Damp and moisture meters
A bathroom fan discharges into the loft rather than outside. What moisture is being put into the roof void, and what fan performance was not tested?
Moist air from the bathroom is being blown into the loft, not carried outside. The fan discharges into the roof void, so steam from washing and bathing can settle on cold felt, rafters and the underside of the covering. That is a moisture path into the void. The home survey has seen where the duct ends. It has not measured how much air the fan moves, how long it runs, or whether the extract rate is adequate. Fan performance was not tested. The finding is the discharge point, not a passed or failed fan.
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?
Warm bathroom air carries vapour. A duct that ends in the loft delivers that vapour onto cold felt and rafters, so wetting can start inside the roof void rather than from a slipped tile. Marks, where they appear, tend to sit near the outlet. The visit has placed the duct. It has not weighed moisture or timed how the void dries after use. Record condensation from this fan as the mechanism, and do not assume a covering failure from position alone.
Ask a ventilation firm, in writing, to say where that duct should terminate and whether the fan unit itself is worth keeping. The survey is not that test, and it will not become one if you reread the loft paragraph. Tell the solicitor the extract discharges inside the roof space, so any reply from the seller about the bathroom fan can be checked against the outlet, not against a claim that the fan was tried and passed.
What should you ask your surveyor?
- Where does the bathroom duct actually end, as you saw it?
- Did you measure airflow, run-on or extract rate, or only the outlet?
- Which loft surfaces should be read with this fan rather than with a roof leak?
What are the typical next steps?
- Treat the loft outlet as the moisture path from the bathroom.
- Do not read the note as a fan performance test.
- Get a written proposal for terminating the duct outside.
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
If the loft already smells musty, is the fan proved to be the only source?
No. A musty loft can have more than one cause, including a cold roof or stored wet belongings. The fan is a path the survey could see. Smell does not isolate it as the only source, and it still is not a performance test.
Should the duct be extended outside before anyone replaces the fan?
The outlet is the defect the survey has actually named. Extending that duct to an outside grille deals with the path into the void. Replacing the fan is a separate choice, and the visit did not show the unit itself had failed.