Roofs and flat roofs
What is the difference between old roofing felt and a breathable membrane, as far as a survey can tell?
Old bitumen felt is a vapour-tight underlay. Many later membranes are meant to let vapour through. A surveyor can sometimes tell them apart by look and age, and sometimes cannot. Neither type proves the tiles are sound, and a torn underlay is a defect of the underlay, not a full description of the covering.
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?
The underlay is the layer under the tiles. Bitumen felt can sag into the eaves and hold water. A breathable membrane is often paler and is supposed to reduce condensation risk if the rest of the roof is detailed properly. From the loft, the surveyor sees the underside: sags, tears, daylight, stains. They do not send the sheet to a lab, and they do not certify a brand.
RICS home surveys are visual. If the report says the underlay is torn or absent, that is what matters more than the trade name. Ventilation at the eaves still matters, especially with an old felt roof. The Building Research Establishment’s condensation note is about vapour in cold voids, which is why the type of underlay is mentioned at all. Ask what was seen, not for a product approval the survey never gave.
What should you ask your surveyor?
- Could you see the underlay, or was the loft boarded?
- Is it torn, sagging into the gutter, or only old?
- Did you see daylight through the covering as well as through a tear?
What are the typical next steps?
- Do not replace a roof because the felt is old and still continuous.
- If it sags into the gutter, include the eaves in any roofer’s visit.
- Ask the surveyor to name what they could see if the report only says “underlay”.
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 breathable membrane mean you can block the eaves?
No. The survey has not redesigned the roof. Blocking vents because a membrane “breathes” is how lofts get wet. Ask before you stuff the eaves with quilt.
Is missing underlay on an old roof a surprise?
Some very old roofs never had any. The question is whether rain is getting in, not whether the roof matches a modern diagram.