Roofs and flat roofs

What is the difference between a torch-on felt flat roof and a single-ply membrane?

Torch-on felt is a bitumen sheet bonded with heat, usually in layers. A single-ply membrane is a single sheet, such as a synthetic rubber or plastic, joined at its seams. They fail in different places. A survey that only says “flat roof” has not told you which one you are buying.

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?

Felt often fails at laps, upstands and blisters. Single-ply fails at seams, punctures and where pipes or balustrades pierce it. From the ground, or from a window, a surveyor may only see the colour and the upstand. They will not always know the product. The seller’s invoice is the document that names it.

There is no standard remaining life you can read off the material. A recent recover can still be poorly detailed at the edge. Ask for the guarantee, whether it is insurance-backed, and whether it transfers. The flat-roof guide explains ponding and edges. This question is only about not treating two products as one.

What should you ask your surveyor?

Could you tell felt from a single-ply sheet, or was the surface hidden?
Where are the upstands, outlets and penetrations you could see?
Did the seller’s papers name the product, or is that outside the inspection?

What are the typical next steps?

Ask the solicitor for any guarantee and invoice. Read who it covers.
Ask a roofing contractor who knows that product to look at the details, not only the middle of the sheet.
Do not compare a felt quote with a single-ply quote as if they were the same job.

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 a fibreglass roof either of these?

No. Glass-reinforced plastic is a different liquid-applied system. It should be named if the surveyor could tell. If they could not, the report should say so.

Does a gravel surface mean felt?

Often the gravel protects a built-up felt roof, but it also hides it. Gravel is not a diagnosis.