Fibreglass Roof installation process explained

Fibreglass roofing, also known as GRP (glass reinforced plastic) roofing, is used as a flat roofing system because it is applied as a liquid. It is a worthy replacement for older felt systems because it is expected to stay leak-free for the duration of its life. This is because the material is sealed and completely solid when dry.

There are four layers of a fiberglass roof:

  • OSB boards (the deck)
  • Fibreglass roll
  • Resin
  • Topcoat
  • Non-slip aggregate (optional)

Here’s how fibreglass (GRP) roofing is installed:


First of all, fibreglass roofing systems can only be installed in dry weather. The ambient temperature should also be no lower than 7°C and no higher than 30°C. These conditions will allow the material to cure properly.

OSB boards

The first stage involves installing OSB boards (unless already installed) and inspecting them to ensure they are flat with the correct pitch where necessary.

It is common practice to use tongue and groove roof decking. If regular OSB boards are used, the joints should be taped using masking tape. This is necessary so the laminate stays in situ and does not run between the joints.

Installing edge trims

Next, we install GRP edge trims which are installed using staples or nails. These create a border around the area. Once these are installed and a good finish is achieved, we can move onto the messy but rewarding wok of laminating.

Laminating prep

Next, a resin is mixed, and a catalyst is added before use. The resin is applied to bandage and trims first. This gives an indication of drying time.

Fibreglass roll

Once the resin around the bandage and trims is dry, we can cut fibreglass roll using the footage of the roof as our template. The fibreglass can be cut in smaller halves so it can be wet out in stages.


Next, the fibreglass roll is ‘wet out’ with the resin we used for the edge trims. This is performed using a roller. The resin seeps into the fibreglass fabric, and once dry, will stick to the OSB boards forming a solid, impenetrable surface.

When wet, the fibreglass can be positioned perfectly. There is a curing time allowing for a margin of error. This is usually 30 minutes.

Laying down the laminate requires further resin application. Approximately 1/3 of the resin is applied to the board and 2/3 to the fibreglass mat. The mat is rolled out and pressed onto the roof boards to create an even surface.


The topcoat is applied once the laminate can be walked on with no stickiness. This is within 24-hours. The topcoat gives the roof a final seal and a smooth finish. It is typical for topcoats to be non-slip, but this is not always the case in wet weather.

If the roof will be walked on in wet weather, a non-slip aggregate can be installed as an additional layer. This is usually a resin with a sand or grit content. Some topcoats have this baked in as standard. It all depends on the product used.

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