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If you're replacing a flat roof on a garage, extension, or outbuilding, you'll quickly run into three main options: EPDM rubber, GRP fibreglass, and traditional felt. The internet is full of conflicting opinions, and every roofer seems to have a strong preference. The truth is that there's no single best flat roofing material for every job. The right choice depends on your budget, how long you want the roof to last, what the surface will be used for, and who's installing it.
This guide cuts through the noise and gives you a clear comparison so you can have an informed conversation with your roofer.
Most domestic flat roofs in the UK today are covered with one of these three systems. Each has real strengths, and each has situations where it's not the right call.
EPDM (Ethylene Propylene Diene Monomer) is a synthetic rubber membrane that has become the go-to choice for domestic flat roofs in recent years. Large sheets bond directly to the roof deck, creating a near-seamless waterproof layer with very few joints to fail over time.
It's flexible enough to handle the natural movement and temperature changes that a flat roof experiences throughout a British year, and it performs well in standing water, which is a real concern on roofs with poor drainage. According to 2026 flat roof cost data, EPDM typically costs between £60 and £95 per square metre installed and carries an expected lifespan of 40 to 50 years. For a typical 25 to 30 square metre garage or extension, you'd expect to pay in the region of £1,800 to £3,200.
It doesn't require open flames during installation, which makes it a safer option for timber-framed buildings. Maintenance is minimal, and if a puncture does appear, a repair kit costs very little and doesn't need a specialist.
The main drawback is appearance. EPDM is available in black, grey, and white, but the finish is plain and rubber-like. It's not the right choice if the roof is visible from above and you want something more refined.
GRP (Glass Reinforced Polyester) is applied as a liquid resin, reinforced with matting, and finished with a hard topcoat. When it cures, you get a completely seamless, rigid surface that's visually clean and can be coloured to match your building.
It's the preferred material for balconies, roof terraces, and bay windows where the roof is visible and may take foot traffic. The seamless finish is its biggest advantage: no joints means no weak points. Industry pricing guides for 2026 put GRP at £80 to £130 per square metre installed, with a lifespan of 30 to 40 years when installed correctly.
The caveat is that installation quality matters enormously. GRP must be applied in dry conditions and at the right temperature. Too cold and the resin won't cure properly. Too hot and it can set too fast. A poorly installed GRP roof can crack or delaminate, and repairs require professional application rather than a simple DIY patch. If your roofer hasn't installed a lot of GRP, this risk goes up considerably.
Three-layer torch-on felt systems have been the industry standard for decades. Modified bitumen sheets are bonded together using a gas torch, building up a multi-layer waterproof covering. It's the lowest upfront cost of the three options, typically around £40 to £70 per square metre, and it has a proven track record across UK homes and commercial buildings.
The trade-off is lifespan. Felt roofs generally last 15 to 20 years before they need replacing, which means over a 40-year period you could end up replacing the roof twice for roughly the same total spend as EPDM installed once. The open flame during installation also creates a fire risk on certain buildings, which some insurers flag.
Felt remains a sensible option for sheds, garages, and budget-sensitive projects where a 15 to 20 year lifespan is acceptable. For a significant extension that you're planning to live with for decades, the modern alternatives usually make more sense.
The right material comes down to a few key questions.
How long do you want the roof to last? If you want to install it and forget about it, EPDM gives the best lifespan for the money. If budget is the main driver and a 15 to 20 year lifespan is fine, felt works. GRP sits in the middle: excellent longevity when installed properly, but highly dependent on the quality of the installer.
Will the roof take foot traffic? If you'll be walking on it regularly, as a roof terrace or balcony, GRP is the right choice. Its rigid surface handles foot traffic far better than EPDM, which can be punctured by sharp heels or dropped tools. Felt is not suitable for regular foot traffic.
Does appearance matter? GRP can be finished in different colours and has a clean, modern look. EPDM and felt are less attractive. If your flat roof is visible from a bedroom window, a neighbouring property, or the street, it's worth paying the extra for GRP.
What's the roof shape like? Flat roofs with multiple angles, upstands, outlets, and complicated detailing suit GRP well because the liquid resin can be worked into any shape before it cures. EPDM sheets are flexible but need careful detailing at edges and penetrations. Simple rectangular roofs suit all three materials.
It's tempting to go with the cheapest upfront option, but flat roofing is one of those investments where the long-term sums matter. Flat roof cost data for 2026 shows that labour accounts for 40 to 60 per cent of the total price, which means the cost of replacing a roof twice in 30 years is almost always higher than choosing a longer-lasting material from the start. If you're already having scaffolding erected and your property disrupted, doing it once with the right material is almost always better value.
If you're not sure which flat roofing material is right for your property, the best starting point is a site inspection. Once we've seen the roof, the deck condition, the drainage, and how the space is used, we can give you a clear recommendation and a transparent quote. Get in touch with C&C Roofing to arrange a visit.
Which flat roofing material lasts the longest?
EPDM rubber has the longest expected lifespan of the main flat roofing materials, typically 40 to 50 years when installed correctly. GRP fibreglass is close behind at 30 to 40 years. Traditional felt systems generally last 15 to 20 years.
Is GRP or EPDM better for a flat roof?
It depends on the application. EPDM is the better all-round choice for most domestic flat roofs due to its long lifespan, flexibility, and value. GRP is better for balconies, walkable surfaces, and roofs where appearance matters, but it requires a skilled installer and the right weather conditions to cure correctly.
How much does a flat roof cost in the UK in 2026?
For a typical garage or extension roof of 25 to 30 square metres, expect to pay around £1,200 to £3,500 for felt, £1,800 to £3,200 for EPDM, and £2,400 to £4,200 for GRP. Prices vary depending on location, access, and deck condition.
Can I walk on a flat roof?
GRP fibreglass can handle regular foot traffic and is the right choice for roof terraces and balconies. EPDM can be walked on occasionally but is not suitable for regular use and can be punctured. Traditional felt is not designed for foot traffic.
Do I need planning permission to replace a flat roof?
In most cases, like-for-like flat roof replacement in England and Wales does not require planning permission. However, if you're changing the height, structure, or use of the roof, it's worth checking with your local authority before work starts.
