For most domestic flat roofs in the UK — typically over extensions, outriggers, dormers and garages — the three coverings worth comparing are EPDM rubber membrane, GRP fibreglass and built-up felt. EPDM is a single-sheet synthetic rubber that suits larger, simpler areas; GRP is a glass-reinforced plastic laminate that produces a seamless, hard-wearing finish; and felt is the traditional layered bitumen system, now usually torch-applied. Each can give 20 years or more when laid correctly, so the right choice depends less on headline lifespan and more on the shape of the roof, the detailing around it, and how the installation is carried out.
What counts as a flat roof?
A flat roof is rarely truly flat. In practice it has a deliberate slope — the "fall" — built into the structure so that rainwater runs towards an outlet rather than pooling. A roof is generally treated as flat when its pitch is below about 10 degrees; anything steeper is a low or shallow pitch and may use different coverings.
The fall matters because standing water is the main enemy of any flat-roof covering. Building Regulations guidance in England and Wales points towards a minimum finished fall of around 1 in 40 to ensure water clears reliably, allowing for construction tolerances. Where ponding (standing water that lingers after rain) appears, it usually signals a fall that is too shallow, a sagging deck, or a blocked outlet rather than a fault with the covering itself.
Drainage is the other half of the equation. Water collected by the fall has to leave the roof through a gutter, an internal outlet, or a chute through the parapet. The covering must be dressed correctly into these points and up against any abutting walls — the upstands and flashings are where most flat roofs eventually fail, regardless of the system chosen. A well-detailed felt roof will outlast a poorly detailed rubber one.
EPDM, GRP and felt at a glance
In practice it has a deliberate slope — the "fall" — built into the structure so that rainwater runs towards an outlet rather than pooling.
Each system reaches the roof in a different form and is fixed in a different way. That difference drives where each one performs best.
- EPDM rubber membrane — a synthetic rubber sheet, often supplied in a single piece cut to the roof size. On smaller roofs it can be laid without any field joints, which removes the most common leak points. It is flexible, tolerates building movement, and is unaffected by frost or UV. The trade-off is the detailing: corners, upstands and outlets rely on adhesives, tapes and pre-formed components, and the quality of those bonded details determines longevity. A realistic working life is commonly quoted in the region of 20 to 30 years or more.
- GRP fibreglass — layers of glass-fibre matting saturated with resin and finished with a coloured topcoat, cured in place to form one continuous, rigid skin. There are no joints anywhere, and the surface is hard enough to take occasional foot traffic. It must be installed in dry conditions within a sensible temperature range, and the timber deck beneath needs to be rigid, because the cured laminate does not flex with movement and can crack if the substrate shifts. Properly laid, it is durable and gives a clean, board-like finish. Expected life is broadly similar to EPDM, often 20 to 30 years.
- Built-up felt — two or three layers of bitumen-based sheet bonded together, the modern versions reinforced with polyester and usually torch-applied with a mineral-finished cap sheet. It is the long-established system and remains widely used. A two-layer system on a small roof has a shorter typical life than the high-performance three-layer build, which can approach the lifespan of the other two. Felt copes well with awkward shapes because it is worked in pieces, though every overlap is a joint that depends on the heat and skill applied to it.
On cost, felt has traditionally been the lower-priced option for straightforward roofs, with EPDM and GRP often sitting higher, but the figures vary so much by roof size, access and region that any general ranking is unreliable. Labour skill weighs as heavily as material price in the long run.
Which system suits which job
The decision usually comes down to roof size and shape, how complex the upstands and penetrations are, and whether the surface will be walked on. None of the three is universally best.
For a large, simple rectangle — a rear extension with few pipes coming through and clean perimeter detailing — EPDM is a natural fit because a single sheet can cover the whole field without joints. The bigger the unbroken area, the more its main advantage counts. On very small or heavily interrupted roofs, the value of that single sheet falls away because so much of the work becomes detailing.
Where the roof will see regular foot traffic, supports a balcony, or wants a hard, smooth finish that looks deliberate from above, GRP fibreglass is often preferred. Its rigidity is an asset for walking on but a liability over a deck that might move, so it suits new, well-built decks more than older structures with uncertain timbers. It is also worth confirming that whoever installs it can guarantee dry working conditions, since GRP cannot be applied successfully in the wet.
For complicated shapes — roofs with several upstands, chimneys, rooflights or changes in level — built-up felt remains a sound choice because it is worked piece by piece and dressed neatly into difficult junctions. High-performance torch-on felt narrows the historical lifespan gap considerably. Felt is also a sensible like-for-like replacement where an existing felt roof has reached the end of its life and the deck and falls are otherwise fine.
Whichever covering is fitted, the same checks apply. You should ask how the fall is being achieved, where the water is intended to drain, how the upstands and any penetrations will be detailed, and what insulation arrangement is being used — a "warm" deck with insulation above the structure behaves very differently from a "cold" deck and affects condensation risk. The covering is only as good as the deck, the fall and the detailing beneath and around it, so those questions reveal more about the likely outcome than the brand of membrane or resin on top.