Flat Roof Maintenance: Felt, EPDM and GRP Looked After Properly

Know which system you have

Built-up felt is the traditional system, usually with a mineral chipping or grey finish and visible overlapping sheets. Lifespan is typically fifteen to twenty years for a modern high-performance felt, less for older two-layer work. EPDM is a single-piece black rubber membrane, seamless across the main field, with a life of thirty years or more. GRP fibreglass is a hard, resin-finished surface, usually grey, laid in one continuous piece with trims.

Each fails differently. Felt blisters, cracks and splits at laps. EPDM is highly durable in the field but vulnerable at seams, corners and where it has been punctured. GRP is tough but can crack where the deck beneath moves, particularly along joins in the boards.

The twice-yearly check

Clear all debris from the surface and, crucially, from the outlet. A blocked outlet on a flat roof creates a pond, and standing water is the single greatest enemy of every flat roof system. Water that remains 48 hours after rain indicates either a blocked outlet or a fall problem.

Then walk the perimeter and inspect the trims, drip edges and any upstands where the roof meets a wall. Look for lifted or split flashing, gaps in mastic, and any point where the membrane has been penetrated by a fixing, an aerial bracket or a satellite dish.

• Clear debris and check the outlet flows freely
• Look for ponding 48 hours after rain
• Inspect all edge trims and upstands
• Check around any penetration or fixing
• Look for blisters, splits or soft underfoot areas

Warning signs that need action

Blistering in felt means moisture is trapped between the layers; small blisters can be monitored, large ones need patching before they split. Soft, spongy areas underfoot indicate the deck below is wet and beginning to rot, which is a more serious finding than a surface defect.

On EPDM, look at the seams and corners — the field of the membrane rarely fails. On GRP, hairline cracking along a straight line usually means deck movement rather than material failure, and needs the underlying cause addressed rather than just a resin patch.

Repair or recover?

Localised repairs are entirely reasonable on a roof that is otherwise sound and within its expected life: a felt patch, an EPDM seam repair, a GRP resin and matting repair. Typical costs run from £150 to £500.

Recovering makes sense when the roof is at or near the end of its life, when the deck is wet or rotten, or when you find yourself patching repeatedly. Full replacement of a small flat roof — a garage or extension of around 20m² — commonly falls between £1,500 and £3,500 depending on the system chosen and whether the deck needs replacing.

Extending the life you have

Keep the outlet clear and keep foot traffic off it. Where access is genuinely needed, for a boiler flue or an aerial, fit proper walkway protection rather than repeatedly walking on the membrane.

Trim back overhanging branches, since leaf litter both blocks outlets and holds moisture on the surface. And attend to small defects in autumn rather than spring — a split that survives one winter usually does not survive two.

The bottom line

Flat roofs fail from neglected drainage far more often than from material failure, which is good news: the most important maintenance task is also the easiest. Clear the outlet, check for ponding, and walk the perimeter twice a year, and a modern EPDM or GRP roof will run its full expected life without drama.

When a repair is needed, match it to the system. Felt patches on EPDM and resin over a moving deck are both false economies that fail within a season or two. And if you find yourself patching the same roof for the third time, price a recover — repeated repairs on a tired membrane usually cost more over five years than replacing it once.

Frequently asked questions