Commercial Flat Roof Systems: Built-Up, EPDM, TPO, PVC

Almost every flat commercial roof falls into one of two families. The first builds the waterproofing on site in layers: built-up roofing, usually bitumen felt bonded together, where the top layer is often torch-on felt, a sheet that is melted onto the layer below with a gas flame. The second brings the waterproofing as one factory-made sheet, the single-ply family, which is rolled out, fixed, and joined at the seams.

Liquid coatings sit slightly apart. They are applied wet over a sound existing surface and cure into a continuous skin, so they suit refurbishment rather than a full strip-out. We come to them later.

Does the name on the roll decide how long it lasts?

Not on its own. The supply chain for a roof covering is only part of the outcome. Preparation of the deck, how the details are formed around pipes, upstands and rooflights, the quality of the seams and the amount of foot traffic from maintenance staff all affect how the covering ages. A roof with plant units on it, where engineers walk daily, wears differently from one nobody visits.

That is also why a survey comes before a price. We look at the deck, the falls, the outlets and the edges, and bring you photographs of what is actually on your roof, before a system is suggested. The covering follows from the condition of the roof, not the other way round.

For the single-ply options in more detail, our answer on single-ply membranes is a useful companion.

Built-up roofing has served flat roofs for a long time, and torch-on felt is its most familiar form. Two or three layers go down, bonded together, so a failure in one sheet is backed up by another. The top layer may carry mineral chips or a reflective finish to protect it from sunlight.

Less, in a flat roof cost guide updated on 26 October 2026, puts torch-on felt at a life of 10 to 20 years. It says careful work can reach 20 years, whereas a poor job may be gone before 10. That spread says a lot on its own. Felt lives or dies by the workmanship at laps, upstands and penetrations.

What are the drawbacks of felt on a commercial roof?

The flame used to bond torch-on felt is hot work on a building with a deck, insulation and perhaps stock beneath. It needs fire precautions, watching time after the last torch is out, and care around anything combustible. A cold-applied or mechanically fixed alternative avoids the flame, which is why it is sometimes specified instead. Built-up felt is also heavier than a single sheet, which matters if the structure was designed for something lighter.

When is felt still the right answer?

On a roof with many awkward details, such as numerous pipes, brackets and small upstands, where the layers can be dressed around each one. It also suits owners who want a repair that can be patched by any roofer who works with bitumen. Where the roof is simple and open, a single-ply membrane often goes down with fewer joints.

When we assess a felt roof, we look first at blisters, splits along the laps and open or lifted edges, because those say whether the roof can be repaired or needs stripping back to the deck.

EPDM is a synthetic rubber sheet, supplied in large rolls so there are few joins. TPO and PVC are plastic sheets whose seams are normally welded with hot air, which fuses neighbouring sheets into one. Each is fixed to the deck by adhesive, by ballast or by mechanical fasteners, and the choice of fixing depends on the deck and on how much wind uplift the roof sees.

Published lifespans for EPDM disagree, and that is worth saying plainly. GreenSpec lists an expected service life of 20 to 30 years. Less now says 30 to 50 years. Treat both as typical ranges, never as a promise, and remember that a warranty period and a lifespan are not the same measure.

For TPO and PVC the price turns on the product and the job rather than on one typical figure. What moves the answer instead is the thickness of the sheet, the quality of the welded seams, the finishing of the edges and upstands and how much the roof is walked on.

Which single-ply suits a roof with plant on it?

That depends on the layout. Rubber is flexible and forgiving around odd shapes, while welded plastics give a seam you can inspect and test. Walkway pads or a protected route between plant and the roof hatch cut wear whichever sheet you choose.

No flat roof is level in practice. Each is laid to a slope called a fall, which carries water to the outlets. BS 6229 deals with felt and single-ply laid on a solid deck. The 2018 edition was withdrawn on 16 December 2025, and BS 6229:2025 is the current one, written for roofs sloping at 10 degrees or less. It does not cover lead.

What fall does the standard expect?

BS 6229:2025 keeps a minimum finished fall of 1:80 across general areas and within internal gutters. To be sure of reaching that once the deck, insulation and covering are in, roofs are designed for a fall of 1:40. A roof designed at 1:80 on paper can end up with ponding in places, because every deck sags a little and every board has a tolerance.

Ponding is standing water that remains long after rain has stopped. It is not always a sign of failure, but it adds weight, collects silt and dead leaves and speeds the ageing of seams and edges. A new covering over an existing deck that already holds water will usually hold it afterwards too. Tapered insulation is a common way of building falls in when the deck itself is level.

Do falls change the choice of system?

Yes, in two ways. Where falls can be improved, almost any system works. Where they cannot, because of fixed outlets or parapet heights, the system with the fewest joints and the best seam inspection usually wins. Our answer on the flat roof minimum fall goes through the maths.

A liquid coating is applied wet by roller or spray and cures into a continuous layer. It has no laps, which suits a roof crowded with rooflights, vents and brackets, and it does not need a flame. The Less guide gives GRP, a glass-reinforced plastic laid as a resin over matting, 25 to 40 years.

When does a coating beat a new covering?

When the existing roof is sound but weathered. The condition is that the surface is dry, clean and firmly attached, with no wet insulation beneath. Coating a roof that already holds water is only a way to seal the problem in. A survey, sometimes backed by moisture checks, decides this before any material is ordered.

Coatings also suit profiled metal roofs, where the sheets are structurally fine but the surface is rusting or the joints weep. Our answer on liquid roof coatings for commercial roofs sets out what goes wrong when the old surface is not prepared properly.

What are the limits?

A coating does not cure a sagging deck, create falls or replace wet insulation. It also needs dry weather to apply and to cure, so timing matters more than it does with a sheet. Colour and reflectivity can be useful on large areas, but they are secondary to the condition of what lies underneath.

If the roof dates from a time when asbestos cement sheeting was used, the sheets must not be sanded, scraped or pressure-washed in preparation. Check first, because a survey comes before the preparation.

Put the questions in order. First, what is the roof like today: sound but tired, or wet and failing? Second, can the falls be improved? Third, how much will the roof be walked on by engineers, cleaners or installers of solar panels and plant? Fourth, can the business keep trading during the work, and does the structure limit weight? Only after those do the system names matter.

Does the answer differ for a shop, a workshop and an office block?

A little. A shop with a small roof and one outlet suits a simple, well-detailed single-ply or felt system. A workshop with many penetrations and ventilation may suit felt or a coating. An office block with a plant deck leans toward a membrane with walkways and tested seams. These are starting points, never rules.

What to ask a roofer

Ask what the system is fixed to, what fall the roof will have when finished, how edges and upstands are formed and what happens at outlets. Ask what lies beneath the old covering and how wet insulation will be dealt with. Ask for the plan for hot work, if any is involved, and for the method statement and the access arrangements, including whether a scaffold or cherry picker is needed.

If the building was built before 2000 and the deck or old sheets are unknown, the first step is an asbestos check. For a roof you want assessed, ask for a survey. Photos of your own roof come before any system is suggested, along with a note of what lies beneath the old covering.