Flat roof systems
Flat roof installation: EPDM, GRP and felt compared, and how each is laid
EPDM, GRP and felt can all keep a flat roof watertight, and each is built differently. What sits under the material and round its edges matters as much as the material itself.
A flat roof is not meant to be dead level. It is a roof with a gentle slope, and the whole system is built to push water off that slope and out through a gutter or outlet. Choosing a covering is the part owners notice, since it is what you can see and what the quotes are named after, but the covering is only the final layer of a build-up that begins at the joists.
Three systems are set side by side here: EPDM, a rubber membrane; GRP, glass-reinforced plastic, which people call fibreglass; and felt, laid in layers and bonded with a flame. They differ in how they arrive on site, how they are joined, what the details at the edges look like, and what the published sources say about how long they last. They also differ in what a roofer has to get right on the day, which is the part brochures leave out.
This page compares the three side by side. It covers how each is laid, what the fall of a flat roof needs to be, what the published lifespans say, how Part L of the Building Regulations treats replacing a membrane, and which price figures are on record. The aim is to help you read a quote, not to choose for you.
01Side by side
Three systems and how each goes down
Whatever the covering, the stages are the same: prepare the deck, set the falls, lay any insulation, apply the membrane, and then finish the edges and upstands. The difference is in the fourth stage.
EPDM arrives as a sheet, so a small roof can sometimes be covered with a single piece and fewer joints. GRP is formed in place, which suits roofs with awkward shapes, but it is sensitive to the conditions on the day: it needs a dry deck and a dry spell while it cures. Felt is the familiar system, and it relies on the skill of the person with the torch at every overlap.
Each system needs the same careful finishing round the edges. Two details carry a lot of weight: the upstand, the upturn of the membrane at a wall, and the drip trim along the edge where water leaves the roof. Both can decide whether the finished roof stays dry. Our page on flat roofing covers how we approach them across all three systems.
None of these is the right answer for every roof. A good survey looks at the deck, the shape, the access and the budget, and then says which system suits.
| System | What it is | How it goes on |
|---|---|---|
| EPDM | A synthetic rubber membrane | Rolled out over the deck and bonded or fixed, with joints and corners sealed |
| GRP | Glass fibre and resin | Laid wet on the deck and left to cure into one surface |
| Felt | Bitumen-based layers | Torched on in layers, each bonded to the one below |
02Key points
The deck and the fall underneath
A membrane can only be as flat and sound as the deck below it. Before anything is laid, a roofer checks that the boards or the existing surface are dry and solid. Soft or damp boards are replaced, not covered, because a covering over a wet deck traps the problem underneath.
The deck also sets the fall, the slope that carries water towards the outlet. The standard here is BS 6229:2025, which applies to flat roofs finished in flexible waterproof coverings that are continuously supported. The previous edition, BS 6229:2018, is shown as withdrawn. The National Federation of Roofing Contractors says the 2025 edition sets a minimum 1:80 finished fall, for general roof areas and for internal gutters. IKO, a flat roofing maker, puts it this way: aim for 1:80, and the code of practice asks for the roof to be designed to a 1:40 fall.
The gap between those two figures is deliberate. Decks settle, boards are never perfectly level, and a roof designed to exactly 1:80 can finish with shallow hollows where water stands. Designing steeper leaves room for the tolerances.
Flat roof systems: your questions
Is EPDM, GRP or felt better for a flat roof?
None is better in every case. HomeBuilding and Less give different lifespans for all three, and each system asks for care in a different place: EPDM at the edges and corners, GRP in dry conditions while it cures, felt at the laps. A survey of the deck, shape and access decides which suits your roof.
- A finished fall of 1:80 is the minimum, so there is no credit for a roof that is flatter than that.
- The designed fall of 1:40 is what the roof is built to, so the finished fall ends up above the minimum.
- The code covers membranes; lead sheet roofs sit outside BS 6229.
Our page on what fall a flat roof needs explains how a roofer measures and corrects a fall, and why ponding water on an old roof is worth taking seriously.
03EPDM
EPDM: a rubber sheet and its details
EPDM is a synthetic rubber, supplied in rolls. It is laid over the prepared deck, with an adhesive or fixings holding it in place, and joints between sheets are sealed with a manufacturer's tape or glue. At the edges, a trim holds the membrane and directs water into the gutter.
The attraction is the simplicity of the surface. A single piece of membrane across a small roof leaves no seams in the field, so there is no joint there to go wrong. The weak points to check sit at the details: the corners, the penetrations for pipes and rooflights, the upstands and the edges.
What a roofer needs to get right with EPDM:
- A clean, dry, smooth deck, since a stone or a screw head can mark the membrane.
- Correct bonding at the perimeter, so wind cannot lift the edge.
- Neat corners and penetration collars, formed to the maker's instructions rather than improvised.
- A trim at the drip edge that matches the fall, so water clears the fascia.
On price, the figures on record are for the finished roof rather than for the membrane. In November 2025, Which? Trusted Traders gave £1,950 to £4,000, excluding VAT, where PVC or rubber EPDM goes on with new boards. MyJobQuote, for rubber roofing, says typical prices of around £60 per square metre mean the average rubber roof costs £1,050 fully installed. Those two do not agree, and each reflects its own sample of jobs. Neither is our price: our written total is a single figure, given following the survey.
If you are weighing a rubber roof against other options, ask for the build-up to be described in the quote, from deck to trim.
04GRP · Felt
GRP and felt: formed in place, or built in layers
GRP, or fibreglass, is a resin laid with glass-fibre matting over the deck, then topped with a finishing coat. It cures into a hard sheet with no laps, and it can be formed round awkward shapes and into corners without joints. That suits small, irregular roofs such as a bay, a porch or an extension with several levels. The trade-off is that it is sensitive to its working conditions. Damp boards, cold or wet weather during laying, and rushed curing can all spoil the finish, which is why a roofer chooses the day with care.
What fall does a flat roof need to drain?
NFRC says BS 6229:2025 sets a minimum 1:80 finished fall for general roof areas. IKO explains that when 1:80 is wanted, the code of practice says the roof should be designed for a 1:40 fall. A roof built to exactly 1:80 can finish with hollows where water stands.
Felt is laid in layers. A base layer is fixed to the deck, and the cap sheet is bonded on top with a flame. Each overlap is a joint that has to be sealed, and the quality of the roof rests on the person laying it, particularly at the laps, the edges and the upstands. Done well, a felt roof is a sound surface. Done badly, the weakness is likely to show early, in the laps and at the edges.
| System | Takes most care |
|---|---|
| EPDM | Perimeter bonding, corners and penetrations |
| GRP | Dry deck and weather on the day it is laid |
| Felt | Laps, edges and the torch work |
On price, Which? Trusted Traders put felt on a typical single garage (6m x 3m) at £1,400 to £3,500, excluding VAT, in November 2025. MyJobQuote's 2026 average for a flat roof replacement runs between £1,500 and £6,000. Both are wide ranges, and the width is the point: the size, the access and the state of the deck move the cost a long way.
05Lifespans
How long each system lasts: two sources disagree
Lifespans are the part of a flat roof comparison where owners most want a single number, and the published sources do not give one. HomeBuilding and Less, two UK guides, differ on all three systems, so we name both rather than choose between them.
HomeBuilding gives approximate figures, and Less attaches conditions to its own, such as maintenance, quality and installation. These are typical ranges, not promises, and nothing on this page is a guarantee of how long a roof will last.
What the table does show is the order: felt at the shorter end, with GRP and EPDM longer. It also shows how much of the number depends on the work. The installer's care at the laps, the corners and the edges, and the maintenance afterwards, are named in the sources themselves.
| System | HomeBuilding | Less |
|---|---|---|
| EPDM | 25 to 30 years | Typically 30 to 50 years |
| GRP | 25 to 30 years | 25 to 40 years with minimal maintenance |
| Felt | 10 to 15 years | 10 to 20 years, depending on quality and installation |
A few habits help any flat roof reach the top of its range: clearing leaves and debris from the outlets, looking for ponding once the rain has stopped, and looking at the edges and upstands in autumn. Our page on how long a flat roof lasts sets out the sources in more detail, and it is worth a read before you compare quotes that promise different lives.
06Regulations
Building Regulations when you replace the membrane
Re-covering a flat roof is a Building Regulations matter, and it catches owners out because it feels like maintenance. Approved Document L, the energy part of the regulations in England, includes among its renovation examples a replacement waterproof layer for a flat roof (para 11.2(d)).
That matters because of the thresholds. Para 11.3 sets two triggers: more than 50% of the surface of the roof element renovated, or a major renovation, meaning more than 25% of the external envelope. Either trigger brings the entire roof element up to column (b) of Table 4.3 in AD L 2021. Table 4.3 gives a threshold U-value of 0.35 for a roof and an improved value of 0.16. Para 4.13 allows a lesser upgrade where reaching the improved figure is not feasible technically or functionally, or would take longer than 15 years to repay.
The Planning Portal's re-roofing test is a separate check. Under it, no building regulations application is normally needed where less than 25 per cent of the building envelope and less than 50 per cent of the roof is involved. A flat roof on an extension may meet it, and a full replacement may not, so ask before you commit.
Whichever system you choose, the quote from us is one written total price, and any quote should show if VAT and scaffold sit inside it. Our page on flat roof building regulations covers the details.
- For a small repair to part of a roof, the threshold is unlikely to be reached.
- For a full recovering of a flat roof, expect insulation to come into the discussion.
- Building control, from the council or a registered building control approver, confirms the route.
Does replacing a flat roof covering need approval?
It can. In AD L, putting a replacement waterproof layer on a flat roof is renovation, and if over half of the roof element is renovated the insulation requirements can come into play. The Planning Portal's re-roofing test and building control decide the route, so check with building control before work starts.
Tell us what's wrong with the roof.
You'll get one written total price.