Flat Roof Types & Costs: EPDM, GRP & Felt

A flat roof, technically anything pitched below 10°, covers more UK homes than most people realise: garage roofs, kitchen extensions, rear additions, bay window tops and porch canopies are all routinely built flat.¹ The choice of membrane system affects not just upfront cost but how long it will last, how well it insulates, and whether it can be walked on. This guide compares the three dominant systems, built-up felt, EPDM rubber and GRP fibreglass, with 2026 UK prices and practical guidance on which suits which situation.

Flat Roof Systems Compared

Built-Up Felt (Torch-On Felt)

Traditional felt, laid in two or three layers with bitumen bonded between each sheet, remains the cheapest flat roof option. Modern torch-on felt is a significant improvement over the old sand-and-felt systems found on 1970s–1990s extensions: the top cap sheet is a mineral-surfaced SBS polymer-modified bitumen, far more UV-resistant than the original.

Felt suits straightforward rectangular roofs where budget is the primary constraint. It is the default choice of many builders doing a rear extension on a tight contract. The weak point is detailing around upstands, flashings and outlets, poorly lapped joints and cracked flashings are responsible for the majority of felt roof failures. A competent installation with properly dressed lead flashings and 150 mm upstands can easily last 15 years; a rushed job on a leaky extension may fail in five.

EPDM (Ethylene Propylene Diene Monomer)

EPDM is a synthetic rubber membrane sold in large single-ply sheets, often covering an entire roof in one piece, eliminating seams. It originated in commercial flat roofing and has become popular for domestic use over the past decade. The material is extremely UV-stable, handles thermal expansion well (rubber stretches and returns rather than cracking), and some manufacturers quote a life expectancy of 50 years or more.

The adhesive bonding process requires skill: a poor bond causes lifting and pooling. Reputable EPDM systems use a fully bonded layout with compatible water-cut outlets and pre-formed corners, so the quality of the installer matters considerably. Look for contractors who are accredited by the system manufacturer (for example, Firestone or Carlisle) rather than those buying generic EPDM off a roll.

GRP Fibreglass (Glass Reinforced Polyester)

GRP is the premium domestic flat roof product. The installer saturates woven glass mat with liquid resin and applies it in situ, then tops with a gelcoat. When cured, it forms a rigid, seamless shell. GRP is the only commonly available system that can be finished to a smooth walking surface without additional decking, and is regularly specified on balconies and roof terraces for this reason.

GRP is hard and brittle compared with rubber, impact from dropped tools or foot traffic without protection boards can crack it, but when undamaged it has outstanding longevity. Well-laid GRP roofs can last for several decades. The main limitation is weather: GRP resins generally should not be laid below about 5°C or in damp or humid conditions, which constrains the working season in the UK.

Cost Comparison

All figures are installed costs including materials, labour, removal of the existing roof covering and disposal, but excluding structural timber repairs. VAT at 20% is included; repairs and replacements to existing domestic roofs are also standard-rated at 20% (a new-build house is zero-rated), check with your contractor.²

A typical UK garage roof is around 20–25 m². At those areas:

• Felt replacement (2-layer): £800–£1,750
• EPDM replacement: £1,100–£2,375
• GRP replacement: £1,300–£2,750

A kitchen extension flat roof at 12–16 m²:

• Felt (2-layer): £480–£1,120
• EPDM: £660–£1,520
• GRP: £780–£1,760

Day-rate labour (excluding materials) for a roofer typically runs about £150–£300 per operative per day outside London, and more in London and the South East.

Lifespan and Maintenance

The cost-per-year calculation often makes EPDM and GRP look better value than their upfront premium suggests. A felt roof at £55/m² lasting 15 years costs £3.67/m²/year; an EPDM roof at £75/m² lasting 40 years costs £1.88/m²/year.

All three systems need periodic maintenance:

• Felt: check flashings and outlets annually, clear debris from gutters, look for blistering or splits after severe winters.
• EPDM: inspect bonded seams and upstand details every 2–3 years; re-apply seam tape if lifting is detected early.
• GRP: inspect the gelcoat for cracks or crazing; a surface re-coat (£15–£30/m²) at 15–20 years extends life significantly.

Insulation for Flat Roofs

Building Regulations Part L in England (Approved Document L, 2021 edition) sets a limiting U-value of 0.15 W/m²K for a new roof on an existing home, such as an extension roof (Table 4.2), and 0.16 W/m²K for a new-build roof or an existing roof that is renovated (Tables 4.1 and 4.3).³ This demands around 140–180 mm of PIR (polyisocyanurate) board insulation in a warm roof configuration, or deeper rafters packed with mineral wool in a cold roof layout.

Warm roof (insulation above the deck, below the membrane) is strongly preferred: it keeps the structure above the dew point, eliminates condensation risk and avoids the complex ventilation required in cold roof designs. Most reputable flat roof contractors will default to warm deck for any new or replacement installation.

Cold roof (insulation between joists, membrane on top of deck) is cheaper to construct but prone to interstitial condensation unless ventilation is correctly detailed. Building Control will scrutinise cold roof designs. Approved Document C accepts a roof as resisting damage from interstitial condensation if it is designed and built to BS 5250 and BS EN ISO 13788, so expect to provide a condensation risk calculation.⁴

Drainage and Falls

BS 6229:2025 sets a minimum finished fall of 1:80 (about 12.5 mm per metre) for general roof areas and internal gutters, so that the roof drains freely.⁵ In practice designers set a steeper design fall, commonly 1:60, so that construction tolerances and deck deflection still leave at least 1:80.

Falls are created by:

• Tapered insulation (most common on new installations): the board manufacturer cuts the insulation to create the required slope. Add £4–£8/m² to insulation costs.
• Firring pieces (timber wedges under the deck): cheaper but less thermally efficient as the insulation above remains level.
• Structural falls (rafters or concrete deck already sloped): ideal but rarely the case in retrofit.

Ponding, standing water that never drains, is the primary enemy of flat roofs. It adds weight, promotes algae, stresses seams and accelerates the degradation of all three membrane types. If your existing flat roof ponds, address the falls at replacement; adding a new membrane over a ponding deck repeats the problem.

When to Choose Each System

• Felt, budget garage or outbuilding roof, short-term landlord fix, area under 15 m², or where you expect to replace the whole extension structure within 15 years anyway.
• EPDM, residential extension or balcony roof where longevity matters and budget is moderate; also where complex shapes (round bays, L-shapes) make seamless application valuable.
• GRP, anywhere foot traffic is expected (accessible balcony, terrace, roof of a below-ground extension), anywhere aesthetics matter (smooth painted finish), or where the client wants the longest realistic warranty.

For extensions in England, Building Regulations approval is required for the structural elements of a new flat roof (Part A) and for the insulation specification (Part L). The membrane itself does not need specific approval but must be installed to the drawings submitted. Most flat roof repairs on existing structures are not notifiable under Building Regulations unless more than 50% of the roof (or more than 25% of the building envelope) is being replaced, confirm with your local building control authority.⁶