• How To Fibreglass a Roof

    | by Holly Wood

    Welcome to our comprehensive guide on “How to Fibreglass a Roof,” where we will walk you through the step-by-step process of fibreglass roof installation, specifically focusing on fibreglass flat roofing.

    Fibreglass roofing, often referred to as GRP (Glass Reinforced Polyester) roofing, is an excellent choice for flat roofs due to its durability and weather resistance.

    Whether you’re a seasoned DIY enthusiast or a homeowner looking to tackle a roofing project, this guide will equip you with the knowledge and techniques necessary to successfully install a fibreglass flat roof.

    What Is Fibreglass Roofing?

    To avoid any misunderstanding, it’s important to distinguish between insulating a roof with fibreglass insulation and the concept of a ‘fibreglass roof.’

    The term ‘fibreglass roof’ specifically pertains to a flat roofing system that utilises fibreglass as a waterproofing layer, typically applied over OSB3 timber.

    This system involves a composite laminate coated with a protective resin topcoat, which is sometimes referred to as a flow coat or gel coat.

    Pre-Installation Preparations

    1. Check the Weather Forecast

    Always start by checking the local weather forecast to ensure the conditions are suitable for roof installation. Additionally, avoid laying fibreglass roofs during rainy or excessively humid conditions, as moisture can cause future problems.

    2. Temperature Control

    Keep the temperature in mind. Do not use resin or topcoat in temperatures exceeding 35 degrees Celsius, as high temperatures can cause the material to melt, resulting in a poor finish.

    3. Mix Small Batches of Resin

    When mixing resin, opt for smaller batches than usual. This allows ample time to apply the resin before it starts to catalyse, ensuring a smooth application.

    4. Apply Laminate in Short Runs

    Always apply the laminate in the shortest runs possible across the fibreglass GRP flat roof. Not only do shorter runs reduce the risk of the resin hardening too quickly before you can work with it, but they also make the process more manageable.

    5. Timing is Key

    If possible, apply the topcoat out of direct sunlight or later in the day. This may extend the installation time, but it prevents the need for re-top coating if the job isn’t done adequately the first time.

    6. Choose the Right Roofing Boards

    Fibreglass roofs are constructed atop a timber decking framework, usually consisting of oriented strand boards (OSB). These boards are installed at right angles to the roof’s joists or rafters.

    Opt for OSB3 boards (2.4m x 0.6m x 18mm tongue and grooved OSB) as they are designed to minimise expansion and contraction, are easy to handle, and do not require edge support between joists or bandaged joints.

    7. Choose the Right Insulation

    Check if your existing roof already has insulation. If it does, you’ll want to determine whether it complies with the necessary standards. If it doesn’t meet these standards, you may need to consider upgrading your roof insulation to meet compliance requirements.

    Step-by-Step Guide to Fibreglass Roofing

    Prepare the Deck

    1. If the substrate is unfit for laying boards directly, remove the surface.
    2. Ensure the deck is dry and free from moisture to prevent board movement and joint failure.
    3. Inspect roofing joists for soundness and replace any rotted ones. Create a fall in the substrate for proper drainage.

    Lay the Deck

    1. Lay 18mm OSB3 boards lengthwise at 90° to the roof joists with the writing side facing up. This enhances adhesion between boards and resin.
    2. Start laying boards furthest from the drip, leaving a 25mm expansion gap along the walls. Trim the last board flush with the fascia.
    3. For gaps over 400mm, use offcuts to start the next row of boards. Stagger and bond them for a strong deck.
    4. Fix boards to timber joists with galvanised ring shank nails (63mm or longer, 200mm apart).

    Fit GRP Edge Trims

    1. Fix edge trims to the decking board with nails or staples, ensuring they bond to the matt finish side.
    2. Additionally, for F300 Flat Flashing and D260 Angle Fillet, use Polyurethane (PU) adhesive instead of nails or staples. Apply adhesive with a skeleton gun and secure the trims.

    Prepare for Fibreglass Roof Laminate

    1. Mask off the entire roof to avoid spillage during laminating.
    2. Mix the resin thoroughly before use and prepare enough to complete the day’s work.
    3. Mix a small quantity of resin (1-2 litres) to laminate corners and bandage trims. This helps determine curing time and catalyst ratio.

    Mixing Resin and Laying Chopped Strand Mat

    1. Mix resin and catalyst based on temperature conditions and manufacturer’s instructions.
    2. Next, lay the chopped strand mat parallel to the drip, overlapping it by at least 50mm. Roll out and cut to fit, ensuring even coverage.
    3. Bandage corners and trim joints using well-mixed, catalysed resin.
    4. Apply resin to the board and mat in a 1:2 ratio (1/3rd on the board, 2/3rd on the mat). Wet out the mat evenly.
    5. Use a paddle roller to consolidate the laminate and ensure even coverage. Check for any pinholes or areas with insufficient resin.

    Preparing for Top Coating on Fibreglass Laminate

    1. Sand corners and trim bandages with 40-grit sandpaper to remove any unsightly fibres.
    2. Cut the excess cured mat protruding beyond the trim with a sharp knife.
    3. Seal edges abutting walls with clear silicone sealant.

    Applying the Topcoat

    1. Calculate the amount of topcoat needed (0.5kg/m²) and add the required catalyst. Stir well.
    2. Apply topcoat immediately after the laminate is semi-cured or within 24 hours to ensure a good bond.
    3. Use a 7” polyester roller for the main body of the roof and a 2½” roller for the trims. Do not apply the topcoat too thickly to avoid cracking.


    With careful preparation, attention to detail, and adherence to temperature guidelines, you can successfully install a fiberglass roof that will provide long-lasting protection.

    Consequently, your efforts will result in a durable and weather-resistant fibreglass flat roof that enhances the overall quality of your property.

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