The short answer
For most garages, extensions and garden rooms, EPDM rubber is the sensible default: one sheet, no joins across the middle of the roof, no flame on site, and a manufacturer's design life of forty years or more if the edges are done properly. Choose GRP fibreglass where the roof gets walked on — a balcony, a terrace, a roof with a flue to service. Choose felt when the budget genuinely decides it, on a roof you are content to redo inside twenty-five years.
What each of the three actually is
All three do the same job: keep water off a deck that has almost no fall. They fail for completely different reasons, and matching the failure to your particular roof is most of the decision.
There is no best flat roof. There is a best flat roof for a 12 square metre garage with a sound deck that nobody will ever stand on, and it is not the same as the best flat roof for a terrace off a first-floor bedroom.
- EPDM rubber
- A single sheet of synthetic rubber, normally 1.2mm or 1.5mm, bonded to a board deck with cold adhesive. On a domestic roof it usually arrives cut as one piece, so there is no join anywhere in the open field. Every join it has is at an upstand, a corner or an outlet.
- GRP fibreglass
- Glass matting rolled out over a plywood deck, wetted through with polyester resin, then finished with a pigmented topcoat. It cures into one rigid shell bonded to the boards. The edge trims are laminated in, so the kerbs and drips are part of the same skin rather than something fixed on afterwards.
- Torch-on felt
- Bitumen sheets, normally three layers, melted onto the deck and to each other with a gas torch. Modern SBS-modified, polyester-based felt is a long way from the tar-and-chippings roofs people remember. It is still a layered system held together by heat, with a lap every metre or so.
The six things that decide it
This is the comparison, in roughly the order it matters on a house.
- Lifespan
- EPDM carries a manufacturer's design life of forty years or more, and sheets laid in Britain in the 1970s are still being inspected rather than replaced — the rubber does not go brittle in sunlight. GRP 25–30 years, and it is the topcoat that ages first, not the laminate. Three-layer torch-on felt 15–25 years. The two-layer pour-and-roll work sold cheap on garages is finished in eight to ten.
- Seams and joins
- GRP has none at all: it is one continuous laminate. EPDM has none across the field, so the whole quality question moves to the corners, the upstands and the outlet. Felt has a lap every sheet width, and every lap is a place water can eventually get under.
- Hot works
- EPDM is cold-applied, no flame. GRP has no flame either, but it cures chemically and warm, and it smells strongly of styrene for a day. Felt is laid with a gas torch inches from a timber deck and a timber fascia. Roof fires start from torch work, which is why insurers and managing agents ask about hot-works procedure and why some buildings will not allow it at all.
- Foot traffic
- GRP is the walkable one, and the only one of the three you should design a terrace around. EPDM takes occasional maintenance traffic but it is soft — a stone under a boot or a ladder foot can nick it. Felt goes soft in summer heat and scuffs; a July mineral-felt roof marks under a heel.
- Repairability
- EPDM is the easiest roof in Britain to repair: clean it, prime it, cold-bond a patch, and the patch is as good as the sheet. Felt patches fine, but the patch is another lap that will lift in its turn. GRP repairs need grinding back, genuinely dry conditions and a topcoat that almost never colour-matches, so a cracked fibreglass roof is a bigger job than it looks from the ground.
- Cost
- Felt cheapest to lay and dearest over 30 years, because you buy it twice. EPDM in the middle. GRP dearest to lay, because it is more labour on site and weather-dependent. Worth it only where the walkability or the shape earns it.
How long each one lasts, and what kills it
Felt blisters. Moisture trapped between the layers, or still in the deck when the roof was laid, turns to vapour as soon as the sun hits a black surface. Vapour expands, the weakest lap lifts, and you get a bubble. The bubble is not itself a leak. It becomes one when somebody walks on it in July, or when a winter of freezing and thawing works the lap open.
GRP cracks where it is asked to move. Cured resin has almost no give, so any movement in the deck below — a badly supported board joint, a rafter that deflects when someone stands on it, ply laid tight with no expansion gap — shows up as a hairline crack running along the line of that movement. Nearly every failed fibreglass roof we open up turns out to be a deck fault, not a resin fault. Fitted over a properly screwed, correctly gapped deck it is a very durable roof.
EPDM rarely fails in the middle of the sheet, and that is the point of it. It fails at the details: an upstand that has released from a rendered wall, a corner cut and bonded in a hurry, an outlet that was not dressed properly, a kerb where the adhesive was rolled out on a damp morning. Every one of those is a workmanship question, not a material question. Ask any roofer quoting rubber how they finish the corners and the outlet.
The deck decides more than the membrane
A flat roof is not flat. It should be laid to a fall of at least 1 in 80 after everything has settled and deflected, which in practice means designing to 1 in 40. Water standing in a puddle two days after rain is ponding, and it shortens the life of all three systems — it holds dirt, it grows algae, and in winter it freezes and works at every join it can find.
If your existing roof ponds, replacing the covering alone will not fix it. The firrings under the deck are what create the fall, and putting a new membrane over a dished deck buys the same problem in a new colour. That is a real cost difference between quotes, and it is worth asking each quoting roofer directly whether their price includes re-decking and new firrings or only the covering.
There is also a regulatory point people get caught by. Replacing a flat roof covering normally counts as renovating a thermal element under the Building Regulations, and once you are replacing more than half the area the insulation usually has to be brought up to current standards. On a warm-roof build-up that means insulation boards above the deck, which raises the roof level and changes door thresholds, upstand heights and the way the roof meets a window. Anyone quoting a like-for-like strip and recover on a habitable room, without mentioning insulation at all, has either checked and can explain why it does not apply, or has not checked.
What moves the cost on a real roof
Small roofs cost more per square metre than big ones, because a day on site is a day on site. A 6 square metre porch roof will not work out at the same rate as a 40 square metre extension, and quotes that pretend otherwise are hiding something in the fixed costs.
- The deck
- New ply or OSB, new firrings and repairs to soft rafter ends are the single biggest swing. Rot is normal on a roof that has been leaking, and it cannot be priced from the ground.
- Insulation
- Going from a cold roof to an insulated warm roof adds PIR boards, a vapour control layer and higher upstands. It is the difference between a covering job and a rebuild of the roof build-up.
- Access and scaffold
- A garage you can reach off a ladder is one price. A second-storey extension roof with an edge protection requirement, or a rear terrace only reachable through the house, is another. In parts of London a suspended parking bay for the skip is a line item of its own.
- Edge detail
- The middle of a flat roof is quick. Time goes into the perimeter: upstands against brick, a chase and lead flashing where the roof meets a wall, rooflight kerbs, outlets and the join with the neighbouring roof. A roof with six upstands is a different job from a rectangle with two.
- Getting the old one off
- Stripping three layers of bonded felt and a rotten deck is heavy, dirty work with a skip attached. An overlay avoids it, and is sometimes the honest answer on a garage — but not on anything you live under.
Which one we would fit, and when
Given a free choice and a sound deck, EPDM covers the majority of South London domestic flat roofs well. That is not loyalty to a material, it is that most of these roofs are simple rectangles nobody walks on, and rubber is the system whose failure mode is easiest to avoid and cheapest to put right.
- Garage or outbuilding, tight budget
- Felt is a defensible choice, and we will say so. If you are selling in three years or the building is a shed with delusions, spending fibreglass money on it is a waste.
- Extension you look down on from a bedroom
- EPDM. It is quiet in the rain, it does not chalk in sunlight, and a black sheet reads better from above than a patchwork of felt laps.
- Balcony, terrace or anywhere with a flue to service
- GRP. It is the only one of the three that shrugs off regular footfall, and it can be laid with a non-slip aggregate in the topcoat.
- Awkward shapes, dormer tops, lots of pipes
- GRP or a liquid system. Both are laid wet and go round complicated detail without joins. Liquid also lets you work over a roof you cannot fully strip.
- No flame allowed on the building
- EPDM or liquid. Cold-applied throughout, which is often the deciding factor on a leasehold flat, a timber-clad building or anything with a managing agent.
- Work happening in December
- EPDM. GRP does not cure reliably below about five degrees and will not go onto a damp deck at all, so a winter fibreglass roof means tenting and heating, or waiting.
Questions
01How long does a flat roof last?
The makers rate EPDM past forty years and the sheet is rarely what gives out. GRP runs 25 to 30 years, with the topcoat ageing before the laminate. Three-layer torch-on felt is 15 to 25. Cheap two-layer felt is closer to eight. In every case it is the deck, the fall and the edge details that decide it, not the brochure figure for the membrane.
02Is fibreglass better than rubber?
Not generally, no. It is better where people walk and where the shape is complicated. Rubber is better where the roof is a plain rectangle nobody stands on, where it has to go down in cold weather, and where you want a repair in future to be a one-hour job rather than a grinding and recoating exercise.
03Can a rubber or fibreglass roof go straight over old felt?
Sometimes on a garage, if the felt is well stuck, dry underneath and the fall is right. Rarely on anything habitable. An overlay traps whatever moisture is already in the build-up, and it hides the rot you were meant to find. If a quote is unusually cheap, check whether it is an overlay.
04Does EPDM shrink or go brittle?
The sheet itself holds up well in sunlight, which is why it lasts. What people call shrinkage is nearly always adhesive failure at the perimeter pulling the sheet away from an upstand or kerb. It is a bonding and detailing problem, and it is repairable without replacing the roof.
05Do I need Building Regulations approval to replace a flat roof?
Usually yes. Re-covering more than about a quarter of a roof is notifiable, and once you replace more than half the area over a heated room it counts as renovating a thermal element, so the insulation has to meet the current standard — around 0.18 W/m²K on a flat roof, which means a warm deck in practice. A detached garage or outbuilding is generally outside that. Your local building control team will confirm for your specific roof.
06Why does water still sit on my flat roof two days after rain?
The fall is wrong, or the deck has dished between rafters. That is a structural fix under the covering — new firrings, sometimes new deck — not something a new membrane cures. Ponding will not sink a good roof on its own, but it shortens the life of every system and it is why the same corner keeps failing.
07Which is cheapest over thirty years?
EPDM on most domestic roofs. Felt is the cheapest first bill and the dearest total, because thirty years means laying it twice with a strip-out and a skip in between. GRP costs most up front and is competitive over the long run, but only where its walkability or the roof shape is actually being used.
08Can you lay a flat roof in winter?
EPDM, yes, on a dry day. Felt, yes, though the deck must be dry. GRP is the awkward one: the resin needs roughly five degrees and above and a genuinely dry deck to cure properly, so a January fibreglass roof either waits for a window in the weather or gets tented and heated, which costs money.