What is GRP / Fibreglass?

Glass Reinforced Plastic, also known as GRP, is a strong, lightweight, corrosion and temperature resistant material. It is easily moulded for complex shapes and structures.  It can be also extremely cost effective compared to traditional building materials.

GRP consists of thermosetting resins and glass fibres which when bonded together result in a solid fibreglass material.

GRP has been used in the UK and Ireland since the 1940’s. Its primary stronghold began in the marine industry for boat building. Once its high strength, versatility and appearance properties were recognised it was soon in demand across a variety of industries including automotive, construction, water tank building and flat roofing.

FibreGlassDirect offer a comprehensive Flat Roofing Materials Calculator to help you work out the exact materials required for your GRP Roof. All you need is a rough size of the area you would like to cover.

GRP Installation of a Roof

Using GRP for Roofing

A GRP Roofing system is a lightweight, single-ply membrane which is applied to secured OSB 3 decking boards. Its core materials consist of Resin, Fibreglass Matting and Topcoat.

Once applied correctly, a GRP Roof will last in excess of 20 years. Once cured fibreglass is inert and it will not rot or corrode. This means little maintenance is required on this type of roofing system.

A GRP Roof can be used for both domestic and commercial applications.

Tips for Laying a GRP Roof

  • Check the weather forecast – A GRP Roof must be laid in dry, mild conditions
  • Do not attempt to lay a roof in conditions under 5 degrees Celsius – The resin and topcoat will not cure adequately
  • Do not Topcoat a roof after 2pm during winter unless it is clear, bright and not too cold. Once the sun sets it will be difficult for the materials to cure
  • Check the temperature of the OSB Boards before applying the GRP materials – if it is too hot,(above 30 degrees) the wax in the Topcoat will melt which will result in a tacky finish
  • If it begins to rain you should stop straight away and cover the roof

Preparing and Laying the Deck

It is important to inspect the existing substrates before proceeding. If replacing an existing roof, ensure the existing boards and joists are free from rot.

We recommend using 18mm OSB3 Decking Boards, laid at 90 degrees to the roofing joists. The boards should be laid with the writing side facing upwards to allow a better key for the lamination. This will also allow the resin to flow into the board joints to effectively glue the boards together.

Start laying the boards from the furthest edge from the drip. If laying boards against a wall, a gap of 25mm should be left between the boards and the wall to allow expansion.

The end of the board should be aligned with the fascia and the last board should be trimmed so it is flush with the fascia.

If the cut off is greater than 400mm, use this to start laying the next row of boards by fitting the tongue into the groove of the boards already laid. The boards will be staggered and bonded by the time you have finished resulting in a strong deck.

After two rows have been laid, align the boards to run straight. The last row can be cut off in line with the fascia.

The boards should be fixed to the timber joists using a compressed air or gas nail gun. Any nails used should be galvanised or sheradised as they will not rust.

63mm galvanised nails should be used at 200mm centres across the boards. This will usually be 4 nails across a 600mm board. Ensure all nails are driven into a joist.

The boards may also be nailed with a hammer, although this will take more time. Care must be taken to avoid any internal damage to the ceiling below.

Applying the GRP Edge Trims

Edge Trims are manufactured from GRP. One side of the trim will have a matt finish for high adhesion and the other side will be glossy. 

Most trims are fixed to the decking board with 13mm galvanised nails or staples. The exception to this is the application of the F300 Flat Flashing and D260 Fillet Trim. These should be fixed with a PU Adhesive.

B260 Roofing Trim

Preparing for Lamination

Before any resin is applied, the fibreglass material should be cut to suit the repair area including any extra pieces needed for corners.

Glass bandage should be used to line the joints and the fibreglass chopped strand matting will be applied to the rest of the roofing area.

The matting should be applied parallel to the drip trim, 1m at a time. It should overlap the trim by 50mm without going over the edge of the trim.

Remove any uneven ends with a Stanley knife.

Laminating the Roof

The resin container should be shaken before use. This will ensure that any styrene or wax settlement at the bottom have been mixed in.

Ideally, the resin should be mixed in batches of 2kg at a time with 2% (40ml) catalyst. At room temperature, this will give a working time of 20 minutes before the resin starts to cure/harden.

The joints and corners of the roof should be applied first.

Taking a brush, generously apply the resin to the joints where the bandage will be placed. Apply the bandage and add more resin on top, working with the brush until it is completely saturated. Take a roller and consolidate the resin ensuring there are no dry patches remaining.

The same principles apply when adding the glass matting to the corners of the roof. Apply the resin to the corner and wrap the glass around it. Cut slits in the side of the glass to help the glass fit into place. Apply more resin to the mat and consolidate with a roller.

When working with the Larger Pieces of Glass, Take the pre-cut matting, roll it out to double-check it will be applied in a straight line and then roll it back.

When applying the resin we like to work to a ratio of 3:1. We recommend one-third of the resin should be primarily applied to the board and two thirds on to the mat.

Follow the resin application with the paddle roller to remove any air pockets. By the end of the laminating process, the glass should be transparent.

Applying Topcoat to the Roof

This is the final stage of the Fibreglass Roofing process.

Before applying the Topcoat, use 40 grit sandpaper sand the corners and bandages. Any excess cured mat which may be protruding may be removed with a Stanley knife. Special care must be given to the corners to avoid any holes appearing.

If you are applying any C100 Simulated Lead Trims, they may be applied at this time prior to topcoating.

Once the glass has semi-cured, the Topcoat can be applied. If possible, it should ideally be done within 24 hours.

Shake the topcoat well before opening. The Topcoat is mixed with catalyst usually at 2%, similar to the resin process outlined above.

The FibreGlassDirect Roofing Kits allow for coverage at 0.6kg/m2 to achieve fire resistance.

Video Guide to Installing a GRP Roof

Where to Buy Fibreglass / GRP Roofing Materials

At FibreGlassDirect we stock a complete range of GRP Flat Roofing Materials. We have been supplying fibreglass materials for over 40 years across a variety of industries.

Our Roofing Materials have over 30 years of tried and tested successful use.

View our different ranges

 Premium Fibreglass Roof Kit