Frequently asked questions

Why do you recommend OSB Sterling boarding rather than plywood?

18mm OSB3 Sterling board, 8x2 T&G all round is the ideal board for GRP roofing. When GRP is applied to one side of any board the glue line shrinks, causing the board to curl up. This causes ‘dishing’ and, if you are using plywood the joints need an additional fibreglass tape to prevent splitting. If you use a T&G board curling and dishing cannot occur and there is no need for the tape, saving time and materials. It also looks a lot neater. Furthermore, GRP will adhere better to ‘upside-down’ Sterling board (writing side up).

Can I use plywood?

Yes, but use a quality grade – WBP 18mm – and sand the surface before you laminate as bonding to plywood can sometimes be difficult. And remember to gap the boards to allow for expansion – around 3-4mm.

Can I bond to other surfaces such as lead, concrete, and asbestos?

Yes – but you will need to prime lead and concrete with G4. Ensure the surfaces are clean and dry, (abrade lead and power wash concrete and asbestos and allow to dry) and brush on a thin coat of G4 primer. When this has gone to tack (do not leave longer than 6 hours) laminate onto the primed surface.

My laminate won’t cure on some small areas of bitumen. What should I have done?

Bitumen inhibits the cure of GRP. You can neutralise bitumen with G4 – brush it on; it will initially soften the bitumen and then it will harden off – but you may do better to replace the boards or overboard with OSB3 Sterling.

Can I join trims with silicone sealant?

Silicone sealants are great for bathrooms but silicone acts as a release agent for fibreglass and any stray sealant could cause you problems. Better to use a PU sealer/bonder such as Adflex or Sikaflex 221.

I put on Topcoat on a hot day and expected it to go off really quickly – but it remained tacky. Why?

If the temperature of the substrate is above 38 deg C, the solvent (which is necessary for the curing process) flashes off and the topcoat remains tacky for 2-3 days. It will cure, but it is better to allow the roof to cool down before applying the topcoat or use Matrix Premium grade topcoat which has an improved curing system.

I opened a second can of standard Matrix Topcoat and it was a slightly different grey. Do colours vary?

Yes. In our standard range the grey can vary slightly from batch to batch. If you want consistent colours either make sure you buy the same batch (or pre-mix sufficient topcoat from different batches). Alternatively use Matrix Premium Grade topcoat – the colour will always be the same.

Do you supply topcoat in any other colour?

Yes – in any colour you wish by special order. If you have a RAL or BS colour reference it helps. Non-standard colours cost more.

My resin is going off in the bucket – how can I slow it down?

Use less catalyst, but always add at least 1% (that’s approx 10ml per kg). Never go below 1%!! If it’s still too fast mix smaller quantities and get it out onto the job rather than leave it in the bucket (it cures quicker in the bucket) or swap over to summer catalyst (but still at least 1%). Store your resin in the shade, don’t work in the hottest part of the day, and wait until it’s cooler. Go for cold beer instead.

My resin won’t go off quick enough. How can I speed it up?

Add more catalyst, up to 4% (that’s approx 40ml per kg). Never go above 4%. If it’s still too slow, allow it to stand 5-10 minutes in the bucket before applying it. Swap over to winter catalyst but still never more than 4%. Store your resins somewhere warmer (not on a cold concrete floor – they become chilled). Try a space heater in the space below – ideal for a garage roof – it will warm the boards. And why are you working when it’s too cold?

I’ve seen professionals lay the mat on the board without ‘wetting out’ the board first with catalysed resin, and putting the resin only on top. Will this work?

No. You will end up with dry patches beneath the mat which will not be fully bonded to the deck.

Why did early fibreglass roofs get such a bad reputation?

There were many reasons, not least of all the resins used in the early days of GRP roofing were often obtained from boatyards. These resins were perfectly good for making stiff yacht hulls, but they were designed to be stiff – not flexible, and would not elongate as the boarding expanded, which resulted in splitting.

All Matrix Roofing roofing resins are plasticised – and accommodate the expansion of the deck without lesions; so much so that up to 50m2 you do not need an expansion joint. Above 50m2 ask our technical dept.

GRP roofs have also improved as roofers have become more experienced and knowledgeable about how to laminate. Matrix Roofing offer technical support and training courses.

Can I lay a ‘warm roof’ with GRP?

Yes. Contact Matrix Roofing for details.

Are the GRP materials dangerous?

If you use these materials carefully and in accordance with the instructions, you will not be at risk. Read the H&S recommendations. The most obvious risks are as follows:

  1. Resins and Topcoats. These materials are flammable and give off styrene – a gas which is harmful in high concentrations and in contained spaces, but will not bother you out on a roof.
  2. Acetone – it’s extremely flammable. Buy a steel bucket and keep the lid on to minimise evaporation.
  3. Dust from sanding. Wear a particle mask if you are doing a lot of sanding.
  4. Catalyst – can be very dangerous and needs treating with respect. Read the data sheet for full information. Keep off skin, and above all do not let catalyst come into contact with your eyes.

What do I do if it starts raining before the laminate is waterproof?

Try not to get into this position! Get a local weather forecast and work around the weather, but if you do get caught out, have some polythene dust-sheet on the wagon and put it over the wet laminate. It can be peeled off later and won’t stick to polyester.

I don’t want join lines – do I have to overlap the joints?

Some CSM is supplied with a ragged edge on end – use this to overlap and you will lose a lot of the joint. Tear the end if both ends are guillotined. Or use two layers of 450gm CSM, butting and staggering the joints (no overlaps).

Can I laminate straight onto the abutment wall, using G4 as a primer on the brickwork?

No. Roofs expand and contract, and you will not be able to produce a lasting waterproof joint by bonding onto the abutment wall. Better to leave a gap 25-50mm) between the deck and wall, and install a D260 trim, finishing with either a lead or GRP flashing.

I am thinking of laying a fibreglass roof on my garage. I am not a professional contractor, but I would like to use the Premium Grade system which has a warranty. Can I buy it for my own use?

No – the Warranty is only available through professional contractors who are experienced and sufficiently competent to lay a GRP roof. You can however buy Premium Grade and take advantage of the quality product without the Warranty.

The boards are wet – can I dry them out with a torch?

No – it may appear that you have dried the boards, but there may be residual dampness which will prevent the laminate from bonding or curing properly. Better to either wait for the boards to dry naturally, or cover with plastic or a tarpaulin with timbers to allow air to circulate below. Better still, don’t lay the boards and leave them open to the weather in the first place.