Flat Roof Materials Compared: GRP, EPDM, Felt and Liquid

GRP is glass reinforced polyester, laid wet onto a new plywood deck and cured into one continuous shell with no joints anywhere in the field of the roof. It is the system we specify most often on domestic extensions, garages and dormers.

Its strengths are hardness and seamlessness. You can walk on it, it resists impact and foot traffic well, and preformed GRP trims give crisp clean edges and drips that look sharp from the garden.

Its limitations are temperature and shape. It needs dry conditions and a reasonable temperature to cure properly, and because it is rigid it wants a stable, well fixed deck. On a large roof, expansion joints need designing in rather than ignoring.

EPDM is a synthetic rubber membrane, usually laid as a single sheet bonded to the deck. On a typical garage or extension the whole roof can be one piece with no seams at all.

It is extremely flexible, copes well with building movement, and it is unaffected by cold at installation, so it can be fitted in weather that would rule out GRP. Lifespan is comparable, twenty to thirty years with a manufacturer guarantee.

It is softer underfoot than GRP, so it is less suited to a roof that will be walked on regularly, and it is more vulnerable to sharp impact. Detailing around numerous penetrations is more fiddly than with a liquid system.

Modern high performance torch-on felt is a long way from the pour-and-roll felt that gave flat roofs their bad reputation. A three layer built-up system with a mineral cap sheet is a legitimate, well proven roof.

It is the cheapest of the four and it is quick, which makes it attractive on large simple areas and on budget jobs. Realistic life is ten to fifteen years, sometimes twenty on a sheltered roof.

It involves hot works, which is a genuine consideration near timber, on occupied buildings and on anything with a fire risk assessment. We will not use it where a cold applied system is the safer answer.

Cold applied liquid roofing, in PMMA or polyurethane, is painted or rolled on and cures into a seamless reinforced membrane. It wraps around anything: pipes, upstands, rooflight kerbs, plant bases, awkward parapets.

Its real strength is overlaying. On a commercial roof where the existing covering is tired but the deck is sound, a reinforced liquid overlay can add twenty plus years without a strip out, without disruption to the business below and with no hot works.

It costs more per square metre than felt and needs a clean, dry, properly prepared substrate. Applied to a damp or contaminated surface it will not bond, which is why preparation is most of the labour.

Falls. Every flat roof needs a fall of at least 1 in 80 designed into the deck, and in practice we build to 1 in 60 to allow for deflection. Ponding water shortens the life of every system on this list.

Upstands. Where the roof meets a wall, the membrane must turn up at least 150mm and be properly terminated. Most flat roof leaks happen at an upstand that is too low or badly finished, not on the flat.

Decks and insulation. A warm deck build-up with insulation above the joists is the current standard and avoids the condensation problems that plague old cold deck roofs. The deck itself must be sound; laying a new covering over soft chipboard is money wasted.

For a new extension or garage roof with a simple rectangle shape and a solid new deck: GRP, for the hardness and the clean trim details.

For a roof that moves, an awkward access situation, or work in cold weather: EPDM.

For a commercial refurbishment, a roof full of penetrations, or an overlay where stripping is impractical: liquid.

For a low budget on a large simple area where a fifteen year life is acceptable: torch-on felt, specified as a proper three layer system rather than a single patch layer.