Materials compared
EPDM, Fibreglass or Felt: Choosing a Flat Roof Build-Up
Rubber, fibreglass and felt differ far less in waterproofing performance than in how they fail, how long they last, and what each one costs per year of that life. Compare flat roof membrane against the attributes that actually decide it, then book a survey. Covered on this page: Torch-on felt · GRP fibreglass · EPDM rubber.
Rubber, fibreglass and felt differ far less in waterproofing performance than in how they fail, how long they last, and what each costs per year of that life. Use settles it before price does.
| System | Per m² | Service life | Per m² per year | Joints in the field |
|---|---|---|---|---|
| Torch-on felt | £40–£60 | 10–20 years | £2.00–£6.00 | Lapped seams, torched on site |
| GRP fibreglass | £60–£100 | 25–40 years | £1.50–£4.00 | Cures seamless |
| EPDM rubber | £80–£120 | 20–30 years, and beyond | £2.67–£6.00 | None in the field on domestic spans |
Torch-on felt costs £40–£60 per m² and lasts 10–20 years, which is £2.00–£6.00 per m² per year. GRP fibreglass costs £60–£100 per m² and lasts 25–40 years, which is £1.50–£4.00 per m² per year. EPDM rubber costs £80–£120 per m² and lasts 20–30 years, and beyond, which is £2.67–£6.00 per m² per year.
Ranked on cost per year of service life the headline order inverts: fibreglass is the cheapest of the three and felt the dearest. All three fail at edges, upstands and outlets before the field, so the installer's edge detailing predicts the outcome better than the system chosen.
3 related pages cover this in more depth: epdm rubber roofing birmingham, fibreglass roofing birmingham and torch-on felt roof installation.
Which flat roof system is best?
Rubber suits most domestic roofs, fibreglass suits any surface taking foot traffic, and felt suits outbuildings on a short horizon. No system is best in general; the intended use and the years required decide it.
- Torch-on felt profile: £40–£60 per m², 10–20 years, lowest capital cost, hot works on site
- GRP fibreglass profile: £60–£100 per m², 25–40 years, rigid and walk-on, deck-dependent
- EPDM rubber profile: £80–£120 per m², 20–30 years and beyond, jointless on domestic spans, cold applied
Which lasts longest?
Fibreglass lasts longest at twenty-five to forty years, followed by rubber at twenty to thirty and beyond, then felt at ten to twenty. Edge detailing moves every one of those figures more than the material does.
Installation cost divided by realistic service life.
- Joint count differentiator: rubber has none in the field; fibreglass cures seamless; felt has lapped seams
- Trafficking differentiator: fibreglass takes foot traffic; rubber and felt do not
Which is cheapest over its lifetime?
Rubber is cheapest over its lifetime. At one hundred pounds per square metre across thirty years it costs less annually than felt at fifty pounds across fifteen. Lowest installation price and lowest lifetime cost are different systems.
All three fail at edges, upstands and outlets before the field.
- Deck dependence differentiator: fibreglass needs a stiff deck; rubber tolerates movement
- Installation conditions differentiator: fibreglass is temperature and moisture limited; rubber is not
Which can you walk on?
Fibreglass takes regular foot traffic and is the system specified for balconies and terraces. Rubber and felt tolerate occasional maintenance access only, and specifying either for a used terrace is a common mis-sale.
Rubber patches bond, fibreglass relaminates, felt is torched.
- Fire risk differentiator: felt applies flame on site; the other two do not
- An installation cost divided by service life ranks systems differently from a headline price.
Where does each one fail?
All three fail at the same places: bonded or lapped edges, upstands terminating short at abutments, and junctions with rainwater outlets. The field of the covering is rarely the component that gives way on any system.
- All three systems fail at edges and outlets before the field.
- A fibreglass takes foot traffic where rubber and felt do not.
- Felt applies flame on site which the other two avoid.
The installer matters more than the material.
Why detailing predicts the outcome
Felt at £40–£60 over 10–20 years is £2.00–£6.00 per m² a year; GRP at £60–£100 over 25–40 years is £1.50–£4.00 — the calculation that inverts the headline ranking. All three fail at the same places, so the installer's edge detailing predicts the outcome better than the system chosen. A trafficked terrace has one answer, a garage has another, and neither is settled by cost per square metre.
Getting a system specified
Hot works against a timber deck attached to a house is a genuine distinction rarely raised in comparisons.
Cost per year of service life, and 5 more
- cost per year of service life
- field
- upstand
- hot works
- deck deflection
- lap
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