GRP vs EPDM Flat Roofing: Which is Better for Lancashire's Climate?

If you're replacing a flat roof on a Lancaster garage, kitchen extension or outbuilding, you'll almost certainly encounter two options in any decent roofer's quote: GRP fibreglass and EPDM rubber. Both are significant improvements over traditional three-layer built-up felt, which is rarely appropriate for new installations in a climate as wet as Lancashire's. But they suit different situations, and understanding the difference will help you make an informed decision rather than just accepting whatever is recommended.

What Is GRP Fibreglass?

GRP stands for Glass Reinforced Plastic — the same material used in boat hulls, water tanks and industrial equipment. In flat roofing, it is applied as a cold-liquid system: layers of fibreglass matting are saturated with polyester resin and cured in place, then sealed with a topcoat. The result is a single, seamless, rigid membrane with no joints, seams or overlaps — anywhere a joint exists is a potential failure point, so eliminating them entirely is a significant structural advantage in Lancaster's rainfall levels.

GRP is hard underfoot, can be walked on freely, and is available in a range of topcoat colours. It is the system we most commonly specify for domestic flat roofs in Lancaster because it performs consistently well in sustained rainfall and freeze-thaw conditions, and the 20-year manufacturer guarantee reflects genuine confidence in the material's performance.

What Is EPDM Rubber?

GRP and EPDM flat roofing systems typically carry manufacturer guarantees of 20 years or more, against 10–15 years for traditional built-up felt.

Source: Manufacturer system warranties (own field data 2025)

EPDM (Ethylene Propylene Diene Monomer) is a synthetic rubber membrane supplied in large factory-manufactured sheets that are bonded to the roof deck. Unlike GRP, EPDM is flexible — it accommodates thermal movement in the deck without cracking, which makes it particularly suited to large spans and to roofs with complex shapes that might cause a rigid system to crack under movement stress.

EPDM has a lower profile aesthetically — it has a matte black or grey appearance in standard form — and is slightly more forgiving on irregular surfaces than GRP. Its performance in sustained damp conditions is very good, and properly installed EPDM systems last 25–30+ years in Lancashire's climate.

Which Is Better for Lancashire?

The honest answer is that both are appropriate for Lancashire's climate when correctly installed. The choice usually comes down to the specific job rather than a universal preference:

• Standard garage or small extension (up to ~20m2): GRP is usually the cleaner choice. The seamless application is ideal for simple rectangular shapes, the finish is neat, and the labour cost difference over EPDM is minimal at this scale.
• Large extension or complex geometry: EPDM's flexibility makes it better suited to large flat areas where thermal movement is greater, or where the roof shape includes internal corners, complex abutments or penetrations that are harder to work around with a rigid GRP system.
• Walk-on roof terrace: GRP is the better choice for any roof surface that will be regularly walked on, as EPDM can be punctured by sharp heel impacts over time and doesn't have the same rigid underfoot feel.
• Budget constraint: EPDM is typically 10–20% cheaper than GRP at equivalent quality. If budget is the primary constraint, EPDM from a competent installer is a sound choice.

Why Not Felt?

Traditional three-layer built-up felt — also known as torch-on felt — has a realistic service life of 10–15 years in Lancaster's climate. It is significantly cheaper to install upfront but generates recurring replacement costs across the life of the building. On a garage or extension in Lancaster that will still be standing in 30 years, the total cost of two felt replacements significantly exceeds the one-off cost of a GRP or EPDM system. We no longer install felt as a primary flat roofing system — only GRP or EPDM where the long-term view is taken.

Typical Lancaster Flat Roof Costs

A standard single garage (~15m2) in Lancaster: £900–£1,800 in GRP, or £800–£1,500 in EPDM. A kitchen extension (~25m2): £2,000–£4,000 in GRP, or £1,800–£3,500 in EPDM. Prices include deck inspection and any necessary boarding, upstand detail and all trim. A free written quote gives you the exact figure for your specific roof area.