Damp and moisture meters
White salts sit on a chimney breast in a room that was dry in summer. Why might those salts be hygroscopic rather than a leak that is active today?
Not white salts on a chimney breast, in a room that was dry in summer, are automatically a leak active today. The salts may be hygroscopic, taking moisture from the air and marking the plaster when no rain is entering. A dry summer visit does not date the salts and does not prove a current roof leak. The survey records the salts and the conditions on the day. It does not certify the flue. It does not decide that the breast must be tanked because the surface looks dusty.
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?
Breasts pick up salts from old flue use. Hygroscopic salts can darken when the air is humid and look quieter when it is dry, which is a different pattern from a leak that wets a ceiling after rain. Summer dryness supports that caution. It does not clear the breast, because the salts are still there, and it does not convict the flashings. Keep salts and an active leak as separate ideas until a proper look joins them.
Useful checks are the flashings and the flaunching outside, and a look at the salts by someone who understands this staining, before any plaster is replaced. An ordinary skim on salty plaster often fails, and the survey has not specified a plaster system. Do not buy a roof repair only because the breast is salty. Do not decorate the breast and call the salts finished. The summer visit left the mechanism undecided.
What should you ask your surveyor?
- Were the salts dry and powdery, or was the plaster itself wet on the day?
- Did you see an outside defect you are linking to this particular breast?
- Are you calling the salts hygroscopic, an active leak, or still undecided?
What are the typical next steps?
- Keep hygroscopic salts as a possible reading, not as a proved live leak.
- Look at the flashings and the flaunching before pricing a roof repair for this breast.
- Do not skim over the salts and treat the breast as finished.
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
Do the white salts mean rain is getting in today?
Not necessarily. Hygroscopic salts can mark a breast by drawing moisture from the air. A dry room in summer does not prove a leak is active now.
Should the breast be tanked because the salts are there?
Not from this note. The survey has not specified tanking. The salts still have to be told apart from an active leak before a plaster system is chosen.