Flat roof systems compared and how long each lasts
Most flat roof failures are not the membrane giving up. They are laps, upstands and outlets that were never detailed properly in the first place.
Felt is cheapest and lasts 10 to 20 years. EPDM rubber is seamless on most domestic roofs and lasts 20 to 25. GRP fibreglass is the toughest underfoot at 20 to 30 years but costs more and dislikes a flexing deck. The deck and the detailing decide the outcome more than the brand.
Torch on felt
Cheapest up front, quick to lay and easy to patch. The weakness is the number of lapped seams, which is where leaks begin. Sensible on a garage or a shed, less so on a habitable extension you want to forget about for twenty years.
EPDM rubber
A single continuous sheet covers most domestic flat roofs with no field joints at all. That is why it is our default on extensions, dormers and garages. It handles movement well and is quick to install, usually in a day on a typical extension.
GRP fibreglass
A rigid resin and glass laminate, seamless and hard wearing, with neat trims. Best where the roof gets walked on or has lots of complicated upstands. It needs a stable, dry deck, because a flexing or damp deck causes cracking.
Warm roof versus cold roof
A warm roof puts the insulation above the deck, which keeps the deck warm and largely removes the condensation risk. It is the better build up on anything habitable below, and it is worth the extra when the covering is being replaced anyway.
Falls, outlets and upstands
Water must be able to leave. Adequate falls to the outlet, correct upstand heights at abutments, proper trims at the edges and an outlet that can be cleared all matter more than which membrane sits on top.
Which we would choose
EPDM for most domestic extensions and dormers, GRP for trafficked areas and complex detailing, felt where budget genuinely dictates and the building is a low value outbuilding.
FAQs
Sometimes, when the existing deck is dry and sound. If there is any trapped moisture, overlaying seals it in and the deck rots underneath.
Occasional maintenance access is fine on all three. Regular foot traffic or a terrace needs GRP or a purpose made walk on system.
A typical 20 to 30 square metre EPDM extension roof is usually a one to two day job including strip and deck repairs, weather permitting.
Related reading
More on comparisons and the jobs that usually come with it.
- uPVC vs timber fascias and soffitsComparisons
- Slate vs tile: which suits your propertyComparisons
- Repair or replace? How to decide honestlyComparisons
- Ridge tile replacement: mortar bedding vs dry-fix ridge
Looking for something specific? See our roofing services, cost guides, common roof problems or the areas we cover.
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