GRP fibreglass
GRP stands for glass-reinforced polyester. It is laid wet: resin is rolled into a glass-fibre mat on a timber deck, building a laminate that cures into a single hard skin with no seams. A coloured topcoat finishes it.
Because it is liquid-laid, GRP follows any shape, which makes it good for dormers, bay roofs, valleys and roofs with lots of corners. Its hard surface is the best of the three for walking on, so it is often used for balconies. The weaknesses are that resin needs dry weather and suitable temperatures to cure, and a rigid laminate can crack if the deck beneath moves, so it needs a stable deck and well-fixed trims.
EPDM rubber
EPDM is a synthetic rubber membrane supplied in large sheets and bonded to the deck with adhesive. On a typical domestic roof it goes on as one piece, with no seams at all. It stays flexible in frost and tolerates small movements in a timber roof well.
EPDM is quick to lay on simple shapes and needs no naked flame. It is less suited to roofs with many penetrations or complex detailing, and it can be punctured by sharp objects, so roofs used regularly need protection or a harder system.
Torch-on felt
Modern torch-on felt is a reinforced bitumen membrane, usually laid in two or three layers and bonded with a gas torch. It is a long-established system, very different from the thin single-layer felt that gave flat roofs a bad name.
Felt is common on larger and commercial roofs and is well understood by experienced roofers. Its drawbacks are the joints between sheets, which must be well lapped and sealed, and the use of a naked flame, which needs care around timber and on occupied buildings.
What really decides how long a flat roof lasts
Whichever system is chosen, these matter as much as the material:
- A sound deck: boards suitable for the system, laid on joists that are not rotten
- A proper fall, so water runs off instead of ponding
- Warm roof construction, with insulation above the deck, so the timber stays dry
- Good edges: trims, upstands and flashings detailed properly
- Outlets and gutters big enough and kept clear
- Laying in the right weather for the system
How to choose
Think about the shape of the roof, whether people will walk on it, what will be fixed through it, and whether you are likely to look down on it from upstairs. A dormer with lots of upstands points to GRP; a rectangular garage points to EPDM; a large single-storey building often suits felt. Ask your roofer why they recommend the system they do, and make sure the quote includes a new deck and insulation where needed.
| GRP fibreglass | EPDM rubber | Torch-on felt | |
|---|---|---|---|
| How it is laid | Resin and glass mat, laid wet | Single rubber sheet, bonded | Layers of bitumen membrane, torched |
| Seams | None | None or very few | Lapped joints |
| Best for | Complex shapes, balconies | Simple rectangles, garages | Larger roofs |
| Foot traffic | Good | Needs protection | Fair with a mineral finish |
| Weather to lay | Dry and not too cold | Dry deck | Dry deck |
| Naked flame | No | No | Yes |
Need this done? See our flat roofing page or fibreglass roofing (GRP), or ask us for a quote.
Questions we get asked
Which flat roof lasts longest?
Lifespan depends more on the quality of installation, the deck and the falls than on the material. A well-laid GRP, EPDM or modern felt roof on a good deck will outlast a poorly laid roof of any type.
Which is cheapest?
It depends on the size and shape of the roof. On a simple garage, EPDM is often quick and economical; on a complex dormer, the labour for any system goes up. Compare quotes that include the same deck and insulation.
Can I walk on a rubber roof?
Occasionally, with care. For regular use, such as a balcony, choose GRP or add a protective walking surface.
Do flat roofs always leak eventually?
No. Most leaks come from poor falls, failed edges or a wet deck. Built properly as a warm roof with a good fall and well-detailed edges, a flat roof can perform well for many years.