Almost every rear extension and garage roof in Hammersmith ends up flat, in one material or another. So the felt-versus-GRP-versus-EPDM question comes up on nearly every extension quote we write. Each material has a genuine place. The right choice depends on the roof’s shape, how much foot traffic it sees, and what you want to avoid repeating in ten years.
Traditional felt: cheap to install, shortest lifespan
Built-up felt roofing is the option most people already know, because it’s been the default on extensions and garages for decades. It’s the cheapest of the three to install, and a competent roofer can lay it quickly. But felt is also the least durable option. Seams and joints are the weak point. UV exposure gradually makes older felt brittle, and that’s when splits and blisters start letting water in. Most felt roofs need attention well before the newer membrane options do. It wins on upfront cost, but it tends to cost more over a roof’s full lifetime.
GRP fibreglass: seamless and hard-wearing
GRP (glass-reinforced plastic, sometimes called fibreglass) roofing is laid as a liquid resin over fibreglass matting. It sets into a single, fully seamless surface. That’s the main advantage: there are no joints for water to find a way through. The finished surface is also hard enough to walk on. That matters if the roof doubles as access to a Juliet balcony or a terrace. GRP suits roofs with a lot of detail, like upstands, rooflights and odd angles. The liquid goes on wet and moulds around them, instead of needing cut joints like a sheet material would.
EPDM rubber: flexible and low-maintenance
EPDM is a synthetic rubber sheet, usually laid in one piece with very few joints. It’s become the most common choice on newer extensions, for good reason. It stays flexible across a wide range of temperatures. As a result, it copes well with a building’s natural movement without cracking, which is a real weak point of older felt roofs. EPDM generally needs very little maintenance once it’s installed correctly. A properly detailed EPDM roof is one of the lowest-maintenance options for a flat roof extension.
Which one should you choose?
Either GRP or EPDM will comfortably outperform felt on a straightforward extension or garage roof. The choice between the two often comes down to budget and whether the roof needs to take foot traffic. Want a hard, walkable surface for a roof terrace? GRP’s rigid finish is usually the better fit. Is the roof purely functional? EPDM is typically the simpler, more flexible install, with the fewest joints.
What this looks like on a Hammersmith property
Hammersmith has a lot of Victorian and Edwardian terraces with single-storey rear extensions. These flat roofs are almost always visible from the upper floors of the house, so appearance and longevity matter more than on a garage roof nobody sees. We generally steer these towards GRP or EPDM rather than felt. A flat roof outside a bedroom window that needs replacing again in eight years is a false economy. The same logic applies in Hammersmith, Putney, or Wandsworth — the housing stock and roof shapes are similar right across the borough.
Warm roof insulation matters as much as the covering
Whichever covering you choose, the insulation underneath makes a real difference to how the roof performs. A modern “warm roof” specification puts insulation above the structural deck. It avoids the condensation problems that used to plague older “cold roof” flat roofs, where insulation sat between the joists instead. Ask your roofer to confirm which build-up they’re specifying. The covering gets all the attention, but the insulation layer underneath is doing just as much work.
Planning a flat roof for an extension or garage in Hammersmith or the wider West London area? Get in touch for a free quote and an honest recommendation on which material suits your roof.