Roofs and flat roofs
Can the underside of a cold flat roof drip when the covering itself has not split?
Yes. The Building Research Establishment has reported that drips in a cold roof are often condensation, not a leak through the covering, because moist air meets a cold deck and water forms on the underside. That can mark a ceiling without a hole in the felt or membrane. A home survey can note drips and where insulation was visible, but one visit cannot always prove which mechanism made a single drip.
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?
A cold flat roof leaves the deck on the cold side of the insulation, with a void that can be chilly and damp. Water on the underside then looks exactly like a leak, and people replace sound coverings. BRE’s point is to separate those causes. The surveyor should say whether they saw the underside, the vents, if any, and the covering, and should not declare a split they did not see. A stain alone is not the split.
The practical difference matters before you commit. A roofer who only sees the top may quote a new membrane. A builder who only sees the drip may quote the same. Ask them to consider condensation if the covering is whole and the void is cold and poorly ventilated. There is no RICS price for either repair. Do not block any vents that do exist while you wait. The survey has not redesigned the roof, and it has not proved the felt innocent in every bay.
What should you ask your surveyor?
- Did you see the underside of the deck, or only a ceiling stain?
- Was the covering split where you could see it?
- Are you allowing for condensation as well as a leak?
What are the typical next steps?
- Do not replace the covering on a drip alone if nobody has looked at the void.
- Tell the roofer the survey raised condensation in a cold roof.
- Do not stuff the void with insulation until someone has said how this roof is built.
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
Is every drip condensation?
No. BRE says drips in a cold roof are often condensation, not that a real hole never leaks. A seen split is still a split.
Does this apply to a pitched loft as well?
The BRE note used here is about cold-roof dripping, typically the flat cold deck. Do not paste it onto every tile roof without reading what the survey actually saw.