Single-ply vs felt vs EPDM — which flat roof is right for you

There are three main families of flat roof covering in regular use on Bath properties: built-up felt, EPDM rubber, and single-ply membrane. There’s also GRP fibreglass, which we don’t install but which you should understand. Here’s how they compare.

Built-up felt

What it is: multiple layers of bitumen-impregnated felt, torched or cold-applied, with each layer bonded to the one below.

How long it lasts: 10-25 years depending on quality of install, exposure, and number of layers. Three-layer felt properly torched onto a sound deck lasts longer than single-layer felt cold-applied.

Pros:

• Cheap
• Most roofers can install it
• Well-understood system

Cons:

• Seams are torched or glued — failure point over time
• Doesn’t like standing water — ponding accelerates failure
• UV degrades the surface over time (mineral surface layer helps)
• Repairs usually mean patches over patches

Where it makes sense: garages, sheds, outbuildings, anywhere the budget is the main constraint and the roof isn’t critical.

EPDM rubber

What it is: a single sheet of rubber (ethylene propylene diene monomer), usually 1.2mm thick, bonded to the deck with adhesive. Seams are joined with primer and seam tape.

How long it lasts: 30-50 years. Properly installed EPDM is one of the longest-lasting flat roof materials available.

Pros:

• Single sheet means no laps on smaller roofs
• Flexible — handles movement well
• UV-stable
• Easy to repair if it does get damaged

Cons:

• Seams (where multiple sheets meet on larger roofs) are the weak point — adhesive can fail over time
• Ponding resistance is good but not perfect
• The aesthetic is plain — a black rubber roof, no colour options

Where it makes sense: small to medium flat roofs, residential extensions, dormers. Particularly good where the roof has an unusual shape.

Single-ply membrane

What it is: a single sheet of synthetic membrane (usually PVC or similar polymer), typically 1.2-1.5mm thick, with seams hot-air welded. The major brands we use are Fatra, Armourplan, and Danosa.

How long it lasts: 30-40+ years. Manufacturer warranties typically run 20-25 years.

Pros:

• Hot-air welded seams — the seam is stronger than the membrane itself, not weaker
• Highly UV-stable
• Resistant to standing water (some products are specifically designed for zero-fall applications)
• Range of colours
• Mechanically fixed or fully bonded depending on substrate

Cons:

• More expensive than felt, similar to EPDM
• Requires a roofer trained in the specific system — not every roofer can install it
• Repairs require the same welding equipment

Where it makes sense: most residential flat roofs. Particularly good on larger flat roofs, exposed sites, and anywhere long-term reliability matters more than upfront cost.

GRP fibreglass

What it is: glass fibre matting laid onto the deck and saturated with polyester resin. Cures to form a single, seamless, rigid surface.

How long it lasts: 20-30 years when installed well. Shorter when it isn’t.

Pros:

• Seamless — no laps to fail
• Can be finished with non-slip coatings
• Looks tidy

Cons:

• The failure mode is bad. When GRP starts to fail, it delaminates from the deck — the whole surface lifts in sheets. There’s no clean way to repair this; the roof essentially has to come off.
• Rigid — doesn’t handle building movement well, can crack at stress points
• Requires dry conditions and skilled installers
• Repairs are difficult to make invisible

Why we don’t install it: the failure mode. A felt roof fails in patches you can repair. An EPDM roof fails at seams you can re-seal. A single-ply roof fails at seams you can re-weld. A GRP roof fails in big sheets that mean starting again. We prefer materials that fail gracefully.

So which do we recommend?

For most Bath flat roofs:

• Single-ply (Fatra, Armourplan, or Danosa) for most situations — extensions, dormers, larger flat roofs, anywhere exposed
• EPDM for smaller or more unusually-shaped roofs where a single sheet works well
• High-quality torch-on felt for outbuildings, garages, and budget-sensitive work
• Not GRP — see above

If you’re not sure which fits your situation, book a survey. We can usually tell from looking at the existing roof what’s been working and what hasn’t, and recommend accordingly.