Damp and moisture meters
Can a bathroom with no working extract grow mould while the masonry behind it stays dry?
Yes. Steam with nowhere to go will grow mould on cold paint, tile grout and ceilings while the masonry behind can still be dry. That is a ventilation failure, not a diagnosis that the damp-proof course has failed. A visual survey does not measure the fan or test the brick behind the tiles.
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?
Showers put a lot of water into the air in a small room. If the extract does not run, or ducts nowhere useful, that moisture stays on the coldest surfaces. Black spots on the ceiling and the silicone are the usual result. They do not require the wall to be saturated, and they do not require water to have risen from the ground. A surveyor sees the mould and whether a fan is even fitted. They do not clock the airflow.
GOV.UK's damp page is written for rented housing providers and is not a buyer discount and not a survey method. Use the survey sentence for what was seen in this bathroom. The Property Care Association's homeowner advice covers damp along with rot and woodworm, which is a reminder that moisture has a source. Cleaning the mould without giving the steam a way out leaves the same surfaces to grow it again.
What should you ask your surveyor?
- Was an extract fan fitted, and did you see it discharge anywhere?
- Is the mould on the finishes, or is the plaster under the tiles visibly wet?
- Are the skirtings in this room dry?
What are the typical next steps?
- Do not inject the walls because the bathroom ceiling has mould.
- Ask the seller whether the fan runs, and where the duct goes, before you rely on it.
- Plan to ventilate the steam. Wiping the spots is not the repair.
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
Does a fan that looks modern prove the duct leaves the house?
No. The grille can be new while the duct stops in the loft or the cavity. The survey should say what it could see of the run.
Is mould on silicone a structural defect?
No. Silicone is a finish. Mould on it is a moisture and cleaning issue unless the joint has opened and is letting water through.