Why the system matters
A flat roof is not really flat: it is laid with a slight fall so water runs to the outlets. The covering has to stay watertight and flexible through summer heat and winter frost, and it has to be sealed properly at every edge and upstand. The system you choose, and how well it is fitted, decides how long it lasts.
GRP fibreglass
GRP, or glass-reinforced plastic, is laid wet and cures into one hard, seamless layer with no joints to fail. It copes well with foot traffic and more complicated shapes, and it gives a clean, durable finish. It needs dry conditions and care to install well, which is where a proper job shows.
EPDM rubber
EPDM is a tough synthetic rubber laid as a single large sheet, which means very few joints on a simple roof. It is flexible, long-lasting and well suited to larger, straightforward flat roofs such as garages and extensions.
Felt, brought up to date
Modern torch-on felt is a world away from the old pour-and-roll felt. Laid in bonded layers it is a cost-effective, reliable covering for many roofs. As with any system, the detailing at the edges and outlets is what makes the difference between a roof that lasts and one that does not.