps?
- Photograph the salts before anyone paints them.
- Ask a damp investigator to separate salt contamination from active water.
- Do not budget a new damp-proof course solely because old salts are present. Ask what has to be removed and why.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Will emulsion paint seal hygroscopic salts in?
Paint hides the mark for a while and can blister. It is not a diagnosis and it is not a damp-proof course.
Are salts the same thing as black mould?
No. Salts are mineral deposits. Black mould is a growth that likes a cold, humid surface. A room can have one, the other, or both, and they do not share one repair.