The short answer
A single ply membrane such as reinforced PVC is a perfectly good covering for a house extension, porch or dormer. It arrives as a factory-made sheet joined along its overlaps by hot-air welding, so no naked flame is used on site. MyBuilder's flat roof guide, updated in August 2026, puts PVC single-ply at £95 to £140 per m² for a full replacement, and only a survey of your roof can place it within that range.
What a single ply membrane actually is
A single ply membrane is one flexible waterproof sheet, in contrast to the stacked layers of bitumen felt that older flat roofs relied on. On a house the usual choice is a reinforced PVC sheet with a polyester scrim buried inside it for strength. The rolls are cut to suit the roof, laid over an insulation board and a sound deck, and each overlap is melted together by a welder blowing hot air. A properly made seam becomes part of the sheet itself rather than a glued joint waiting to peel.
Rubber sheet such as EPDM is also a single ply product, but it is normally stuck down with adhesive and jointed with tape. The two behave differently at corners and upstands, which is why we compare them on our flat roof materials page.
The sheet is thin, light and fairly flexible, so it follows a gentle curve or wraps around a small dormer cheek without the fuss that rigid coverings bring. Colours are limited to a handful of greys, and the finish is plain rather than pretty, which most people prefer on a roof they seldom look at.
Where it suits a house and where it does not
Single ply earns its place on roofs with a lot of detail: a rear kitchen extension with a parapet on two sides, a porch with a rooflight, or a bay window roof that is a polygon rather than a rectangle. Factory-made corners and welded upstands deal with those shapes more tidily than a patchwork of felt strips.
It is less happy where people will be walking about. Stiletto heels, dropped tools and furniture feet can scuff or puncture the sheet, so a roof used as a terrace needs a different build-up, which we describe in our page on walking on a flat roof.
Small dormer roofs are another good fit, because the membrane can run up the cheeks in one continuous piece with very few joints to fail. If you are weighing it against fibreglass for a dormer, our dormer flat roof page and the notes on GRP fibreglass roofing set out the other side of the argument. Neither wins every time, because the geometry of the roof usually tips the balance, and a quick sketch of the plan with the main dimensions helps us advise you before any visit.
What goes under the sheet matters more than the sheet
Most single ply failures start below the membrane. The deck has to be dry, flat and firmly fixed, because a thin sheet will show every ridge and hollow in the boards. Water that sits in a dip will test the seams for years, so the fall matters. BS 6229 recommends designing for 1:40, so that once tolerances and deflection are taken off, a finished minimum of 1:80 is still achieved. Our page on ponding on flat roofs explains what happens when the falls are missing.
Insulation is the next layer. Part L counts a fresh waterproof layer as renovation of the roof's thermal element, and insulation is usually improved during that same visit. That can raise the roof edge slightly, which is worth planning before door thresholds and upstands are set. Our notes on upgrading flat roof insulation go through the options and the thicknesses involved.
Typical cost and lifespan
MyBuilder's flat roof cost guide, updated on 21 August 2026, lists PVC single-ply at £95 to £140 per m², against £70 to £100 for bitumen felt, £90 to £130 for EPDM and £95 to £150 for GRP. Each range describes a full replacement across the UK, and it says little about the cost of any one house. A small extension is dominated by fixed items such as access, edge trims and stripping the old covering, so the per-metre figure matters less than the total on the written quote.
On lifespan, MyBuilder says a flat roof typically lasts 15 to 30 years in total, and where yours lands in that range depends on the quality of the deck, the insulation and the details far more than on the brand of sheet. Cover offered by the sheet maker is a separate matter from lifespan, and neither equals the workmanship guarantee from the firm laying the roof. Ask what each document actually covers, who gives it and for how long.
Seams are the usual place for trouble to start, so look along them once a year for lifting edges or dirt-filled channels. Clear leaves from outlets, keep branches off the surface and let a roofer check the upstands every few years. Small splits in a PVC sheet can often be patched by welding on a new piece, which is one practical advantage over felt.
What we check before recommending it
We look at the deck first, because a spongy or stained board changes the whole job, and our guide to a spongy flat roof explains why. Then we check the falls, the point where water leaves, the height of the upstands at walls and doors, and how the edge will look once a trim is fitted.
If single ply is the right answer for the shape of your roof, the photographs from the survey will show why and the written quote will say so. If a different covering suits the building better, we tell you that instead. Our advice leans towards modest work when it will do, and our workmanship guarantee on finished work lasts ten years and comes in writing. Booking a visit takes one call to 01793 626096, a note on WhatsApp or the enquiry form. Everything else we do on this kind of roof is set out under flat roofing.