Flat roof systems compared: torch-on, EPDM and GRP

The short version

• Torch-on felt: best for larger or complicated roofs, and the easiest of the three to patch later.
• EPDM rubber: best for simple rectangles, quick to fit, and no joints across the middle of the roof.
• GRP fibreglass: best where the surface will be walked on, and the least forgiving to install.

Torch-on felt

Two or three layers of modified bitumen membrane bonded together with a gas torch. This is not the felt of the 1970s: modern membranes are reinforced with polyester or glass and behave completely differently under heat and movement.

It handles complicated shapes well, which is why it is the usual specification on roofs with several penetrations or awkward junctions. Its real practical advantage is repairability — a damaged area can be cut out and a new piece bonded in, and the repair is as sound as the original.

The thing to ask about is the flame. It is applied next to timber and often next to a neighbour’s wall, so heat protection at abutments and a proper watch period afterwards are not optional.

EPDM rubber

A single sheet of synthetic rubber, cut oversize, relaxed, and bonded down. On a garage or extension roof the whole field can be one piece with no seams at all, which removes the most common place for a flat roof to leak.

It goes down quickly and can be fitted in weather that would stop a GRP job. It is relatively soft, so it does not want foot traffic, and the details at corners and upstands are where the skill is. When it is damaged it patches readily.

GRP fibreglass

Glass matting laid into resin and cured in place, forming one continuous laminate including up the upstands and around the outlets. It sets hard, so it takes foot traffic — which makes it the usual answer for a balcony or a roof terrace.

It is the fussiest to install. It needs dry weather and a workable temperature to cure, and it needs a rigid, properly fixed deck: a deck that flexes will crack an otherwise perfect laminate. That usually means a new deck as part of the job, which is a genuine cost difference rather than an upsell.

What actually decides how long a flat roof lasts

Not the material. Roofs fail at the falls, the upstands and the outlets, and they fail because water is standing on them or because the covering was terminated badly at an edge. A cheaper system detailed well outlasts an expensive one detailed badly, every time.

So the questions worth asking any flat roofing quotation are: what falls are you achieving and how; how high are the upstands; what happens at the door threshold; and what are you doing about the deck. If the answer to the last one is "we will see when we get there", get that in writing as a rate.

And insulation

If the covering is coming off anyway, that is the moment to upgrade the insulation. Doing it as part of a re-cover costs a fraction of doing it as its own job later, and moving from a cold deck to a warm deck removes the ventilation problem that causes condensation people mistake for leaks.

More on flat roofing work.