Most flat roof problems are not caused by the system chosen. They are caused by falls, edges and outlets. That said, the three systems in common use behave differently, and matching the system to the job genuinely matters.
Felt
Modern torch on felt, laid in layers, is a long way from the pour and roll felt that gave flat roofs their reputation. It is economical, it copes well with complex shapes and awkward details, and a competent installer can dress it neatly around anything. It is the most forgiving system on a roof with lots of upstands and penetrations.
GRP fibreglass
GRP is laid wet and cures into one seamless piece, so there are no joints to fail. It gives a hard, walkable surface and a very clean finish, which makes it a good choice for a balcony or a flat roof overlooked from a window above. It needs a sound, dry deck and settled weather to install properly, and it is less tolerant of movement than the alternatives.
EPDM rubber
EPDM is a single rubber membrane, often laid in one piece on a domestic roof, so again there are no seams across the field. It stays flexible in cold weather and copes well with slight movement in the deck. It is quick to install on a simple rectangle such as a garage, and it has a long service life with very little maintenance.
Which should you choose
For a straightforward garage or extension roof, EPDM is usually hard to beat. For a roof with many upstands, pipes and awkward junctions, felt often detail better. For a surface that will be walked on or looked at closely, GRP tends to win.
What matters more than any of it is that the falls run to the outlet, the outlet is at the low point and clear, and the edges and upstands are properly detailed. Get those wrong and every one of these systems will eventually leak.
We install all three across Portsmouth. If you would like an honest recommendation for your particular roof, call 023 9355 3347 and we will come and look.