Flat roofs on extensions, garages and dormers get replaced more often than a full pitched roof. As a result, the “per square metre” question comes up a lot from homeowners in Kettering and across the area. Since the short answer depends heavily on which of the three common materials you choose, we cover all three below.
The three main flat roofing materials
Felt (built-up or torch-on) is the traditional, cheapest option. It typically runs around £55–£95/m² installed, though some guides put fully installed felt jobs at £80–£120/m² once labour is included. It’s a proven system. But the lowest-cost felt products have a shorter working life than the alternatives below.
EPDM rubber is a single-sheet synthetic rubber membrane, usually priced around £80–£120/m² installed. It has very few joints, often none on a small roof. That’s one reason it tends to be more durable and lower-maintenance than felt over the roof’s life.
GRP fibreglass is laid as a liquid resin that cures into a single seamless surface. It typically costs £95–£140/m² or more, depending on size, access and detailing. It’s usually the most expensive of the three per m². But the seamless finish is popular on roofs with lots of upstands or awkward junctions, where a jointed material is harder to detail well.
(Figures from MyBuilder’s 2026 flat roof cost guide and Checkatrade’s 2026 flat roof replacement guide — always confirm against a written quote for your own roof.)
What a typical project costs in total
Materials make up roughly 60–70% of the total cost, since labour covers the rest. As a rough guide, a small 20 m² EPDM replacement — a typical extension roof — often lands around £2,800–£3,600 installed. A larger 50 m² flat roof can run closer to £7,000–£9,000. Most UK flat roof projects overall fall somewhere between £1,500 and £7,500.
Why flat roofs need replacing more often than pitched roofs
Flat roofs shed water more slowly than a pitched roof. As a result, they’re less forgiving of poor detailing, standing water or UV damage over time. If you’re seeing ponding water, bubbling, or a felt surface gone brittle and cracked, that’s usually a sign it needs replacing rather than another patch repair. A leak that seems to move around after rain is also a classic flat-roof symptom. See our guide on common causes of roof leaks for how we trace it, or our roof repairs service if you’d rather book a survey straight away.
Choosing between the three
If budget is the main driver and the roof is straightforward, felt remains a sound, well-proven choice. Although it costs more upfront, EPDM is usually the better value over 20+ years, since it has fewer joints and needs less maintenance. If your roof has a lot of detailing — rooflights, pipes, changes of level — GRP’s seamless finish often justifies the extra cost. It avoids weak points at every joint.
Whichever material suits your roof, we’ll only recommend what the roof actually needs. Since we cover EPDM, GRP and felt for homes across Wellingborough and the surrounding area, we’re never limited to just one system.
Get a free survey and written quote. We’ll measure the roof, check the deck and existing insulation, and give you a clear price for the material that actually suits your roof.