Home / Guides / Flat roof options compared
GRP, EPDM or felt: which flat roof and why
Every flat roofer has a favourite, and it is usually the one they were trained on. Here is what separates them on the things that decide whether a roof lasts.
Fibreglass GRP
Laid wet and cured in place, so the finished roof is one seamless skin with crisp moulded edge trims. It looks the neatest of the three and it is genuinely tough underfoot.
It is also rigid. Over a deck that moves, or on a dormer that flexes, GRP finds the movement and cracks, usually at an upstand. It needs a dry spell and a properly prepared board to go down on, which in a British autumn is a scheduling problem as much as a technical one.
EPDM rubber
A single sheet, often with no field joints at all on a domestic roof, bonded down and dressed into the detailing. It stays flexible in cold weather and tolerates movement better than anything else here, which is why it is the sensible default over timber that will shift.
It will go down in weather that would stop a GRP job. It is less resistant to sharp impact, so if the roof is a route to a flue or a plant deck, say so and it gets a walkway.
High-performance torch-on felt
People dismiss felt because they remember what failed on their parents' garage. A modern two or three layer system with a mineral cap sheet is a different product: tough, proven, and the easiest of the three to repair neatly in ten years' time.
On a big roof with lots of penetrations, upstands and awkward junctions, it is frequently the best answer, because the detailing is worked in layers rather than fought with a rigid material.
What matters more than the choice
Falls. A consistent fall to an outlet, achieved with firrings if the deck does not have one, does more for the life of a flat roof than any material decision. Standing water will eventually beat all three.
Upstand heights and terminations. Water gets in at edges, thresholds and pipes far more often than in the middle. A quote that mentions neither falls nor upstands has not looked at your roof.
Ventilation. A cold deck needs a clear path, or the ceiling below gets wet from condensation and everybody blames the covering that is working perfectly.
So which one
Small, square, sound deck, dry week: GRP or EPDM. Anything that moves, or work in winter: EPDM. Large, complicated, many details, or a roof you want to be able to repair cheaply later: felt.
If somebody is quoting one system for every roof they ever see, that is a supplier relationship talking, not your building.
Questions
Can a flat roof be insulated at the same time?
Yes, and on an extension it is usually the moment to do it. It changes the build-up, so it has to be in the quote rather than added later.
How long do they last?
Properly laid on correct falls, decades. Badly laid on flat falls, a few years, whichever product is on top.