Fibreglass, or GRP (glass reinforced plastic), is a popular covering for flat roofs on extensions, dormers, garages and porches. Laid well, it forms a hard, seamless skin with no laps to open up. Laid badly, or on a deck that moves, it cracks, and a crack in fibreglass lets water straight through to the timber below.
Newcastle Roofs repairs GRP roofs of all sizes. We work out why the roof has failed before we touch it, because a crack along a board joint calls for a different repair from a crazed topcoat or a trim that has come away. Where a repair will last, we repair. Where the deck underneath is wet or rotten, we will tell you plainly that relaying is the better use of your money.
Why fibreglass roofs crack and leak
Most GRP failures we see trace back to one of four causes.
- Deck movement – GRP is rigid, so if the boards underneath expand, shrink or flex, the laminate cracks above the joints. Straight cracks running in a grid pattern are a giveaway.
- Poor trim fixing – drip and kerb trims need bonding with the right adhesive and laminating over. Trims fixed with mastic or too few fixings work loose and crack at the edge.
- Laid in the wrong conditions – polyester resin needs a dry deck and suitable temperatures to cure. Resin laid on damp boards or on a cold day may never cure fully and can delaminate.
- Topcoat wear – the coloured topcoat protects the laminate from sunlight. A thin or tired topcoat crazes, lets moisture into the matting and allows the surface to deteriorate.
How a GRP repair is done
A lasting repair means bonding new laminate to sound, clean material. We grind back the damaged area and a margin around it, wipe it with acetone to remove dust and contamination, then lay new chopped strand mat wetted out with resin and rolled to drive out air. Once the laminate has cured, it is sealed with a fresh layer of topcoat.
For cracks over board joints, we may reinforce the joint first, because simply laminating over will often crack again as the deck keeps moving. Loose trims are removed, rebonded and laminated back in. Where the whole surface is crazed but the laminate is sound, a new topcoat across the full roof can restore protection. Some colour difference between old and new topcoat is normal, as the original will have faded.
Weather, seasons and GRP in the North East
Fibreglass work depends on the weather more than most roofing. Resin manufacturers set a minimum air temperature for laying, typically somewhere around 5–8°C, and the deck must be completely dry. Rain or heavy dew on uncured topcoat can spoil the finish. With short winter days and frequent damp spells on Tyneside, there are times when proper GRP work has to wait.
If your roof is leaking in midwinter, we can make a temporary repair to keep water out, then return in a suitable spell to laminate it properly. Many GRP roofs here cover the rear offshoots and dormers of terraced houses and Tyneside flats, where the roof drains onto another roof or into a shared gutter. We check those outlets and junctions, because a blocked outlet causing standing water speeds up wear on any flat roof.
Repair or relay?
When water has been getting in for some time, the boards under the GRP can swell and rot. You may notice a spongy feel underfoot or a sagging ceiling below. Patching laminate onto a wet deck traps the moisture and fails quickly. In that case we strip the roof, replace the deck with new boards suitable for GRP, and lay a new laminate and topcoat with fresh trims. Replacing the waterproofing over most of a roof can also mean the insulation should be upgraded under Building Regulations, and we explain what that involves.
