Roofs and flat roofs
What is a warm-roof build-up, and can a survey confirm that the insulation sits above the deck?
A warm-roof build-up puts the insulation above the deck, with the waterproofing above the insulation, so the deck is on the warm side. A home survey can confirm those layers only if an edge, an upstand or a drawing actually shows them. The look of the top surface does not prove the order. It is not a remaining-life figure, and it is not a reason to replace a covering the survey could not see through.
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?
Where the insulation sits changes how the roof deals with moisture, but the buyer’s question here is what was proved. A smooth membrane might cover insulation, a deck, or both, and the surveyor cannot core it on a standard inspection. If a drip edge or a cut at a rooflight shows boards, insulation and felt in that order, they can describe that sample. One sample is not every bay. A seller’s diagram is useful only if it matches this roof.
Do not assume a modern-looking covering is a warm roof, and do not assume an old-looking one is not. RICS home surveys stay with what is visible. They do not calculate condensation risk for a build-up they have not seen. If you need the layers confirmed before you commit, agree an inspection that can see an edge, or ask for the specification. There is no RICS price for rebuilding a warm roof, and no duty to rebuild one that has not been shown to be defective.
What should you ask your surveyor?
- Did any edge show insulation above the deck, or only the top sheet?
- Are you inferring the build-up from age, or from something you saw?
- Which parts of the flat roof were not visible at all?
What are the typical next steps?
- Ask the seller for any specification or invoice that names the layers.
- Do not replace a flat roof to “make it warm” on appearance alone.
- If an edge is exposed, ask the surveyor to describe that edge rather than the whole roof from guesswork.
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 warm roof need eaves vents like a cold loft?
Not in the same way. The deck is meant to be warm. Do not punch vents through a warm covering because a loft detail looks familiar. Ask before anyone cuts the roof.
Can insulation under the plasterboard still be a warm roof?
No. Insulation only at ceiling level is a different arrangement. The point of this question is insulation above the deck, and only if it was shown.