A single-ply roof is called that because the waterproofing is a single layer of manufactured sheet, rather than the stacked plies of a built-up felt roof. The sheet is made in a factory to a controlled thickness, rolled up, delivered to site and laid over an insulated deck. The joins between rolls are the weak point of any flat roof, so how they are made matters more than the name on the roll.
Three materials dominate commercial work: PVC, TPO and EPDM. This guide explains what each is, the three ways of fixing a membrane, how seams and details are formed, what published guides say about life and cost, and what a survey checks on an existing membrane. It deals with the system rather than with comparing every roof type, which is the job of our guide to commercial roof systems. The wider service is at commercial roofing.
What PVC, TPO and EPDM actually are
The three names describe three different polymers, and they behave differently on a roof.
PVC
Polyvinyl chloride sheet is reinforced with a polyester or glass scrim, which gives it dimensional stability and tear strength. It is a thermoplastic: heat softens the surface, and two sheets can be fused together with hot air. PVC is widely used on large commercial and industrial roofs.
TPO
Thermoplastic polyolefin is also a thermoplastic that welds with hot air, so the fixing and seaming methods resemble PVC. The Building Centre describes TPO life in excess of 35 years, a figure from a trade body article rather than a guarantee. Formulations differ between makers, which is a reason to read the manufacturer's own data sheet for the product in question.
EPDM
In EPDM the letters abbreviate a long chemical name; the result is a synthetic rubber. It is not a thermoplastic, so it is not welded: joints are glued or sealed with taped splices. Its great virtue is that it can be made in very large sheets, which means few joints on a big roof. Rubber stays elastic over a wide temperature range, though it can be marked by sharp objects and some chemicals.
| Material | Type | How joints are made |
|---|---|---|
| PVC | Thermoplastic, reinforced | Hot-air welded |
| TPO | Thermoplastic | Hot-air welded |
| EPDM | Synthetic rubber | Adhesive or taped splice |
Houses and extensions follow different rules and scales, and are covered under flat roofing. This page is about commercial buildings.
Three ways of holding a membrane down
A membrane is no use if the wind can lift it. Single-ply is held in one of three ways, and the right one turns on the deck, the building height, the exposure of the site and what the roof carries.
Mechanically fixed
Fasteners and plates, or fixings in the seam allowance, hold the sheet through the insulation into the deck. This is quick, works over most decks and is common on large steel-deck industrial roofs. The fixings are counted and spaced to resist the wind loading calculated for the roof, with extra density at the corners and edges, where uplift is greatest.
Fully adhered
The membrane is stuck over its whole area to the insulation or the deck with contact adhesive. This suits smaller roofs, curved roofs and roofs where fixings through the deck would be a problem. It depends on a clean, dry, compatible surface, and weather during laying matters a good deal, because adhesive and damp do not mix.
Ballasted
The membrane lies loose on the insulation and is held down by weight, usually washed stone or paving slabs. It needs a structure that can carry the extra load, and any change in roof weight is a question for the Building Regulations. The Planning Portal sets the line at an increase of 15 per cent or more in covering weight. Ballast also hides the membrane, which makes inspection harder, so access for checking is planned from the start.
Sometimes more than one method is used on the same roof: adhered at the perimeter and upstands, mechanically fixed across the main area. Each method has to be written into the specification, with the fixing pattern and the wind uplift it was designed for, so the person laying the roof and the person checking it are working to the same document. When we survey an existing membrane, we begin by establishing which method was used, because that decides how a repair is made and whether the sheet can be lifted without damaging the insulation below.
Seams, laps and details: where a single-ply roof is won or lost
A membrane covers a flat area easily. The care goes into the seams and into the details: upstands, corners, pipes, outlets, rooflight kerbs and edges.
Welded seams
On PVC and TPO, a hot-air welder fuses the overlap of two sheets. A good weld is probed along its edge to check it has fused fully, and the weld width and temperature are matched to the material and the weather. Welding is skilled work, and the quality of the person with the welder is what the roof finally depends on.
Glued and taped seams
EPDM joins are made with adhesive and splice tape or a seam tape, pressed down with a roller. Weather and cleanliness again matter. The mating faces must be clean and primed, and a seam made over dust, oil or damp is a leak waiting to happen. Roller pressure along the full width of the splice is part of the job, not an afterthought.
Details and upstands
At a wall junction, the membrane turns up as an upstand and is finished under a flashing or a metal coping. Pre-formed corners, pipe collars and outlet boots are used where the sheet would otherwise have to be cut and folded. Edge trims are formed from coated metal, and the same cut-edge issues that affect metal cladding apply to them if the coating is damaged, a fault covered in our guide to cut-edge corrosion on metal roofs.
If we are reroofing an existing building, we photograph each detail before stripping and write them into the quote, so the same junctions are formed again properly.
How long published sources say single-ply lasts
Lifespan figures for single-ply vary a good deal, and published guides disagree. The table collects what we found, with the source and its date.
| Material | Published figure | Source |
|---|---|---|
| EPDM | 20 to 30 years | GreenSpec |
| EPDM | In excess of 25 years | Building Centre, October 2017 |
| EPDM | 30 to 50 years | Less, updated March 2026 |
| PVC | 20 to 30 years | GreenSpec |
| PVC | 20 to 40 years | Building Centre, October 2017 |
| TPO | In excess of 35 years | Building Centre, October 2017 |
Less also reports that EPDM manufacturer warranties usually run 20 to 25 years. A warranty is about the product's manufacture, usually with conditions. It is not a forecast of service life. Neither should be taken as a promise about your roof.
Why the spread is so wide
Part of it is the product: thicknesses and formulations differ between makers and between ranges. Part is the building: a roof with heavy foot traffic, ponding or a lot of plant gets harder use. Part is workmanship at the seams and the details. And part is maintenance: clearing outlets, checking seams and replacing damaged trims all extend a roof's service. For the longevity comparison between single-ply, felt and metal, see our answer on commercial flat roof lifespan.
Falls, ponding and the deck underneath
A single-ply membrane is waterproof, but it is not a cure for water that stands on it. IKO explains that BS 6229 recommends a minimum finished fall of 1:80 on a flat roof, and that the code advises designing for 1:40 to allow for tolerances and deflection. Rockwool, citing BS 6229:2018, makes the same point. On a new roof, the falls are built into the structure or formed with tapered insulation. On a refurbishment, they depend on what the deck allows.
What ponding does to a membrane
Standing water is not always a fault in itself, but it adds weight, it holds dirt and algae against the surface, and it concentrates stress on the seams. On a thermoplastic membrane it can lead to staining and surface growth, and on any membrane it hides small defects until they open. On a roof that already has a sagging deck, it can get worse each season as the extra water loads the structure further.
Insulation and deck
The sheet sits on insulation, which sits on a deck of metal, concrete or timber. A deck that is rusted, rotten or damp cannot hold fixings properly, so a membrane laid over it fails early. A vapour control layer under the insulation protects the deck from moisture rising from inside the building. We check the underside of the deck where we can reach it, and we measure and photograph any ponding before pricing.
If ponding is the problem you are chasing, our answer on commercial roof ponding goes into causes. Here, the point is that the membrane and the falls are one system.
Cost, repairs and refurbishing with single-ply
MyBuilder's flat roof guide, updated in August 2026, gives national replacement ranges of £90 to £130 per square metre for EPDM and £95 to £140 for PVC single-ply. Checkatrade's repair guide, with a June 2026 body date, gives £120 to £170 per square metre for EPDM repair, excluding VAT and access. Those are published ranges for roofs on houses and small buildings. A large commercial roof is priced on its own survey, and the drivers behind that price sit in our guide to commercial roof costs.
Repairing a membrane
Small punctures and failed seams on a thermoplastic can be repaired by welding in a patch of compatible material. On EPDM, a patch is glued or taped over a clean area. Repairs need the right material for the right membrane, which is why mixing products from different makers on one roof causes problems.
Overlaying with single-ply
A new membrane is sometimes laid over an existing roof covering, which is described in our guide to commercial roof overlays. The old roof has to be dry and sound for this to work, and the existing layers add weight.
Workmanship and what we put in writing
After the survey, the quote covers access, strip, insulation, membrane, details, drainage, waste and making good. Expect photographs taken on your own roof, a cleared site afterwards, and a 10-year workmanship guarantee on paper. The first step is a message with the address, the current covering and a few photographs.