Three systems cover almost every domestic flat roof in Oxfordshire: EPDM rubber, GRP fibreglass and high performance torch on felt. They are not interchangeable. The right one depends on the size and shape of the roof, whether anyone will walk on it, how long you intend to keep the property and what the deck underneath is doing.
A side by side comparison with a straight recommendation for each type of flat roof.
If it is a garage or a simple rear extension and nobody will walk on it, EPDM is usually the best value over its life. If it is a balcony, a terrace or anything with regular foot traffic, GRP. If the budget is genuinely the deciding factor and the structure below is not living space, modern felt done properly is not a bad roof.
Lifespan and cost over time
Felt costs the least to fit and lasts the least time, at roughly 15 to 20 years. EPDM and GRP both sit in the 25 to 30 year bracket and beyond when installed correctly.
On a 25 square metre extension roof, the felt option might save you a four figure sum today and cost you a full replacement a decade before the alternatives would need one. On a detached garage you may never care. On a roof over a kitchen you almost certainly will.
Seams, movement and where roofs actually fail
EPDM has no seams across the field of most domestic roofs, which removes the classic failure point, and it stretches with building movement.
GRP is seamless too, but it is rigid, so it transfers movement into the deck rather than absorbing it. That is fine on a properly built new deck and a problem on an old one.
Felt has laps throughout. Modern torch on laps are strong when they are properly fused, but there are simply more joints, and every joint is an opportunity.
Walkability, appearance and repair
Only GRP is designed to be walked on regularly. EPDM handles occasional maintenance access but is not a terrace surface. Felt sits between the two and will scuff.
For repairs, felt is the easiest and cheapest to patch. EPDM patches cleanly with the same material. GRP repairs are perfectly possible but need matching resin work and a cure window.
Visually, GRP takes a coloured topcoat and looks the sharpest from a window above. EPDM reads as a flat dark surface. Felt shows its mineral finish and its laps.
What we would specify, roof by roof
These are the recommendations we give most often across Oxfordshire, and we will always explain why on your specific roof rather than defaulting to the same answer.
Detached garage, no living space below: felt if budget led, EPDM if you want to forget about it
Single storey rear extension over a kitchen or living room: EPDM
Balcony or roof terrace: GRP with anti slip topcoat
Dormer top and cheeks: EPDM, or lead where the property is period
Porch or bay top on a period frontage: lead, or GRP where lead is not viable
Large commercial deck: mechanically fixed single ply rather than any of the three
Which flat roof system lasts longest?
EPDM and GRP are broadly comparable at 25 to 30 years or more when correctly installed. Felt is shorter at 15 to 20 years.
Which is the cheapest flat roof?
Felt, upfront. Over 30 years the picture usually reverses, because a felt roof will need replacing at least once more than an EPDM or GRP roof in that time.
Can I put a new system straight over the old roof?
Sometimes, but it hides whatever the old roof is doing. If the deck is wet or soft, overlaying simply buries the problem, and any guarantee is worth much less.
Does a flat roof replacement need building regulations approval?
A full replacement of more than half the roof area normally triggers a thermal upgrade requirement, so insulation comes into it. A like for like repair does not.
Why does my flat roof hold water after rain?
The falls are wrong or the outlet is blocked or badly placed. Standing water shortens the life of any system, so it is worth correcting the falls during a replacement rather than laying a new membrane on the same problem.
The condition and stiffness of the deck rules some systems in and out before anything else is decided.
Foot traffic changes the answer more than any other single factor.
Weigh cost against how long you are staying
A ten year horizon and a thirty year horizon lead to different sensible choices, and neither is wrong.
Check the falls and the outlet
No membrane fixes a roof that ponds. Falls get corrected during the work or the new roof inherits the old problem.
System, thickness, insulation and trims named on the quote so you can compare quotes properly.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.