Why Do Some Fibreglass Roofs Crack or Pinhole?

1. Laying Resin Over Wet Timber or the Wrong Boards

Polyester roofing resin hates moisture. Before a single drop of resin is mixed, the timber deck underneath must be bone dry—below 18% moisture content when checked with a proper two-pin moisture meter. If a roofer turns up after a frosty morning in Reading or a damp autumn morning in Oxfordshire and starts glassing over slightly damp boards, that trapped moisture has nowhere to go.

As soon as the sun comes out and heats the dark grey fibreglass surface, the trapped water turns to vapour, expands, and pushes the laminate away from the wood. That is what causes hollow-sounding blisters and peeling sheets of topcoat.

Just as bad is using the wrong sheet material. You can never lay GRP over old bitumen felt, standard builders' plywood, or interior chipboard. The glue in standard plywood and chipboard reacts with styrene in the resin and stops it bonding. A proper GRP system must go onto brand-new 18mm OSB3 sterling board with tongue-and-groove (T&G) edges, fixed down with ring-shank nails or decking screws so the joints cannot bounce up and down when someone walks on the roof.

2. Why Pinholes Appear in Fibreglass Flat Roofs

If you climb up to inspect a leaking garage or extension roof and spot tiny craters or needle-sized holes across the grey topcoat, you are looking at pinholing. Left alone, rainwater sits in those tiny pits, freezes in winter, and slowly splits the resin layer open.

Pinholes happen during the wet lay-up stage for three main reasons:

• Skipping the paddle roller: When saturating 450g or 600g chopped strand fibreglass matting with resin, you cannot just roll it on with a fluffy paint roller and walk away. You have to work the wet mat thoroughly with a metalribbed consolidation roller (a paddle roller) to break down the binder in the glass fibres and drive out every microscopic air bubble.
• Starving the mat of resin: Trying to stretch a 20kg drum of resin too far leaves dry white patches of glass fibre poking up. When the coloured topcoat is rolled over those protruding fibres, tiny air pockets form around the bristles and pop as it cures, leaving pinholes behind.
• Laying in blazing midday sun: On a hot summer day, dark roof boards heat up fast. Air inside the timber expands and blows tiny bubbles up through the wet resin before it can gel. Experienced roofers shade the deck or lay resin early in the morning before the sun hits the roof.

3. Loud Banging Noises and Cracking on Large Roofs

Have you ever heard a fibreglass roof make a loud "crack" or "bang" on a hot afternoon or a freezing night? GRP cures into a very rigid, hard shell. On a standard garage, bay window, or dormer roof under 30 square metres, normal thermal expansion is handled easily by a gap left around the perimeter trims.

However, if an installer lays one continuous sheet of fibreglass across a huge kitchen extension without leaving a 10mm–15mm expansion gap around the walls, or forgets to install flexible expansion joints on larger spans, the roof is locked tight. When the temperature swings from -4°C in January to 30°C in July, the timber and fibreglass move at slightly different rates. Because the rigid GRP can't slide, the stress builds up until it snaps along a board joint or rips the drip trims clean off the fascia.

4. Bad Catalyst Mixing ("Hot Mixes" vs Under-Cured Resin)

Fibreglass resin cures through a chemical reaction triggered by a liquid hardener (catalyst), which has to be measured accurately at 1% to 4% depending on the outside temperature.

When someone tips catalyst straight out of the bottle by eye instead of using a graduated measuring dispenser, two things go wrong. Pour in too much catalyst (a "hot mix") to beat incoming rain, and the resin gets scorching hot in the bucket, shrinks aggressively, and turns as brittle as glass—meaning it will spider-crack within its first winter. Put in too little catalyst on a cold day in Tadley or Basingstoke, and the roof stays tacky for weeks, washing white streaks down your brickwork the first time it rains.

Repair vs Full Strip and Re-Deck: How to Tell Which You Need

Building Regulations When Replacing a Failed Flat Roof

If you are just sanding and over-coating a localised crack, you don't need council paperwork. However, if more than 50% of a flat roof above a heated room (like a kitchen extension, bedroom, or home office) has to be stripped back to the joists because the boards are rotten, Building Regulations Part L kicks in.

That means you are legally required to upgrade the thermal insulation to current U-value standards—typically converting it into a warm roof with rigid PIR insulation boards—unless it is an unheated detached garage or porch. As an NFRC and TrustMark accredited contractor, we advise you upfront on whether your roof needs building control sign-off and handle the specification properly so you aren't caught out when you come to sell your house.

Common Questions About Fibreglass Roof Problems