A plain account of the three common single-ply materials, how a membrane is laid and fixed, what BS 6229:2025 says about falls, and why outlets and upstands are the places to check on any quote.
A single-ply membrane is a factory-made waterproof sheet, laid in one layer over the roof and joined at its edges to form a continuous covering. The name sets it against built-up roofing, where several layers of felt or asphalt are laid one on another. On a commercial building the single layer is rolled out in wide sheets, which means fewer joints than a felt roof over the same area.
Three materials do most of the work. PVC is a plasticised vinyl sheet, TPO is a thermoplastic polyolefin sheet, and EPDM is a synthetic rubber, short for ethylene propylene diene monomer. TPO is covered here as a single-ply membrane, a sheet that is welded, not a coating. Each has its own way of being joined, its own tolerances and its own price, and each is only as good as the detailing around the edges.
This guide describes the three in general terms for shops, offices, schools, warehouses and units in Broadstairs, Herne Bay, Deal and Faversham, and across East Kent. It follows the code of practice for falls, then looks at the junctions where leaks start. It sits within our commercial roofing work, and the price is one written total, set once the roof has been surveyed.
What a single-ply roof is made of
Below the membrane there is always a structure. On a commercial flat roof that is a deck of profiled steel, concrete or timber boards, a vapour control layer where the specification calls for one, and insulation on top. The membrane is laid over the insulation, or over a separating layer that keeps the sheet from touching incompatible materials.
A roof with insulation above the deck is called a warm roof, because the deck stays on the warm side of the insulation. A cold roof puts the insulation below the deck and leaves the void above it ventilated. Single-ply membranes suit warm roofs well, since the insulation board gives the sheet a firm, continuous surface.
The membrane can be fixed in three ways:
- mechanically fixed, with screws and plates through the sheet edges into the deck
- fully adhered, with the sheet bonded to the insulation over its whole area
- ballasted, held down by a layer of stone or paving loose-laid on top
The choice rests on the deck, how tall the building stands and what load the structure can take. Ballast is heavy, so a structural check comes first. Fixing numbers and spacing are worked out for the site's own exposure, not set from habit. Met Office averages for Manston, from 1991 to 2020, show a mean wind of 9.86 knots, with January at 11.21 knots, and a roof on an open industrial estate in East Kent should be treated with that in mind.
PVC, TPO and EPDM in general terms
The three materials differ in how they are made and how the seams are formed.
| Membrane | Material | Seams joined by |
|---|---|---|
| PVC | Plasticised vinyl sheet | Hot-air welding |
| TPO | Thermoplastic polyolefin sheet | Hot-air welding |
| EPDM | Synthetic rubber sheet | Seam tape or adhesive |
A welded seam fuses the two sheets into one piece of plastic, so the joint is the same material as the sheet. EPDM is a thermoset rubber and does not weld, so its laps are bonded with a tape or adhesive, and the quality of that bond depends on a clean, dry surface on the day. Large EPDM sheets are available, which cuts the number of seams on a big roof.
Whichever material is chosen, seams are only as good as the work on them. A welded seam needs a clean sheet, the right temperature and steady pressure, and every welded seam is probed along its length before the roof is handed over, so a gap is found while the crew is still on site.
For EPDM, two published guides give different lives, so both are named. HomeBuilding gives 25 to 30 years for EPDM, while Less gives 30 to 50 years. These are typical ranges, not promises, and a warranty is not a lifespan. No single figure fits every roof, because a roof that is walked on, ponds water or has poor detailing will age faster than a tidy one.
We compare PVC and TPO side by side in our guide to PVC and TPO membranes, and the rubber option has its own page on EPDM commercial roofs.
Falls under BS 6229:2025
A flat roof is not flat. It is laid to a fall, a gentle slope that takes water to an outlet or gutter. BS 6229:2025 is the current standard for flat roofs under flexible membranes, and the 2018 edition is withdrawn. It deals with membranes only; lead sheet follows BS 6915 by way of Approved Document C.
Two figures matter. The finished roof keeps a fall of at least 1:80 across general areas and in internal gutters, which is a floor for the built result and not a design aim. IKO, a membrane maker, advises designing a single-ply roof for 1:40.
In plain terms, 1:80 means a drop of 12.5 mm for every metre of run, and 1:40 means 25 mm for every metre. A roof that is dead level, or has a dip in it, holds a pond after each rain. Ponds collect dirt, freeze in winter and add weight to the deck.
A survey measures the existing fall with a level and marks ponding with chalk after rain. Where the deck is level, tapered insulation boards build the slope in, and where the deck already slopes the boards follow it. Either way the slope has to carry on unbroken to every outlet, so the insulation layout is drawn up before the first board is laid.
Detailing at outlets
An outlet is the point where water leaves the roof for a downpipe or a hopper. It is the lowest point of the roof, so every leak path leads there, and the membrane must be sealed into it with a purpose-made fitting. A cut-out with a smear of sealant is not the same thing.
Good outlet detailing includes:
- the outlet set at the lowest point of the finished fall, with the insulation tapered towards it
- a membrane flange or collar bonded or welded to the sheet, never just sealant
- a leaf guard, so that autumn debris does not block it
- an overflow or second outlet, where the roof would flood if one outlet failed
- a clear path from the outlet to the downpipe, with no kink or restriction
A blocked outlet is a simple fault to fix and a costly one to leave. A roof that cannot drain loads the deck with water, and a membrane that sits under standing water suffers at its seams.
Gutters on commercial buildings can be hidden, set behind a parapet or inside the roof edge, and a leak from one may show as a stain on the ceiling below. Brett Martin gives a gutter fall as 1mm for every 600mm, which is the figure to check on the visible rainwater run. Hidden gutters need a lining or a membrane carried up the sides of the box, which is why the survey opens the parapet to look.
Detailing at upstands, edges and penetrations
An upstand is the upturned edge of the membrane where the roof meets a wall, a parapet or a plant base. The membrane is carried up the wall and sealed beneath a flashing or a termination bar, which is a metal strip fixed to the wall to hold the sheet's top edge. Water shed down the wall must run over that edge, never behind it.
Checks on a quote or a finished roof:
- The upstand is tall enough to keep water out when the roof ponds briefly. The product data sheet gives the minimum.
- The top edge is covered by a flashing, a coping or a sealed termination bar.
- Corners are formed with factory-made corner pieces, not folded and glued sheet.
- Pipes and flues pass through boots made for the purpose and sealed to the sheet.
- Roof edges and drip trims are fixed along their length and sealed at the joints.
Penetrations are the part of a roof that gets disturbed later. An engineer who fits a new extract fan or an aerial can cut the membrane and not repair it. A roof plan that marks every penetration and every sacrificial walkway, with a note about who may open the covering, protects the work after we leave.
Lead flashings on a masonry parapet follow the lead sheet codes, and the code for each detail is set in the specification. Code 3 is for soakers only. Where a membrane meets a lead flashing, the junction is a detail the roofer draws up in advance, and it is the one to ask a bidder to show you.
Choosing and pricing a membrane roof
The choice between PVC, TPO and EPDM rests on a handful of building facts. The shape of the roof matters, since many penetrations and small upstands favour a material that can be hand-welded on site. The use of the roof matters, since a roof with regular foot traffic wants walkway sheets laid along the route. The programme matters too, since adhesive-bonded work wants dry weather and welded work wants a dry sheet.
Surveying comes before choosing. The survey records the existing covering, the deck, the insulation, the falls and the details, and it checks for wet insulation by core samples, because wet insulation left under a new membrane stays wet.
Price is set by the roof area, the access, the condition of the deck, how much insulation is replaced, the number of outlets and details, and whether the existing covering is stripped or overlaid. A rate per square metre is not quoted here, because commercial work is priced after a survey. The survey also looks at what the roof carries afterwards, such as plant, walkways and access ladders, because each one changes the design. Ask for the build-up in writing, covering deck, vapour layer, insulation thickness, membrane and fixing method, so that two quotes describe the same roof. Check that any quote covers scaffold or access equipment, VAT at the standard rate of 20% and the removal of waste.
A written price is one total, set after we have seen the roof. Our commercial roof survey page explains what that visit records.
