Three flat roofing systems account for the vast majority of work we carry out on Ipswich extensions, garages and outbuildings: GRP fibreglass, EPDM rubber membrane, and various types of felt. They differ significantly in cost, lifespan, installation requirements and suitability for different applications. This guide gives you the information you need to make an informed decision when your flat roof needs replacing.
GRP (glass reinforced polyester) is our standard recommendation for most domestic flat roofing applications in Ipswich. It is a seamless system — the laminate is laid wet and cures to form a continuous surface with no joints. Joints are the weak point in any waterproofing system; removing them removes the primary failure mode. Design life with correct installation: 25 years or more.
GRP handles the East Anglian thermal cycle well. The material expands and contracts slightly with temperature change, and the glass fibre reinforcement gives it the flexibility to do this without cracking — provided the deck beneath it is stable. On a sound, correctly ventilated deck, GRP performs reliably. On a soft deck or one with inadequate ventilation, it will fail earlier. We check deck condition as standard on every survey and include deck replacement in the quote where it's needed.
The limitation of GRP: it requires specific installation conditions. Below approximately 5°C, the resin cures slowly and the result is compromised. In very hot conditions (above 25°C), it cures too fast. This means GRP installation has a preferred temperature window, and in Ipswich's variable climate we schedule accordingly.
A correctly installed GRP (glass reinforced polyester) roofing system carries a typical service life in excess of 25 years, subject to correct deck preparation and resin curing.BS 6229 and GRP manufacturer system specifications
EPDM is a synthetic rubber membrane — a single sheet laid over the deck and adhered at the edges. It handles temperature extremes well (better than GRP), is more forgiving of imperfect deck conditions, and is cost-effective for larger simple areas. Design life: 20+ years.
EPDM is our recommendation for larger flat areas — outbuildings, flat roofed garage ranges, larger commercial applications — where the cost advantage of EPDM over GRP on a simple square area is significant. For smaller, more complex shapes with multiple penetrations (pipes, outlets, upstands), GRP's seamless application is superior.
The limitation of EPDM: the joins between sheets and the edge detailing are the potential failure points, as with any lap-jointed system. On larger simple areas with few joins, this is not a significant concern. On complex roofs with many features, GRP's seamless nature is a meaningful advantage.
Modern torch-on felt (SBS modified bitumen) is a significant improvement over the original cold-applied bitumen felt systems installed on most 1960s–80s Ipswich extensions. Torch-on felt, properly installed in multiple layers with correct laps, has a design life of 15–20 years and performs adequately on simpler applications. It is the lowest-cost option upfront.
Original single-layer cold-applied felt, which is what most Ipswich extension flat roofs carry from the 1960s–80s, has a design life of 10–15 years. On properties where this original felt has been on the roof for 30–40 years, we are almost always dealing with a failed system. We do not re-felt with single-layer systems; where felt is the specified system, we use multi-layer torch-on.
For rear extensions and integral garages: GRP fibreglass. Seamless, durable, handles the East Anglian climate well, and the 25-year design life makes it the best long-term value for standard domestic applications in Ipswich. For larger outbuilding roofs where cost matters and the shape is simple: EPDM. For tight budget situations where a 15-year solution is acceptable: multi-layer torch-on felt.
The three main flat-roofing options suit different situations, and the right choice depends on the roof rather than a blanket preference. Felt — specifically modern high-performance built-up felt, not the old-style stuff — is the cheapest, and still has a place on budget jobs and outbuildings, but it has the shortest life and the most joints to fail, so it's rarely our first recommendation on anything you want to forget about for twenty years. GRP fibreglass gives a seamless, rigid, hard-wearing surface with no joints at all, which makes it excellent where the roof is overlooked from above, might occasionally be walked on, or needs to look neat — a garage roof, a flat-roofed extension visible from an upstairs window. EPDM rubber is a single continuous membrane with outstanding movement tolerance and a very long life, and it comes into its own on larger areas and more exposed positions where its flexibility and durability pay off.
Broadly, felt is cheapest to install but costs most over time because you replace it soonest; GRP and EPDM cost more up front but last far longer, so the whole-life cost is usually lower. On a small, simple, budget job — a shed or a garage you're not precious about — felt can be the sensible economic choice. On an extension over living space, where a leak means water in your home and a replacement means disruption, the longer-lived systems are almost always worth the extra. The exposure matters too: on a more weather-beaten position, EPDM's flexibility handles the movement and temperature swings better than a rigid system, while GRP's seamless hardness suits a sheltered, walked-on surface.
Whichever system goes on, the detailing and the drainage decide how long it actually lasts. A cheap system well-installed with proper falls, clean upstands and a clear outlet will outlast an expensive one fitted badly. This is the part homeowners can't see in a quote and where corners get cut — which is why the specification and the roofer matter as much as the material. On an Ipswich extension or garage we'll recommend the system that genuinely suits your roof and how it's used, explain the trade-off in plain terms, and detail it properly so it delivers the life the material is capable of.
Whichever of the three systems you choose, the quality of the installation matters as much as the material, and it's the part homeowners can't judge from a photograph of the finished roof. The falls have to be correct so water runs to the outlet rather than ponding; the upstands where the flat roof meets a wall have to be properly dressed and high enough; the outlet has to be sized and positioned to clear the water the roof sheds. A cheap system installed with these details right will outlast an expensive one installed carelessly. This is why the specification and the roofer matter so much on flat roofing, and why the lowest quote — which often reflects corners cut on exactly these unseen details — frequently proves the most expensive over time.
GRP for most garage applications — seamless construction is ideal for the typical garage roof with multiple penetrations and a gutter outlet. EPDM is cost-effective on larger, simpler areas. We recommend after surveying the specific roof.
GRP: £60–£90 per m². EPDM: £50–£75 per m². Torch-on felt: £35–£55 per m². For a 20m² garage roof the difference is approximately £200–£500 between systems.
GRP: 25 years+. EPDM: 20 years. Torch-on felt: 15 years. Original 1970s single-layer felt: likely at or past end of life if not replaced in the last 15 years.
We don't install GRP below approximately 5°C — it affects the cure quality. In Ipswich's climate this means winter installation may need to be scheduled for milder periods. EPDM and torch-on felt are less temperature-sensitive.
Original 1960s–80s cold-applied bitumen felt was a single layer with a 10–15 year design life. At 30–50 years old, it has been past its design life for decades. Nothing went wrong specifically — it simply ran its course. Modern systems are significantly better specified.
We check deck condition as standard on every survey. If the deck has softened from slow water ingress — very common on Ipswich properties where the felt has been failing gradually — we replace it as part of the job. This is included in our survey quotes, not added as a surprise on the day.
Only if the existing deck is completely sound. In most cases we remove the old felt to check deck condition properly. Over-laying without checking the deck is a shortcut that causes early GRP failure.
Whatever your roofing question, we're happy to help. Call us for a free survey and honest assessment.
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