Flat roof options: EPDM vs GRP vs torch-on felt compared

Roofers laying new tiles on a pitched roof

For most homes, the choice comes down to three systems: EPDM rubber, GRP fibreglass and torch-on felt. On 2026 published estimates, felt is the cheapest to lay but has the shortest life. EPDM and GRP cost a little more per square metre and typically last two or more times as long.

All prices here are estimates from 2026 guides. Your quote will depend on the size of the roof, the condition of the deck underneath and how many details (rooflights, upstands, outlets) it has.

The three systems at a glance

	Torch-on felt	EPDM rubber	GRP fibreglass
Typical cost per m², supply and fix	£50 to £80	£60 to £90	£70 to £100
Typical lifespan (estimate)	15 to 20 years	30 to 50 years	25 to 40 years
How it goes on	Layers of bitumen felt bonded with a gas torch	Usually one large rubber sheet, bonded down	Resin and glass mat laid wet, then a topcoat
Joints	Overlaps between layers	Few or none on small roofs	Seamless once cured

Costs and lifespans are from Trade2Base’s June 2026 pricing guide. Deck replacement is extra.

What a flat roof typically costs

Which? surveyed its Trusted Traders in November 2025 for a single garage of about 6m by 3m, keeping the existing structure. Prices include labour and materials but exclude VAT.

New felt flat roof: about £1,400 to £3,500
New flat roof in other materials (new boards, PVC or EPDM rubber): about £1,950 to £4,000

Trade2Base (June 2026) gives about £1,200 to £2,500 for a single garage in EPDM, and about £4,000 to £8,000 or more for a house extension roof with parapets, rooflights and upstands.

If the timber deck is soft or rotten, it has to come off. That is often where a quote grows, so ask your roofer to price deck replacement as a separate line.

EPDM rubber

EPDM is a synthetic rubber membrane. On a garage or small extension it can often go on as a single sheet, so there are few joints to fail.

Best for: simple rectangular roofs, garages, dormers and extensions.
Watch for: sharp objects and heavy foot traffic, and badly finished edges and corners, which is where most problems start.
Guarantees: one widely sold EPDM system carries a BBA certificate with a 35 year service life. Its seller says a 20 year materials guarantee is available when a trained installer fits it. Terms vary by product, so ask which one your roofer uses.

GRP fibreglass

GRP (glass reinforced plastic) is built up on site from resin and a glass fibre mat, then finished with a coloured topcoat. Once cured it is one seamless, hard skin.

Best for: roofs with lots of corners, outlets or rooflights, and roofs you need to walk on occasionally, such as for window cleaning or maintenance.
Watch for: the finish depends heavily on the installer’s skill, so ask to see recent GRP roofs they have laid.
Guarantees: these come from the system manufacturer and usually depend on an approved installer. Ask for the BBA certificate and the guarantee in writing before work starts.

Torch-on felt

Torch-on felt roofs are typically built up in three layers of bitumen felt, bonded with a gas torch.

Best for: tight budgets and roofs you may rebuild within 15 to 20 years anyway.
Watch for: the system is laid with a naked flame, so use an experienced roofer with public liability insurance.
Lifespan: at about 15 to 20 years, it typically needs replacing sooner than the other two.

Building Regulations: the insulation rule

This is where many flat roof quotes differ, and it is worth understanding.

When the whole roof needs insulating

Approved Document L, the guidance on energy efficiency, lists “replacing the waterproof membrane on a flat roof” as renovation of a thermal element (paragraph 11.2). Paragraph 11.3 then says the whole roof should be improved to at least a U-value of 0.16 W/m²K where either:

more than 50% of that roof’s surface is renovated, or
the work is a major renovation, meaning more than 25% of the surface area of the whole building’s external envelope is renovated.

In practice, re-covering an entire flat roof usually triggers the upgrade. Approved Document L’s worked examples suggest insulation placed between and over the joists to reach the target, where existing insulation is less than 100mm or in poor condition.

When a lower standard is allowed

If reaching 0.16 is not technically or functionally feasible, or would not pay back within 15 years, paragraph 4.13 lets the roof be upgraded to the best standard that is. An upgraded element should generally not be worse than 0.7 W/m²K.

Adding insulation on top of the deck raises the roof level. That can affect upstands, door thresholds and flashings, so a good quote will deal with these details.

When you need approval

The Planning Portal says no building control application is normally needed for repairs or re-covering where less than 50% of the roof and less than 25% of the whole building envelope is affected. Bigger jobs need approval, as do structural alterations and coverings that add 15% or more to the roof’s weight.

A roofer registered with a competent person scheme can self-certify the work instead. gov.uk lists CompetentRoofer, run by the NFRC, and NAPIT as authorised for replacing roof coverings on pitched and flat roofs. The NFRC scheme says it issues Building Regulations compliance certificates automatically and offers a 10 year insurance backed guarantee where required.

A new edition of Approved Document L takes effect on 24 March 2027. Until then, the 2021 edition (with 2023 amendments) applies.

For more on the rules, see our guide to planning and Building Regulations for a new roof.

Which should you choose?

Garage or simple extension, best value over time: EPDM is a common choice.
Complex roof, rooflights or occasional foot traffic: GRP is often the better fit.
Short term or lowest upfront cost: torch-on felt, accepting a shorter life.

Whatever you choose, ask for a written specification that names the system, the insulation, the deck and the guarantee. If you are also comparing pitched roof work, our roof replacement cost guide and repair or replace guide may help.

Replacing a flat roof? Get a quote from a checked local roofer.

Common questions

Which flat roof lasts longest?

On published 2026 estimates, EPDM rubber has the longest typical life at about 30 to 50 years, ahead of GRP fibreglass at about 25 to 40 years and torch-on felt at about 15 to 20 years. Quality of installation and the deck underneath matter as much as the material.

Do I need Building Regulations approval to replace a flat roof?

Often, yes. The Planning Portal says no application is normally needed where less than 50% of the roof and less than 25% of the whole building envelope is affected. Beyond that, the work needs approval and the roof should normally be insulated to current standards.

Is EPDM better than GRP?

Neither is better everywhere. EPDM is usually a single sheet with few joints and suits simple shapes. GRP is laid as a liquid resin and glass mat, so it suits roofs with lots of details and gives a hard surface that copes better with light foot traffic.

How much does a new garage flat roof cost?

Which? Trusted Traders put a new felt roof on a single garage of about 6m by 3m at £1,400 to £3,500, and a roof in other materials such as EPDM or PVC with new boards at £1,950 to £4,000, both excluding VAT.