Flat roof cost: GRP versus EPDM versus felt
The covering is the part everyone compares, yet it is rarely the biggest line on a flat roof quote. The deck underneath it, the insulation build up and the number of upstands and outlets usually matter more to both the price and how long the roof lasts.
Torch on felt is cheapest to install and shortest lived. EPDM rubber suits larger, simple areas in one seamless sheet. GRP costs most and looks the sharpest on visible bays and porches, but it is weather dependent to lay and unforgiving of movement.
Torch on felt
Modern high performance felt in two or three layers is a long way from the bitumen sheets people remember. It is the most affordable option, copes with awkward shapes, and can be laid in cooler weather. It has the shortest realistic service life of the three and fails at laps and upstands first.
EPDM rubber
A single sheet of rubber membrane, cut to size, with no seams across the field on most domestic roofs. That seamlessness is its real advantage on garages and larger extensions. It tolerates building movement well and is quick to lay. Detailing at corners and outlets is where the skill sits.
GRP fibreglass
A laid in place resin and matting system that cures to a hard, seamless shell with neat trims. It looks best where the roof is visible from a window or a bay, and it takes foot traffic better than the others. It must be laid in dry conditions above a minimum temperature, and it needs a stable deck because it does not flex.
The deck is the hidden cost
Old chipboard or soft OSB has to come off. A new deck of exterior grade ply, plus new firrings to create a fall, is routine on older flat roofs and can be a third of the job. Any quote that has not looked at the deck condition is guessing.
Warm roof build ups and regulations
Replacing more than half a flat roof covering brings insulation standards into play. A warm roof puts the insulation above the deck, which raises the finished height and means the door thresholds, upstands and trims all have to be reworked. It is the right build up, and it is why a modern flat roof is not a like for like swap.
Cost of ownership, not just installation
Divide the installed price by realistic years of service and the ranking changes. Felt is cheapest today. EPDM and GRP usually work out lower per year, and both need far less attention in between.
Which we would specify where
Large simple garage or rear extension roof, EPDM. Visible bay, porch or small roof that people look down onto, GRP. Complex shape, tight budget, or a roof with an uncertain future because of planned building work, felt.
FAQs
Manufacturers publish their own figures, and installation quality moves them more than the brand does. In practice we see felt needing attention first, with EPDM and GRP both going considerably longer when the deck and falls are right.
Sometimes, but we rarely recommend it. Overlaying traps moisture in the old build up and hides deck rot, and it makes the insulation upgrade harder to achieve properly.
The falls have flattened, usually because the firrings or the deck have sagged. Re laying the covering without correcting the fall just moves the problem to a new membrane.
Yes, in writing, and we set out what is covered. We also tell you honestly when a roof has enough life left that a repair beats a replacement this year.
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