There is no universal winner. A good flat roof system is one that suits the roof shape, substrate, details and budget and is installed properly. Here is the useful difference between the common options.
EPDM rubber
EPDM uses a flexible rubber membrane and is particularly useful on simple roof shapes where large sheets can minimise joints. Detailing around outlets, edges and penetrations still determines how well the finished roof performs.
GRP fibreglass
GRP forms a rigid, seamless surface when installed correctly. It can produce a crisp finish around edges and complex shapes, but the deck and installation conditions need to be suitable because movement and poor preparation can lead to cracking.
Modern roofing felt
Torch-on and other modern felt systems are very different from the thin shed felt many homeowners picture. Multi-layer systems are widely used and can be robust, repairable and well suited to many domestic and commercial applications.
Choose the system after checking the roof
Drainage, deck condition, upstand heights, insulation and penetrations often matter more than the marketing name on the membrane. A survey should come before the product decision.
Related help
Start with four facts about the roof
Before comparing materials, establish whether the room below is heated, whether the deck is sound, how water drains and how complicated the edges are. A simple garage and an extension with several rooflights can call for different systems. The available height beneath window sills and wall flashings also matters if insulation is being added. These constraints narrow the options more usefully than choosing the material with the longest advertised lifespan.
Compare complete specifications
Ask each quotation to describe the preparation, retained or removed layers, deck repairs, insulation and perimeter details. EPDM needs compatible bonding and junction components; GRP needs a suitable stable deck and curing conditions; felt needs the correct layer and installation system. None of those requirements is captured by a price for “a new flat roof”. Compare access, waste handling and the extent of outlet and rooflight work too.
Why an overlay is a separate decision
Keeping an existing covering may reduce disruption, but the retained roof must be suitable. Trapped moisture, weak decking and poor adhesion can make an overlay inappropriate. The finished height at edges and thresholds also changes. Ask what has been inspected and why retention is justified. A designed refurbishment can be sensible; concealing an uncertain roof is not the same thing.
Think about the next ten visits to the roof
Will window cleaners, aerial installers or maintenance staff need access? Are there trees dropping debris into an outlet? Will equipment be added later? The installation should include a practical maintenance plan and any protection needed for expected access. Keep the system name and guarantee paperwork so future repairs use compatible materials.
The decision to make with your roofer
Choose the system that can be detailed reliably on your building and supported by a clear specification. Ask how the roof will drain, how moisture is controlled and what is included at the edges. Our materials guides explain EPDM, GRP and felt separately, and an inspection can establish the condition that a general comparison cannot.
This guide is general information, not a substitute for inspecting the individual roof or checking current planning/building requirements for a specific property.