Flat-roofed extensions, garages, and outbuildings almost always come down to a choice between two materials: traditional bitumen felt, or GRP (fibreglass) roofing. They look broadly similar once installed, but they behave very differently over time.
Lifespan: the biggest practical difference
This is where the two materials diverge most. A well-installed felt roof typically lasts somewhere in the region of 10 to 20 years, depending on the specific product (basic three-layer felt sits at the lower end of that range; modern polymer-modified torch-on felt performs somewhat better). A properly installed GRP fibreglass roof, by contrast, can reasonably be expected to last 30 to 50 years with minimal maintenance — roughly double the service life of felt, and sometimes considerably more.
Why felt tends to fail first
Felt roofing is applied in overlapping sections, which inevitably creates seams — and those seams are usually where a felt roof fails first, whether through thermal movement, water tracking under a lifted edge, or the bitumen simply drying out and cracking with age and UV exposure over the years. GRP, by contrast, is applied as a continuous, seamless membrane, which removes that particular failure point entirely — there’s no joint for water to exploit.
Maintenance and general upkeep
Fibreglass roofs are genuinely low-maintenance day to day — an occasional clean with soap and warm water is usually all that’s needed, and the protective top (gel) coat can be renewed periodically to extend the roof’s life further without needing to replace the whole system. Felt roofs need more regular attention, since surface chippings can wear thin over time (leaving the bitumen more exposed to UV degradation), and any lifted seam or puncture typically needs prompt repair before it lets water in.
Cost: cheaper upfront, more expensive over time
Felt is the cheaper option to install initially, which is exactly why it remains common on smaller extensions and outbuildings where upfront cost matters most. But because GRP lasts roughly twice as long, the real lifetime cost often favours fibreglass once you account for the fact that a felt roof is likely to need replacing (or substantially repairing) at least once in the time a single GRP installation would still be performing well.
The practical takeaway
If a property has a felt flat roof that’s approaching the upper end of its expected lifespan, it’s one of the more predictable “known future cost” items a survey can flag — not usually an emergency, but a sensible one to budget for, and a genuine case where switching to GRP at replacement time is likely to be the better long-term decision.
