We get asked about this. GRP fibreglass flat roofing is widely advertised, it’s perfectly serviceable when installed properly, and there’s no shortage of roofers who install it well. So why don’t we?
The short answer
We prefer materials that fail gracefully. GRP doesn’t.
The longer answer
Every roofing material fails eventually. The question is what failure looks like, and whether you can do anything about it.
Felt fails in patches. You can patch a patch.
EPDM fails at seams. You can re-seal seams.
Single-ply membrane fails at seams. You can re-weld seams.
GRP fails in sheets. When the bond between the GRP and the deck fails, the whole surface delaminates in one piece. The roof effectively has to come off and start again.
What delamination looks like
You start to see bubbles in the surface. Then lifting at the edges. Then the whole top layer comes away from the deck, often in one or two large pieces. It’s dramatic, sudden, and not repairable in any practical sense.
When it happens — and it does happen, particularly on roofs that were installed in less-than-ideal conditions or with cheaper resin — there’s nothing to do except strip and replace.
What causes GRP to fail
The usual culprits:
- The deck moves more than the GRP can flex with. GRP is rigid once cured. Timber decks in particular move with seasonal humidity. Over years, that movement breaks the bond.
- The resin was mixed off-ratio. Common with less experienced installers. The mix ratio matters more than people realise.
- The deck wasn’t prepared properly. GRP needs a very specific surface condition to bond properly.
- The original installation wasn’t fully cured when finished. Rushed jobs.
Skilled GRP installers mitigate most of these. But the failure mode, when it comes, is the failure mode.
When GRP is the right answer
Honestly, sometimes. If you want a particular finish (non-slip coating, a specific colour, a particular architectural look), GRP gives you options that the other systems don’t. If you’ve got a skilled installer who regularly does GRP work, and they guarantee it, the system can perform well.
For most residential flat roofs in Bath, though — extensions, dormers, garage conversions, bay windows — we don’t think GRP is the right trade-off. The risk of the bad failure mode outweighs the upside.
What we install instead
Single-ply membrane (Fatra, Armourplan, Danosa) is our default for most flat roofs. Welded seams, long service life, well-understood failure mode.
EPDM for smaller or more complex shapes.
Torch-on felt for budget work and outbuildings.
All three have failure modes that are repairable. All three are systems we know thoroughly.
What to ask a roofer who offers GRP
If a roofer quotes you for GRP and you want to take that forward, ask:
- How many GRP roofs have you installed in the last year?
- What resin system do you use, and is the installer accredited?
- What’s the guarantee, and what’s the failure mode if it fails?
- Have you got any GRP roofs in service longer than 15 years that I can look at?
If the answers are vague, walk away. The same questions apply to any roofing system, honestly — GRP is just the one where the answers matter most.
If you’ve already got a GRP roof
Don’t panic. Most GRP roofs perform fine. The bad failure modes tend to show up on:
- Roofs installed more than 15 years ago
- Roofs on timber decks with significant movement
- Roofs that have had standing water issues
If any of those apply, book a survey and we’ll tell you honestly whether yours is at risk.
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