What are the best boards for a GRP flat roof


OSB3 Tongue & Groove ( Oriented Strand Board ) is the roof deck of choice for laying a new fibreglass roof. It’s rough textured surface allows the resin to get a firm grip and key into the board, meaning there is very little chance of delamination ( the matting peeling away form the surface of the wood ). With standard square edge boards allow 3mm gaps between adjoining boards for expansion.

When using square edge be prepared to

  • Leave a 3-4mm gap between each board edge (for internal movement and expansion)
  • Vigorously abrade plywood to improve bond with 40-60 grit red aluminium oxide paper
  • Apply masking tape to all joints to prevent resin ingress when the bandage is applied *ensure resin can soak into the tape do not use duck tape this is an old style method,
  • Apply an additional glass bandage (450g x 75mm) prior to the main laminate.

T&G – The best is tongue and groove OSB3 but this can be tricky to get hold of. Some fibreglass suppliers and timber yards will deliver T&G ( tongue & groove ) but this can make costs spiral upwards which can be a problem in a competitive market. For any medium to large roof though make the effort to acquire OSB T&G as it has expansion gaps built in by design, which is nice and will make for a better job.

  • T&G profile prevents the boards from distorting (or“dishing”) as the laminate cures. 
  • T&G boards do not require an additional CSM ‘bandage’ on the joints.

  • T&G boarding saves time and materials. 

  • T&G boards do not have to be joined on beams or noggins.

  • Used writing side up the fibreglass bond is much stronger than onto plywood.

  • No surface preparation is required (plywood must be sanded before applying laminate)

  • 8x2 boards are easier to handle and to transport. 

  • Writing side up! Note the small gap formed along the joints - resin will run into the gap and bond the boards together when the laminate is applied.