The Short Answer
GRP for anything you might walk on or anything with awkward shapes. EPDM for big simple roofs like garages where one sheet covers the whole roof. Felt where the existing roof is felt and the deck is sound. All three, laid properly on a dry deck with the right falls, will last 20 years and more.
GRP Fibreglass
Resin and glass mat laid wet over a plywood deck and cured hard into a single seamless skin. No joins, so nothing to fail at a seam. Walkable. Takes complicated shapes, upstands and outlets well. Needs a dry deck and a dry day to lay. 25 years and more when it is done right, which is why it is our first choice on extensions and dormers.
EPDM Rubber
A single sheet of synthetic rubber, glued to the deck. Quick, flexible, unaffected by cold, and very good on a large simple roof where one sheet does the whole thing with no joins. Less happy on complicated shapes with lots of upstands, where the joins multiply. 25 years is normal.
Torch on Felt
Modern high performance felt in two or three layers, bonded with a torch. Still the right answer on plenty of roofs and the right repair on many existing felt roofs. Fifteen to twenty years. The old three layer felt from the 1970s and 80s, which is most of what we replace, was good for about fifteen.
Falls and Outlets
Standing water is the enemy of every flat roof, whatever it is made of. A flat roof is not flat; it should fall gently to an outlet. We build the falls in with firrings or tapered insulation and put the outlets where the water actually wants to go. Ponding on a new flat roof means someone skipped this.
The Edges
A flat roof fails at its upstands, its trims and its outlets. Never in the middle. The detailing around the edge is the whole job and it is the part that gets skipped when a roof is done cheaply. We photograph the edges on every flat roof we finish, because that is where the work is.
Repair or Replace
If the deck is dry and the failure is local, repair. If water has got under the covering the deck is wet and a patch on top traps it. We check the deck before we recommend either. A soft or springy feel underfoot means the deck is wet.