Step-by-step guide to fibreglass flat roofing installation process

Fibreglass flat roofing, also known as GRP (Glass Reinforced Plastic) roofing, has gained popularity due to its durability, longevity, and ease of maintenance. This guide provides a detailed, step-by-step approach to installing a fibreglass flat roof, ensuring a professional and watertight finish.

1. Preparation and Safety Measures

Before starting the installation, it’s crucial to prepare adequately and follow safety measures:

  • Safety Gear: Ensure you have appropriate safety gear, including gloves, goggles, and a dust mask.
  • Weather Conditions: Choose a dry day for the installation to avoid complications from rain or moisture.
  • Workspace Preparation: Clear the roof surface of any debris, dirt, and old roofing materials. A clean surface ensures better adhesion of the fibreglass.

2. Measuring and Cutting Materials

Accurate measurement and cutting of materials are essential for a smooth installation process:

  • Measure the Roof: Measure the dimensions of the roof accurately, including any parapets, skylights, or other protrusions.
  • Cut Plywood: Based on the measurements, cut the plywood boards to fit the roof. These boards will act as the base for the fibreglass.
  • Dry Fit: Lay the plywood on the roof to ensure a correct fit. Make any necessary adjustments before securing the boards.

3. Laying the Plywood Base

The plywood base provides a stable foundation for the fibreglass layers:

  • Apply Adhesive: Apply a suitable adhesive to the roof joists to secure the plywood boards.
  • Secure with Screws: Use corrosion-resistant screws to fix the plywood boards to the joists. Ensure the screws are flush with the board surface to avoid any unevenness.
  • Sealing Joints: Seal the joints between the plywood boards with a PU (polyurethane) adhesive or jointing compound to prevent any movement or gaps.

4. Installing the Edge Trims

Edge trims are crucial for a neat finish and preventing water ingress:

  • Measure and Cut: Measure and cut the edge trims to fit the perimeter of the roof.
  • Fixing the Trims: Fix the edge trims using nails or screws. Ensure they are secure and level.
  • Corners and Joints: Pay special attention to corners and joints, ensuring they are well-sealed to prevent leaks.

5. Laying the Fibreglass Matting

The fibreglass matting forms the main waterproof layer of the roof:

  • Cut the Matting: Cut the fibreglass matting to fit the roof dimensions. Allow for some overlap at the edges and seams.
  • Mixing the Resin: Prepare the fibreglass resin according to the manufacturer’s instructions. Typically, this involves mixing the resin with a hardener.
  • Applying the Resin: Pour the resin onto the roof and spread it evenly using a roller. Work in small sections to ensure the resin doesn’t cure before the matting is applied.
  • Laying the Matting: Lay the fibreglass matting onto the wet resin. Use a roller to embed the matting into the resin, ensuring there are no air bubbles or wrinkles.
  • Additional Layers: Apply additional layers of resin and matting as needed, especially in areas prone to higher stress or potential leaks.

6. Curing and Finishing

After the fibreglass matting is laid, the roof needs to cure and be finished:

  • Curing Time: Allow the resin to cure according to the manufacturer’s instructions. This usually takes 24 to 48 hours.
  • Inspect for Imperfections: Once cured, inspect the roof for any imperfections or areas that need additional resin.
  • Final Layer: Apply a final layer of topcoat resin. This topcoat provides UV protection and gives the roof its finished appearance.

7. Post-Installation Checks

After the installation, perform thorough checks to ensure the integrity of the roof:

  • Leak Test: Check for leaks by spraying water over the roof. Inspect the interior for any signs of water ingress.
  • Edge Trim Inspection: Ensure all edge trims are secure and properly sealed.
  • Clean-Up: Remove any excess materials or debris from the roof.

Maintenance Tips for Fibreglass Flat Roofs

To extend the life of your fibreglass roof, regular maintenance is essential:

  • Regular Inspections: Inspect the roof at least twice a year and after severe weather events.
  • Clean the Surface: Remove debris, leaves, and dirt from the roof surface to prevent water pooling and potential damage.
  • Check for Cracks: Look for any cracks or blisters in the fibreglass layer and repair them promptly to prevent water ingress.
  • Maintain Drainage: Ensure that all drainage points are clear and functioning correctly to prevent water accumulation.


Fibreglass flat roofing is an excellent choice for its durability, low maintenance, and watertight properties. By following this step-by-step guide, you can achieve a professional installation that will provide long-lasting protection for your property. Remember, the key to a successful fibreglass roof is thorough preparation, accurate application, and regular maintenance.

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