If you're replacing the flat roof on an extension, garage, dormer or porch, you'll quickly hear three names: GRP fibreglass, EPDM rubber and torch-on felt. All three can give you a watertight roof, but they differ in lifespan, appearance, cost and how well they suit your building. Here's a straightforward comparison to help you choose.

The Quick Answer

For most homes, GRP and EPDM are the best modern choices. GRP gives a hard, seamless, walkable finish that looks neat and suits roofs with lots of detail. EPDM is a single sheet of rubber that's fast to install and very forgiving of temperature changes. Torch-on felt is a proven, lower-cost system that still has its place, especially on larger roofs, but it needs more maintenance over its life.

GRP Fibreglass

GRP (glass reinforced plastic) is built up on site from resin and fibreglass matting laid over a timber deck, then finished with a coloured topcoat. Once cured, it forms one continuous, seamless surface with moulded trims at the edges.

  • Pros: No seams or joints for water to get through. Hard-wearing and suitable for light foot traffic. Neat, clean finish in a choice of colours. Handles complicated shapes, upstands and roof lights well. Typically lasts 25 years or more.
  • Cons: Must be laid in dry weather and within a temperature range so the resin cures properly. Needs an experienced installer, because a poor laminate can crack. The deck underneath has to be sound and stable.
  • Best for: Extensions, dormers, balconies and roofs you'll see from upstairs windows.

We recently replaced a blistering felt roof on a Thames Ditton extension with an insulated GRP system. You can see that job here.

EPDM Rubber

EPDM (ethylene propylene diene monomer) is a synthetic rubber membrane, usually laid as a single sheet and bonded to the deck, with the edges finished in trims or upstands.

  • Pros: Very flexible, so it copes with expansion and contraction across hot summers and frosty winters. Often laid in one piece on smaller roofs, so there are few or no joints. Quick to install and can be done in cooler weather than GRP. Long lifespan, often 25 to 40 years.
  • Cons: The black finish isn't to everyone's taste. Can be punctured by sharp objects or careless foot traffic. Joints on large roofs need careful sealing.
  • Best for: Garages, extensions with simple shapes and roofs that aren't walked on.

Torch-On Felt

Modern torch-on felt is very different from the old single-layer felt that gave flat roofs a bad name. It's a multi-layer system of reinforced bitumen membranes, bonded with a gas torch, with a mineral-finished cap sheet on top.

  • Pros: Proven and widely used. Usually the lowest cost upfront. Good on large commercial-style roofs. Repairs are straightforward.
  • Cons: Shorter lifespan, typically 15 to 25 years for a good multi-layer system. Can blister and crack with age and UV exposure. Uses a naked flame, which needs care near timber.
  • Best for: Larger roofs and tighter budgets, or where matching an existing felt roof.

Side-by-Side Comparison

  • Typical lifespan: GRP 25+ years · EPDM 25 to 40 years · torch-on felt 15 to 25 years
  • Seams: GRP none · EPDM few or none · felt overlapping layers
  • Foot traffic: GRP good · EPDM light only · felt moderate
  • Weather needed to install: GRP dry and mild · EPDM dry · felt dry
  • Look: GRP smooth, choice of colours · EPDM matte black · felt mineral finish

Don't Forget What's Underneath

Whichever covering you choose, the roof is only as good as the deck, insulation and drainage beneath it. A new flat roof is the ideal time to replace rotten boards, add insulation to bring the roof up to current building regulations, and make sure there's a proper fall so water runs off instead of ponding. Getting the falls right matters more than the choice of membrane.

Need a Flat Roof Replaced?

We install GRP, EPDM and torch-on felt roofs across West Molesey, Elmbridge and Surrey, and we'll recommend the system that genuinely suits your building. See our flat roofing service, or read our comparison of flat vs pitched roofs. Call 07470 744247 for a free, written quote with no deposit.