Flat roofs have a bad reputation in Scotland, and it’s half deserved. A badly laid flat roof will fail in five years. A properly laid one will outlast the extension underneath it. The difference is almost entirely in the detailing — the falls, the upstands, the edges — and none of it is visible from the ground.
Roofs don’t have an expiry date stamped on them. They have symptoms. The reliable ones:
A "flat" roof should never actually be flat. It needs a fall — a slope — so water runs off. If water sits in a puddle for days after rain, it will find a way in eventually, and it will accelerate the breakdown of whatever it's sitting on. Ponding is the most common defect we find, and it's a design fault, not a wear fault. It was built wrong.
Water gets into a flat roof at the edges — where it meets a wall, where it turns up at a parapet, where it goes over a drip at the fascia. The middle of a flat roof almost never fails. The perimeter almost always does.
Old torch-on and pour-and-roll felt systems become brittle with UV and thermal cycling. Blisters form where moisture is trapped underneath, then split.
Fibreglass is rigid. If it's laid over a deck that moves — or without proper expansion detailing at joints — it cracks. Well-laid GRP on a sound deck doesn't do this.
The cheapest failure to prevent and the most common to find. Leaves, moss, a tennis ball.
A liquid resin and glass matting, laid wet and cured into a single seamless sheet. No joints anywhere.
Good for: Small to medium roofs, complex shapes, anywhere you want a seamless finish. Walk-on balconies.
Watch out for: It's rigid, so it needs a sound, non-moving deck (usually new OSB3 boarding). It cannot be laid in the wet or the cold — which in Perthshire genuinely restricts the working window. Anyone laying GRP in December is doing it wrong.
A single sheet of synthetic rubber, usually laid in one piece for a domestic roof, bonded to the deck.
Good for: Most domestic extensions and garages. One sheet means no joints, and it's flexible, so deck movement doesn't crack it. It's also far more tolerant of being laid in Scottish weather than GRP.
Watch out for: The quality is all in the edge detailing and the trims. A cheap EPDM job with badly bonded upstands will leak just like anything else.
Multiple layers of bitumen membrane, heat-bonded.
Good for: Larger roofs, commercial, budget domestic. It's proven — this system has been in use for decades.
Watch out for: Joints exist, so workmanship matters. Cheap single-layer felt is a false economy. A proper three-layer system is a good roof.
For a typical Perthshire extension or garage, EPDM. It handles our weather during installation, it handles our weather afterwards, and it doesn't care if the deck moves a bit.
If you're replacing a flat roof over a habitable room, you have an opportunity you'll only get once to fix thermal performance before closing it back up.
Warm decks keep insulation above the deck to prevent condensation. Cold decks put insulation below and rely on ventilation gaps that frequently block and cause rot.
When stripping a flat roof over a living space, we almost always recommend a warm deck build. It costs more upfront, but avoids a structural problem later.
If it’s a localised failure on an otherwise sound roof, we can repair. If the roof is ponding badly, or the felt is brittle across the whole area, or the deck underneath is soft, replacement is the honest answer. We’ll show you photographs and let you decide.
Most domestic flat roofs are one to three days.
EPDM and felt, yes. GRP, realistically not — it needs dry conditions and reasonable temperature to cure properly, and we won’t lay it into weather that will compromise it. If someone offers to lay you a fibreglass roof in a Perthshire January, that tells you something about them.
Depends on the system and the build-up. If you want a usable balcony or terrace, say so at survey — it changes the specification.
