EPDM or GRP: which flat roof should you choose?

1Cost and expected life side by side

These are typical national ranges from published guides. They describe the market, and a contractor on the Sussex coast may sit above or below them. The cost gap is narrow and the ranges overlap, so price alone rarely settles the question.

The life figures come from GreenSpec and the Less guide (March 2026). Less gives EPDM a higher figure of 30 to 50 years, so the sources disagree at the top. Both materials depend on workmanship, exposure and upkeep, and a lifespan is never a guarantee. A sound deck with careful details matters more than the choice of material, and a poor deck will shorten the life of either. A manufacturer's warranty is a separate thing again, and should not be read as a lifespan.

2How each is laid and what the weather does

EPDM comes as a large rubber sheet, rolled out over the deck, glued down or fixed along the edges, with trims and upstand details finishing the job. Because one sheet can cover most of a roof, there are few joints in the open field.

GRP is built in place. Glass matting is laid on the deck, saturated with resin, and finished with a topcoat. The resin cures by chemical reaction, which is why GRP is the more sensitive to the weather during installation. Rain, heavy damp or cold at the wrong moment can spoil a layer that cannot then be put right by waiting.

EPDM needs a dry deck too, because the adhesive will not grip a wet one, and strong wind makes handling a large sheet awkward. On the Sussex coast, where weather can change within a morning, good timing and a dry spell matter for either material. We plan the job around the forecast rather than around a fixed date, and we would rather move a start than lay over damp boards.

3Movement, cracks and repairs

EPDM is flexible. It moves with the roof as temperatures change, and most damage comes from punctures, a dropped tool, a sharp edge or a heavy foot, or from the glued details at upstands, which can lift. A small puncture is repaired by cleaning the area and fitting a patch, which is often a quick job if the membrane is otherwise sound.

GRP is a rigid shell, so it relies on a firm, stable deck. If the boards flex, hairline cracks can open, usually at joints or around the edge, and those let water in. Patching GRP means abrading the area and laying new resin and matting, and a repair can show as a different shade until the whole surface is retopped. A crack that keeps returning in the same place usually means the deck is moving, and the cure is to deal with the deck.

On repair prices, Checkatrade gives £120 to £170 per m² for EPDM and £130 to £180 for GRP, both excluding VAT. Our page on flat roof blisters explains a fault that can affect either covering when moisture is trapped beneath it.

4Appearance, edges and foot traffic

EPDM is black and matt as standard, finished with trims at the edges. Some owners like its plain look. It tolerates occasional foot traffic, but a roof walked on regularly usually needs walkway tiles or boards to protect it, and gutters or outlets need to be reached for cleaning without stepping on the edges.

GRP has a smooth, hard surface that can be coloured through its topcoat, and it creates a tidy, continuous edge with its own trims. It is a natural choice for a small balcony or a roof used for access. It is not immune to wear, though, and grit carried across it by shoes will scuff the topcoat over time.

Neither covering corrects a roof that does not drain. Both need a properly built fall, and our page on what fall a flat roof needs explains why a finished 1:80 is the aim. Choose the covering after the deck is dealt with, not before, since the best sheet or resin cannot make up for a hollow beneath.

5Choosing for your roof

As a rule of thumb, a large roof with a simple shape tends to suit EPDM, because one sheet covers it with few joints. A small roof with many corners, steps or pipes often suits GRP, because it is moulded around each detail. These are tendencies, not rules, and local conditions such as shade from trees, access for repairs and whether people will use the roof as a terrace can all shift the balance.

Look at what is already there before choosing. If the existing roof is sound apart from one failed area, a repair may be the right answer, and we will tell you that plainly. If the covering has simply run its course, our page on repairing or replacing a flat roof helps with the decision.

At the survey we photograph the deck, the edges and the details, and the quote that follows is in writing, naming the covering we recommend and the reasons for it. The full range of flat roofing work is on its own page, and the final choice stays with you.