Put EPDM, GRP and torch-on felt on the same roof and each will keep the rain out for a while. The differences show up later, at the edges, in the winters, and the first time somebody has to repair a leak. They are built differently, they fail differently, and they cost different amounts. This comparison goes through the three in turn on what they are, how they cope with movement, how they handle awkward details, what repairs involve and what the typical UK price ranges are. It is written for the moment you are holding two or three quotes with different materials on them and need to know what is behind the names.
If you are new to the options, the short answer on the three types of flat roof explains the basics first.
What each one physically is
EPDM is a synthetic rubber sheet, short for ethylene propylene diene monomer. It comes on a roll, is stuck down with adhesive and is joined with tape. GRP is glass-reinforced plastic, which most people call fibreglass. Resin and glass matting are laid wet over a timber deck, cure into a single hard shell with no joins, and get a coloured topcoat. Torch-on felt is bitumen reinforced with polyester, supplied in rolls and melted onto the surface with a gas torch. A proper felt roof has two or three layers, finished with a cap sheet that is often faced with mineral granules.
So one is a flexible rubber sheet, one is a rigid moulded shell and one is a layered bitumen blanket. Nearly every difference below follows from that.
Movement, weather and wear
Timber decks move. They swell when it is wet and shrink when it is dry, and a flat roof also expands and contracts with temperature. EPDM is elastic, so it follows that movement and returns to shape. Felt is flexible too, though the bitumen stiffens in cold weather and softens in heat, and cheaper felts can split along the line of a deck board. GRP is rigid, so it relies on a sound, well-fixed deck and on correctly formed expansion details. Laid over a deck that flexes, it can develop hairline cracks.
On weather, all three do their job once they are properly installed. GRP and felt with a mineral cap resist sun and light foot traffic well. EPDM is UV stable and unaffected by frost, though it dislikes oil and solvents. Bitumen products carry a risk of blistering when moisture trapped in the deck turns to vapour in summer sun, which is why a dry deck matters whatever the covering.
Detail work: edges, corners and penetrations
Roofs leak at the details far more often than across the open field. An upstand is the section of membrane carried up a wall at the edge of the roof, and a flashing is the overlapping band that sheds water over its top. Every roof has them, along with edge trims, rooflight kerbs and pipe collars.
GRP is laid as a liquid, so it flows around a kerb, a pipe or an inside corner in one continuous skin. That makes it the neatest choice for a roof full of awkward shapes, and a good laminator can finish a curved fascia edge that rubber would struggle with. EPDM is cut and folded, with preformed corner pieces and collars, and in practised hands the result is tidy and watertight. It simply takes more pieces. Felt is dressed with extra strips, and the fire risk of the torch means we avoid it against old timber, sarking boards or tight cavities where a flame cannot be controlled.
If you are comparing quotes, ask how each detail will be formed. A vague answer for the lantern kerb or the wall junction is a warning.
Repairs and ageing
A repair is where the three part company most clearly. A split or puncture in EPDM is patched with a bonded piece of rubber or self-adhesive tape, a job that is simple on a clean, dry sheet. A crack in GRP is ground back, filled with fresh resin and glass, and re-coated. The patch is hard, strong and sometimes slightly visible if the topcoat colour has faded. Felt repairs are the most variable. A small tear can be patched with a bonded piece of cap sheet, but old felt that has become brittle may not accept a patch and tends to fail again alongside it.
All three need the cause of the leak found before any patch goes on. Water seen on a ceiling is rarely under the damage on the roof, because it travels along the deck and the insulation first. We trace it, show you photographs, and only then say whether a repair is enough. Our guide to EPDM rubber roofing and our guide to GRP fibreglass roofing each go further into how that material ages.
Cost and lifespan side by side
Two third-party cost guides report different figures, and both are worth showing because they bracket the real picture. One reports typical UK ranges of £60 to £100 per m² for felt, £80 to £120 per m² for EPDM, with GRP the dearest at £100 to £160 per m². A trade-facing guide reports £70 to £100 per m² for EPDM, £80 to £120 per m² for GRP and £45 to £75 per m² for modified bitumen felt. These are national figures for orientation only. The fixed quote comes after the survey.
| Covering | Typical UK range, first guide | Typical UK range, trade guide | Character |
|---|---|---|---|
| Torch-on felt | £60 to £100 per m² | £45 to £75 per m² (modified bitumen) | Lowest outlay, shortest life of the three |
| EPDM | £80 to £120 per m² | £70 to £100 per m² | Flexible sheet, few joints on simple roofs |
| GRP | £100 to £160 per m² | £80 to £120 per m² | Hard shell with no joins, best for complex shapes |
Price per m² hides a lot. A small roof with many details costs more per square metre than a big plain one, and the deck, insulation and access can outweigh the covering. On lifespan, modern reinforced felts last far longer than the thin felts that gave flat roofs their poor name, but they generally have the shortest life of the three. EPDM and GRP can both last for many years when properly laid and kept clear of debris. We do not quote exact lives, because the quality of the deck and the standard of the detailing change them more than the material does. Our page on torch-on felt roofing explains where felt still makes sense.
Fire, foot traffic and where each one struggles
Each covering has a weak point worth knowing before you sign. Torch-on felt carries the fire risk of its installation method, as described above, and an installer who rushes the cooling and checking period afterwards can leave a smouldering problem behind. EPDM is vulnerable to sharp objects and to solvents, so it asks for care around trades working on or near the roof afterwards, for example a satellite engineer or someone cleaning a gutter. GRP is vulnerable to a poorly prepared deck and to a topcoat applied in damp or cold conditions, which can leave it looking patchy or prone to crazing.
Foot traffic splits them again. A roof you step onto every few months to clean a window suits the hard surface of GRP, which can take an anti-slip topcoat. A bonded EPDM roof can be walked on with soft-soled shoes and care, and a path of walkway pads protects the route. Felt with a mineral cap is a fair middle, though the granules loosen under regular traffic. If anyone plans to use the roof as a balcony or terrace, it becomes a separate question about guarding, privacy and structure, so speak to the planning team at your council and to your building control body before you commit.
Trees and wildlife matter as well. Leaves and moss hold water against any covering and block outlets, and birds working at a felt surface can pull at loose granules. Whichever material you choose, clear the outlets each autumn and look over the edges after storms.
Questions to put to each quote
Whatever the material, the quote tells you more than the brochure. Ask what the deck is made of and what will be done if boards are soft. Ask what the fall will be and how it is formed. Ask whether the insulation is above the deck or between the joists, and what happens at the edges. Ask for the make and thickness of the membrane or the number of felt layers, and how each wall junction will be finished. Ask who will be on site, how weather delays are handled, and what the guarantee covers in writing. A price that is much lower than the others often hides a thinner build-up, a skipped detail or an overlay on a deck that should have been stripped.
Be wary of any firm that dismisses one of the three out of hand. EPDM, GRP and felt each have a proper place, and the honest answer depends on the roof. We would sooner explain why a material is wrong for your roof than sell you something because it is what we happen to carry.
Choosing between them
A plain rectangle with one wall junction suits EPDM. A roof with a lantern, vents and a curved edge suits GRP. A small outbuilding, a tight budget or a short stay in the house suits felt. Those are tendencies, and the roof in front of us decides. Walking traffic, nearby trees, the state of the old deck and any plan to add insulation can each tip the balance, and a firm that only fits one material will find a reason to recommend it.
The covering also matters less than the roof beneath it. Falls, deck condition, insulation and edge trims decide whether any of these three succeeds, and these are the items worth questioning in a quote. If you want to see the whole roof choice laid out by shape and use, the page on the flat roofing in Basingstoke hub is the place to start.
When we survey, we photograph your roof, explain what each option would involve and give a fixed written quote. If the existing roof has years left, we say so and price a repair instead. The workmanship is guaranteed for ten years in writing, and we leave the site tidy with the old covering taken away. To arrange the survey, use 01256 213956 or WhatsApp.
