These are the two systems we are asked to compare most often, and the honest answer is that they suit different roofs. One is a flexible sheet bonded down; the other is a rigid laminate cured in place. The right choice follows from how the roof is built, how it will be used and what the deck is doing.
Movement and deck stability
This is the single most useful question. Timber decks move, with load, with moisture, with temperature. EPDM stretches and returns; GRP cannot, so movement it cannot absorb becomes a crack. On an older garage roof with joists of uncertain history, EPDM is the safer specification. On a well-built, properly fixed new deck, GRP has nothing to fight against.
How the roof will be used
If anyone is going to walk on it regularly, a balcony, a terrace, a roof accessed for plant or maintenance, GRP is designed for that and EPDM needs a protective walkway layer added. If the roof is only ever seen from an upstairs window, that consideration disappears.
- Balcony or terrace: GRP, or EPDM with a walk-on layer
- Garage or extension, foot traffic only for maintenance: either
- Roof visible from above where colour matters: GRP
- Complex shape with many penetrations: often felt or EPDM detailing
Water, weather and installation
EPDM tolerates standing water better, which matters on a roof where the falls cannot be perfected. GRP needs a genuinely dry deck and a suitable weather window to cure properly, so it is more constrained in winter. EPDM can be installed in a wider range of conditions, though the deck still has to be dry for adhesive to work.
Repairs and long-term view
Both repair well when done by someone who knows the system. GRP repairs are chemically integral; EPDM patches bond with the same adhesives and tapes used to build the roof. Expected life is broadly comparable, EPDM commonly quoted at 25 years or more, GRP around 20 to 25, but in practice the deck, the falls and the detailing decide the outcome far more than the choice between these two.
Common questions
Which is cheaper?
They are usually close for a domestic roof, with the deck and insulation work outweighing the difference between the coverings. We price both on the same scope so you can see the real gap for your roof.
Which lasts longer?
On paper EPDM has a slight edge. In practice, both outlast a badly built deck and both fail early over one, which is why we spend the survey time on the substrate rather than debating membranes.
Can either be laid over an old roof?
GRP cannot, it needs a clean, dry board deck. EPDM occasionally can where the existing covering is sound and dry, though we usually recommend stripping so the deck can be inspected.
Which looks better?
GRP can be finished in a colour and gives a hard, even surface. EPDM is a matt dark sheet. If the roof is overlooked from a bedroom window, that is worth deciding before ordering.