What single-ply means
Older flat roofs were built up in layers, as with several sheets of felt bonded one over another. A single-ply roof reverses that: the weatherproof layer is one flexible sheet, manufactured in a factory to a controlled thickness and delivered to site in large rolls. The sheet is unrolled over the roof build-up and fixed down, either bonded with adhesive, held with mechanical fixings or weighted with ballast.
The roof below the sheet still matters. Under it sit the deck, a vapour control layer if the roof needs one, and the insulation. The membrane keeps the water out, and everything under it keeps the building warm and stops moisture building up.
Because the sheet is a single layer, workmanship concentrates in the joins, the edges and the details around outlets, corners and wall upstands. A well-made sheet that is badly finished at those points will still let water in there, so the detailing deserves as much attention as the product. Falls matter too: IKO's guidance for its single-ply roofs is a design fall of 1:40, and the minimum fall answer explains why.
Comparing single-ply types: PVC, TPO and EPDM
TPO stands for thermoplastic polyolefin, a plastic single-ply membrane. Here is how the three types differ, in general terms:
| Type | Material | Joined by |
|---|---|---|
| PVC | Plastic membrane | Welded with hot air |
| TPO | Plastic membrane | Welded with hot air |
| EPDM | Rubber membrane | Glued, with tape |
Ask whether the sheet is reinforced and how thick it is, because both vary by product and maker. With the plastic types, welding fuses the sheets into a single joint, and corners and details are formed with pre-moulded pieces. With rubber, one large sheet is often enough for a small roof, so there may be no seam in the middle at all, and the corners are folded or fitted with patches.
For EPDM, the rubber type, HomeBuilding gives 25 to 30 years and Less gives 30 to 50 years, and the EPDM lifespan answer explains the gap. For a plastic membrane, ask the installer for the maker's data rather than relying on a general claim, and ask how the membrane is protected where people may step on it.
Where single-ply suits a house extension
A rear extension with a flat roof, a garage conversion or a porch is a natural fit, for several reasons. The roof is a simple shape, so the sheet can often be laid with few cuts. The membrane is light, which keeps the load on the joists down, and the roof can be finished in a short working window. It can run up a house wall as an upstand without a separate joint, and it can be cut around a rooflight kerb or a vent pipe with pre-formed pieces.
Single-ply is less suited to a roof that people walk on regularly, a roof with many small penetrations and odd angles, or one exposed to sharp objects and heavy foot traffic, where a tougher finish might be chosen. A roof terrace would also need a proper walking surface laid over the membrane.
The choice between types is really a choice about the details: how the edges are trimmed, how the sheet is held down and how the joins are made. Ask the roofer to describe those, and to explain how the roof will drain. The rear extension answer covers the whole job from strip to finish, including access and the insulation rules that apply.
Questions to put to an installer
Because single-ply roofs are marketed under different names, a few plain questions cut through the sales language:
- What is the membrane made of: PVC, TPO or EPDM?
- How is it fixed to the roof: adhesive, mechanical fixings or ballast?
- How are the joins made, and how are corners and outlets detailed?
- What fall will the finished roof have, and which standard is the design based on?
- What insulation goes under it, and how does the build-up meet the Building Regulations when the membrane is replaced?
The last point matters on a replacement. Under Approved Document L a new membrane on a flat roof is renovation, so Table 4.3 and its upgrade values come into the discussion, along with the question of how thick the insulation must be.
For a comparison with other flat coverings, see the lifespan by material guide, and for what we do on flat roofs, flat roofing. Photos of the existing roof, and of the wall where it meets the house, are the best way to start a conversation about which type suits the extension, the deck and the budget.
