Single-Ply vs Felt vs GRP: Which Flat Roof Is Best?
If you're replacing or building a flat roof, you'll come across three main options: traditional felt, GRP (fibreglass) and single-ply PVC. They're not equal — here's an honest look at how they stack up, and why JLH only fits single-ply.
Felt (built-up bitumen)
Felt is the old-school option: layers of bitumen sheet bonded with heat or adhesive. It's cheap up front, which is its main appeal. The downside is lifespan and reliability — felt is prone to blistering, splitting and lifting at the laps as it ages and moves with temperature. Realistically you're looking at a shorter service life, and repairs tend to beget more repairs.
GRP (fibreglass)
GRP is a glass-reinforced resin laid and rolled by hand, then topcoated. Done well it gives a seamless finish, but it's heavily weather- and skill-dependent: it needs dry, warm conditions to cure, and quality varies a lot between installers. Because it cures rigid, GRP can become brittle over time and is prone to cracking — especially on roofs that flex or have movement in the deck.
Single-ply PVC
Single-ply is a flexible, factory-made membrane laid in large sheets and joined with hot-air welded seams — the seams fuse into one continuous, homogeneous skin. No glued laps to lift, no brittle resin to crack. It stays flexible and UV-stable for decades, fits in almost any weather, and gives a clean, architectural finish.
How they compare
| Factor | Felt | GRP | Single-ply |
|---|---|---|---|
| Lifespan | 10–20 yrs | 20–25 yrs | 30–40+ yrs |
| Joints | Lapped | Hand-laid resin | Hot-air welded |
| Flexibility | Moves/blisters | Goes brittle | Stays flexible |
| Install weather | OK | Fussy | Almost any |
| Finish | Dated | Good | Cleanest |
So which is best?
For the vast majority of flat roofs, single-ply PVC offers the best balance of durability, reliability and value. That's why we specialise in it and don't fit felt or GRP. One caveat: single-ply is only as good as the installer — the performance is in the welds, so use a specialist. If cost is your main question, see our flat roof cost guide.
Get expert advice on your roof
JLH are single-ply flat roofing specialists covering Rotherham and across South Yorkshire, backed by a 20-year workmanship warranty. Book a free, no-obligation survey and we'll give you honest advice and a clear, fixed quote.
Frequently asked questions
Why don’t you fit GRP or felt? +
We chose to master the best system rather than dabble in several. Single-ply’s welded seams, longer life and cleaner finish make it the better long-term choice, so it’s all we fit.
Is single-ply more expensive than felt? +
A little more up front, but it lasts two to three times as long, so it’s usually cheaper over the life of the roof.
Can single-ply go over my existing roof? +
Often yes — if the structure is sound, an overlay with new insulation and membrane avoids a full strip-out. We’ll confirm at survey.
Need a flat roof done properly?
Book a free, no-obligation survey across Rotherham & South Yorkshire.