GRP roof decking

Choosing the right decking and how to apply

Recommended Decking

When re-boarding for a fibreglass roof, we recommend using only 18mm 8X2 OSB3 (Oriented Strand Board) these boards not only provide the perfect surface for the resin to bond with but also provide a number of other labour and monetary savings.

Most of these boards also carry FSC Forestry certification, which means the wood used to manufacture the boards has been sourced from well-managed and maintained forests.

Perfect bond

The textured surface allows the resin to secure a firm bond and key into the board, virtually eliminating de-lamination.

Prevents distortion

The tongue & groove profile will stop the boards from distorting as the laminate cures.


The smaller 8×2 tongue and grooved boards have been designed to reduce expansion and contraction and a much easier to carry up ladders.

No jointing required

These boards do not require any additional fibreglass bandage on the joints saving both time and money.

Fixing the deck

For optimum performance a GRP roof should always be applied to fresh OSB3 decking and you should be wary of contractors who suggest boarding over the existing surface. Boarding over is a serious cut corner and will save money but any underlying issues will still be there and may come back to bite you.

If your roof has been holding water (ponding) then you need to fix it at this stage, your roof should have a minimum fall of 1:80 which can be corrected with firring strips on the rafters, before laying the boards. Do not prepare the deck in bad weather, the boards must be completely dry before the resin is applied so if you are leaving overnight make sure you protect from rain or moisture.

The easiest way to fix the boards to the joists is to use a nail gun with a 63mm or longer galvanised ring shank nail at 200mm centres with 4 nails across a 600mm board, if you don’t have a nail gun then you can use screws or nails with a minimum penetration of 40mm.

Start and place your boards from the furthermost edge of the drip section, staggering the joints to create a strong structure, if your boards are going to be laid along a wall or any abutting up stands, always leave an expansion gap of 25mm.

Lay the boards lengthways at an angle of 90° to the roof joists, it’s important to lay the boards “writing side up” this sides rougher texture not only allows the Fibrelast GRP roofing base coat to flow better getting a firm hold and key into the board it also allows the base coat material to run into the boards joints giving a better bond.