If you have a flat roof on a garage, extension, bay window, or porch, the chances are it was originally covered in felt. And the chances are equally high that it has leaked at least once — or is showing signs of deterioration that your roofer has suggested addressing sooner rather than later.
When that conversation happens, you'll typically be offered three main options: a like-for-like felt replacement, an EPDM rubber system, or a GRP fibreglass system. The costs are different. The lifespans are very different. And the right choice for your situation depends on factors that a roofer who's only interested in completing a quick job may not walk you through properly.
This guide gives you the complete honest comparison — how each system works, how long it lasts, what it costs, what it's best suited to, and what the real 20-year economics look like — so you can make an informed decision before signing any quote.
System One: Traditional Roofing Felt
Built-Up Felt Roofing
Standard 3-layer bitumen felt / Torch-on SBS modified felt
Traditional roofing felt is a bitumen-saturated or bitumen-coated fabric — usually polyester or glass fibre — that is laid in multiple overlapping layers on the roof deck. In a standard 3-layer system, a base layer is cold-bonded to the deck, an intermediate layer bonds to the base, and a mineral-surfaced cap sheet forms the weathering surface. In a torch-on system, a gas torch is used to heat-weld the layers rather than using cold adhesive, creating fully fused laps that are more resistant to moisture ingress.
Felt was the dominant flat roofing material in the UK for most of the 20th century, and the majority of existing flat roofs on residential properties were originally installed with it. Its primary failure mechanisms are well understood: UV degradation makes the bitumen brittle over time; thermal cycling opens cracks at lap joints; and moss and standing water accelerate deterioration. The result is a roof that typically requires attention every 10–15 years for standard felt, or every 15–25 years for quality torch-on systems.
- Lowest upfront cost of any system
- Widely available from most roofers
- Quick to install — often one day
- Easy to patch small localised damage
- Torch-on version significantly more durable
- Shortest lifespan of the three systems
- Lap joints are the primary failure point
- Requires more frequent inspection
- UV-sensitive — degrades in direct sun
- Higher 20-year total cost vs EPDM or GRP
System Two: EPDM Rubber Roofing
EPDM Rubber Membrane
Ethylene Propylene Diene Monomer — single-ply rubber sheet
EPDM is a synthetic rubber membrane manufactured in large factory rolls and cut to size for each roof. Unlike felt, which is applied in multiple overlapping layers, EPDM is typically laid as a single sheet with no seams across the main deck — only at the edges and upstands where the sheet terminates. This single-sheet application is its most significant advantage: it eliminates the lap joints that are the primary failure point of any felt system.
The membrane is bonded to the roof deck using a specialist contact adhesive, applied after thorough priming. Edges are trimmed and tucked under the perimeter trim, then sealed with EPDM-compatible flashing tape. Upstands — the vertical sections where the membrane meets a wall or parapet — are bonded with additional adhesive and finished with seam tape.
EPDM remains flexible at temperatures from -45°C to +150°C, meaning the freeze-thaw cycling that cracks felt has essentially no effect on it. It is highly resistant to UV degradation, ozone, and most forms of biological attack. The material is the same product used on commercial and industrial flat roofs worldwide, and its performance data over 30+ years of commercial use is extremely well documented. When installed correctly by an experienced contractor using a quality brand — Firestone and Carlisle are the market leaders — an EPDM roof requires virtually no maintenance for decades.
- Longest proven lifespan — 25–50 years
- No seams on main deck — primary failure point eliminated
- Highly flexible — unaffected by freeze-thaw
- Easy to repair — patch kit and adhesive
- Proven commercial track record globally
- Can be installed in lower temperatures than GRP
- Black appearance — not to everyone's taste (white and tan available)
- Softer surface — more susceptible to puncture than GRP
- Quality varies significantly by installer and brand
- Adhesive bond can fail if substrate poorly prepared
System Three: GRP Fibreglass Roofing
GRP Fibreglass Roofing
Glass Reinforced Polyester — laminated rigid composite system
GRP fibreglass roofing is an entirely different technology to both felt and EPDM. Rather than applying a pre-manufactured sheet, GRP is built up as a composite on-site. Chopped strand glass fibre matting is laid over the primed roof deck, then saturated with liquid polyester resin and left to cure to a solid, rigid laminate. Once cured, the surface is abraded and a coloured gelcoat topcoat is applied — typically in light grey, slate grey, or white — which provides UV protection and the finished appearance.
The result is a monolithic, fully bonded, rigid surface with no seams, laps, or joints anywhere on the main deck — fundamentally different from the multi-layer approach of felt. GRP is the hardest of the three main flat roofing materials: it resists puncture from falling debris, handles light foot traffic comfortably, and — unlike EPDM — can be colour-matched and refinished with a fresh topcoat if the surface becomes weathered or scratched after many years.
The primary requirement for GRP is temperature — the resin will not cure correctly below approximately 5°C, which means installation is typically restricted to spring through autumn in the UK. GRP also requires a trained and experienced applicator: the resin-to-catalyst ratio, application technique, and curing conditions all affect the final result, and a poorly laminated GRP roof can blister, delaminate, or crack at joints — not from material failure but from installation error.
- Hardest and most impact-resistant surface
- Fully seamless — no joints on main deck
- Handles light foot traffic well
- Gelcoat can be refreshed after many years
- Clean, professional appearance
- Not susceptible to puncture like rubber membranes
- Highest upfront cost of the three systems
- Cannot be installed below ~5°C
- Highly installer-dependent — poor application causes failures
- Less flexible than EPDM — can crack under significant structural movement
- Requires specialist applicator — not all roofers are trained
Head-to-Head: Every Factor Compared
| Factor | Felt | EPDM | GRP |
|---|---|---|---|
| Lifespan (quality install) | 10–25 yrs | 25–50 yrs | 25–40 yrs |
| Single garage installed cost | £600–£1,400 | £800–£1,600 | £1,000–£2,000 |
| Seams / lap joints on deck | Multiple | None | None |
| UV resistance | Poor–Medium | Excellent | Excellent |
| Flexibility / freeze-thaw | Poor | Excellent | Good |
| Impact / puncture resistance | Medium | Medium | Excellent |
| Foot traffic suitability | Not recommended | Light only | Light–medium |
| Ease of repair | Easy (patch) | Very easy (EPDM patch) | Moderate (resin repair) |
| Winter installation | Yes (above freezing) | Yes (above freezing) | No (above ~5°C only) |
| Appearance | Mineral surface / grey | Black (or light grey) | Smooth — colour options |
| Maintenance required | Annual inspection | Very low | Very low |
| Installer skill dependency | Low–medium | Medium | High |
| Best for longevity | — | EPDM wins | GRP close second |
| Best for impact resistance | — | — | GRP wins |
| Best value upfront | Felt wins | — | — |
| Best 20-year value | — | EPDM wins | GRP very close |
The 20-Year Cost Reality
This is where the decision becomes very clear for most homeowners. Using a standard single garage as the example:
Over a 20-year period, EPDM is the most economical option on a typical single garage — despite costing more than felt upfront. GRP is a close second. Standard felt, despite appearing cheapest at point of purchase, is the most expensive over any realistic planning horizon for a flat roof.
These figures also don't account for one of the most significant costs associated with felt failure: internal damage. A felt roof that develops a slow leak — as many do from year 8 onwards — can cause ceiling board damage, insulation saturation, and damp that adds £500–£2,000 to the total cost if left undetected. EPDM and GRP, with their seamless decks and much longer failure-free periods, dramatically reduce this risk.
Which System for Which Situation?
Standard flat garage or extension roof, no foot traffic required
EPDM is the best choice for most homeowners in this situation. The single-sheet system with no deck seams, the proven commercial lifespan, and the ease of repair if a puncture does occur make it the best combination of performance and value. It costs slightly more than torch-on felt but the 20-year economics are decisively in its favour.
Flat roof that doubles as an accessible terrace or walkway
GRP is the better choice where regular foot traffic is expected. Its rigid surface handles weight and movement far better than EPDM and is far less susceptible to puncture from shoes, furniture legs, or plant containers. GRP can also be laid with additional non-slip aggregate for safety on regularly used surfaces.
Roof with complex geometry — multiple penetrations, awkward upstands, irregular shape
Both EPDM and GRP handle complex geometry, but GRP has a slight edge because it is applied as a liquid that conforms perfectly to any shape before curing. EPDM requires more cutting and joining at complex details, and the quality of those details depends heavily on installer skill. For a very simple rectangular deck with one outlet, both systems are equivalent.
Winter installation required — work needs to happen in November–February
EPDM is the only real option during cold months. The adhesive system can be applied down to near-freezing temperatures with care. GRP requires temperatures consistently above 5°C for the resin to cure correctly — attempting winter GRP installation risks a failed laminate that blisters or delammates. Felt can also be applied in winter but cannot be torch-on below approximately 3–5°C safely.
Tight budget — need to keep immediate cost as low as possible
Torch-on SBS felt is the most defensible budget choice — significantly more durable than standard felt, widely available, and at roughly 60–70% of the cost of EPDM or GRP on the same area. If the budget genuinely doesn't stretch to EPDM or GRP right now, torch-on felt is a reasonable 15–20-year solution. Standard 3-layer cold-bonded felt should be the option of last resort and treated as a 10-year temporary solution, not a permanent installation.
What to Ask a Roofer Before Committing to Any System
- ✓ For EPDM: which brand of membrane are you using — and what is its stated design life? (Firestone and Carlisle are the established market leaders)
- ✓ For GRP: are you a trained applicator for the specific system you're quoting? Ask to see examples of previous GRP jobs
- ✓ For all systems: is the existing decking being checked and replaced where necessary? New membrane on rotten decking will fail early regardless of system quality
- ✓ For all systems: is the fall (drainage slope) being assessed? Ponding water on any flat roof reduces lifespan significantly
- ✓ For all systems: what is the workmanship guarantee — separate from the material guarantee — and how long does it run?
- ✓ For any system: what is the specific detail at upstands, outlets, and edges? This is where most flat roof failures originate — ask how the roofer handles these
- ✓ Compare quotes on a like-for-like basis — a cheaper quote may exclude decking check, waste disposal, or edge trim, while a higher quote may include them
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Frequently Asked Questions
Which flat roof system lasts longest — EPDM, GRP, or felt?
EPDM rubber and GRP fibreglass both significantly outperform traditional felt. EPDM has a proven lifespan of 25–50 years; GRP typically lasts 25–40 years. Standard 3-layer felt lasts 10–15 years, and even torch-on SBS felt lasts 15–25 years. For longevity, EPDM has the slight edge due to its extreme temperature tolerance and seamless single-sheet application that eliminates the lap joint failures common to felt.
Is EPDM or GRP better for a flat roof?
Both are excellent — the right choice depends on your situation. GRP provides a harder, more rigid surface that handles foot traffic and falling debris better, and its seamless solid finish can be colour-matched and refinished. EPDM is more flexible, easier and cheaper to repair if punctured, and has a longer proven commercial track record. For a simple flat garage or extension roof with no regular foot traffic, EPDM typically offers the best combination of performance, repairability, and 20-year value.
Why does felt flat roofing fail so quickly?
Traditional felt fails through UV degradation, thermal cycling, and moisture absorption. The bitumen binder becomes brittle over time as UV exposure breaks down the polymers. When the roof expands and contracts with temperature changes, cracks form — particularly at lap joints where two sheets overlap. Once moisture enters through a crack it accelerates deterioration. Torch-on SBS felt is more resilient but follows the same failure pattern over a longer period.
How much does EPDM cost compared to GRP?
On a standard single garage (approximately 15–18m²), EPDM typically costs £800–£1,600 installed and GRP costs £1,000–£2,000. The cost difference narrows on larger areas. Both systems cost more than felt upfront, but the 20-year total cost including repeat felt replacements makes both EPDM and GRP considerably more economical over any realistic planning horizon.
Can I walk on a GRP or EPDM flat roof?
GRP can handle light foot traffic comfortably — it cures to a hard, rigid surface and is used extensively on accessible rooftop terraces. EPDM can also support light foot traffic but is more susceptible to puncture from sharp objects or stiletto heels. For a roof used regularly as a terrace or walkway, GRP is the better choice. For occasional maintenance access only, either system is suitable.
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