How To Fibreglass a Roof
Installing a fibreglass (GRP) roof is one of the most effective ways to create a durable, leak-proof finish for flat roofs. This guide covers the end-to-end installation process for homeowners and tradespeople looking for a professional-grade result.
Quick Answer: Key Takeaways
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Best Substrate: Always use 18mm OSB3 boards (specifically Tongue & Groove) for the deck.
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Weather Matters: Never install in the rain or if the temperature is below 5°C or above 35°C.
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The “Sandwich”: The system consists of timber decking, a resin/chopped strand mat laminate, and a protective topcoat.
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Precision Mixing: Resin cures fast; mix in small batches (1â2 litres) to avoid wastage.
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Safety First: Use GRP-specific tools (paddle rollers) to remove air bubbles, which are the leading cause of roof failure.
What is Fibreglass Roofing?
Fibreglass roofing, technically known as GRP (Glass Reinforced Polyester), is a cold-applied liquid roofing system that hardens into a single, seamless waterproof membrane.
When You Need It / Why It Matters
GRP is the gold standard for flat roofs on extensions, garages, and balconies. Unlike traditional felt, it has no seamsâmeaning there are no joints to perish or leak. It typically lasts 25â30 years and is incredibly resistant to foot traffic and UV damage.
Fibreglass Roofing Material Options
Choosing the right base is critical for the resin to bond correctly.
| Component | Recommended Type | Purpose |
| Decking | 18mm OSB3 T&G | Provides a stable, moisture-resistant base. |
| Reinforcement | 450g or 600g Chopped Strand Mat | Provides structural strength to the resin. |
| Finish | UV-Resistant Topcoat | Protects the laminate and provides the colour. |
| Alternative Deck | 18mm Hardwood Plywood | Used in specific structural builds (requires extra sanding). |
How to Choose Your Materials
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If itâs a standard extension: Choose OSB Board (specifically OSB3). The textured surface allows the resin to “key” or grip the wood perfectly.
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If there is heavy foot traffic (e.g., a balcony): Use a heavier 600g glass mat instead of the standard 450g.
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If you need a premium finish: Use 25mm Hardwood Plywood for maximum rigidity, though OSB3 remains the industry standard for GRP.
Step-by-Step Installation Guide
1. Prepare and Lay the Deck
Remove the old roofing material. Ensure the joists are dry and sound. Lay your 18mm OSB3 T&G boards at a 90-degree angle to the joists. Leave a 25mm expansion gap against any walls. Fix the boards using 63mm galvanised nails at 200mm intervals.
2. Fit Edge Trims
Install GRP edge trims (drip trims and upstands) around the perimeter. If your fascia boards are uneven, you can first install a thin strip of 11mm OSB or 15mm OSB as a “backing” to ensure the trims sit perfectly flush. Nail them to the decking every 300mm. For joints between trims, use a polyurethane adhesive to ensure a watertight seal before laminating.
3. Laminating (The Fibreglass Layer)
Lay out your chopped strand mat and cut it to size, allowing a 50mm overlap at joints. Mix your resin with the liquid catalyst (hardener). Apply resin to the board, lay the mat on top, and apply more resin until the mat becomes transparent.
4. Consolidate the Surface
Use a metal paddle roller to roll over the wet laminate. This is the most important step; it removes air bubbles and ensures the resin fully saturates the glass fibres.
5. Applying the Topcoat
Once the laminate is “tack-free” (firm but slightly sticky), sand down any rough fibres. Mix the topcoat with catalyst and apply it with a roller at a rate of roughly 0.5kg per square metre. This provides the final waterproof seal and UV protection.
Common Mistakes to Avoid
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Moisture Traps: Never apply resin to damp boards; the roof will eventually delaminate (peel off).
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Too Much Catalyst: Adding too much hardener in hot weather will cause the resin to “cook” and crack.
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Poor Rolling: Failing to use a paddle roller leaves air pockets that expand in the sun, causing “star cracking.”
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Wrong Board Side: Always lay OSB3 with the writing side up to ensure the best resin adhesion.
FAQs
Can I apply fibreglass over old felt?
No. Fibreglass must be applied to fresh, clean timber (ideally OSB3) to bond correctly. Applying it over felt will lead to immediate failure.
How long does a fibreglass roof take to dry?
Depending on the temperature and catalyst ratio, it usually becomes “tack-free” in 20-60 minutes. The full cure takes 24 hours.
When should I call a professional?
If the roof is over 40 square metres or requires complex structural changes to the joists to create a “fall” (slope), hiring a pro is recommended to ensure proper drainage.
Summary + Next Steps
A GRP roof is a “fit and forget” solution if installed correctly. The secret lies in the quality of your timber deck and the thoroughness of your paddle rolling.
