Frequently asked questions
Although we are happy to give advice on the basis of our experience we are unable to accept responsibility for advice given. Furthermore we recommend you become proficient in the application of GRP before commencing work and engage an architect or engineer to assist you as required.
On this page:
- Boards and boarding
- Bonding GRP to other materials
- Trims
- Resin and topcoat
- Rumours and past errors
- How much material do I need?
- Do I need expansion joints?
- Warm roofs and insulation
- Health warnings
- Weather
- Getting a better finish
- Premium grade warranty
- Approvals
- Waste disposal and old materials
- I did it my way
Section 1 – questions about boards and boarding
Why do we recommend OSB3 T&G 18mm 8’x2’ boarding rather than plywood?
18mm OSB3 T&G 8’x2’ boards (from Sterling or Smart Ply) are the preferred board for GRP roofing. They are available throughout the UK from timber yards and builders’ merchants and although we stock them at our Bristol branch we are unable to send boards beyond the BS postal area as they are often damaged in transit. T&G OSB3 boards have several important advantages:
- Due to their T&G profile the boards do not curl or dish as the fibreglass cures
- There is no need for additional glass bandage on joints which improves final finish
- T&G boarding saves time and materials and there is no need to join on a beam
- Better adhesion – used writing side up GRP will bond better to OSB3 board
- No surface preparation (plywood which requires abrading before applying laminate)
- Easier to handle and transport – 8x4 boards get heavier every year!
If you are unable to obtain T&G boards use 8x4 OSB3 standard boards but be prepared to gap and bandage each joint.
Can I use plywood?
Yes, but use a good 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 – and apply masking tape to the joints to prevent the ingress of resin when applying the 75mm bandage.
Someone else fixed the ply boards and there are no gaps. Do I have to take them off and re-fix with gaps?
No – but it is essential to create a 3mm gap on every board joint using an electric saw.
Can I use chipboard?
NO. NO. NO.
Can I use insulation with ply attached?
Yes but not as you would imagine. PU insulation is available with 6mm plywood glued to one side. It would seem reasonable to fix it PU side down and apply the GRP to the 6mm ply. Unfortunately as the resin cures it shrinks and the 6mm ply is not stiff enough to prevent curling and the panel will dish. If however you use it ply-side-down and fix an 18mm T&G OSB 3 board on top of the insulation (fixed through to the beams) you will be able to apply the laminate without distortion.
Section 2 - bonding GRP to other materials
Can I bond to other surfaces such as lead, concrete, and asbestos?
This is a big question so we’ll take it one at a time.
Lead
Lead can be primed with G4 Dampseal (which will prime many surfaces and facilitate adhesion when applying fibreglass). The G4 should be thinly applied with a brush or roller and allowed to tack before applying the laminate. The G4 should be left no more than 2hrs before the application of the laminate as it becomes less effective as it moves towards a full cure.
An alternative method of bonding to lead is as follows: abrade the lead with 40 grit aluminium oxide. Apply a thin film of Matrix PU trim sealant (around 1mm) and gently “pad in” a strip of pre-cut glass mat. Do not force the PU sealer through the mat – rather engage the mat in the PU and let it go off (around an hour). You now have a piece of dry mat flexibly bonded to the lead but available to receive more mat and resin on the top (dry) surface thus bonding the lead to the new laminate. This method can be applied to pipes and up-stands and other difficult areas where it is difficult to achieve a bond between disparate materials. It can also be used to “anchor” a grp liner into an old lead gutter. Ring us for more information on this application.
Concrete
You can bond to concrete using G4 Dampseal as a primer (see above). The concrete should be clean and dry and fully cured (2-3 weeks minimum) and there should be no small pieces of loose aggregate which may compromise the laminate and cause poor adhesion to the surface. We recommend G4 as a primer before laminating a pond liner in situ. For smaller areas you may use Matrix PU trim sealant to obtain a good bond.
Asbestos
GRP laminates bond well to asbestos but the asbestos must be clean and dry. We do not recommend power washing or abrading asbestos (it produces dangerous slurry or dust) and we strongly recommend engaging an asbestos professional for advice.
Can I laminate onto bitumen and asphalt?
Bitumen inhibits the cure of GRP but on small areas you can neutralise it with a coat of G4. Applied with a brush or short pile roller it will initially soften the bitumen and then it will harden off sufficiently to accept a GRP laminate – but you may do better to replace the boards or even overboard with 18mm OSB3 Sterling. We do not recommend laminating onto asphalt – although some of our customers tell us they have managed to do so successfully. However we are not satisfied that the laminate will achieve full cure evenly and we can only recommend that you over-board with 18mm OSB3.
Section 3 – trims
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. We make a specific sealant – Matrix PU Trim Sealant – which can be used to joint and fix trims and for many other applications (see bonding to lead and concrete).
What is the best way to shorten, lengthen and join trims?
Trims can be cut with hand saws, hack saws, jig saws (there are special down cut/fibreglass blades available for most models) or best of all a 4” diamond wheel blade. Trims can be lengthened by joining them with Matrix PU Trim sealant – overlap one trim into another by around 50mm – and apply two jointing layers of sealant – one under each edge back and front.
If I don’t use pre-made corners what’s the best way to join trims?
As the configuration of the roof gets more complex the cost of premade corners can increase significantly (especially around multiple skylights). There are several options:
- Mitre corners using a diamond wheel cutter. Seal any gaps or tunnels with masking tape and then apply a layer of 450csm and resin which extends beyond the edge of the masking tape and bonds to the trim. Sand when hard and topcoat to finish.
- As above but use colloidal silica to thicken the resin and increase its tack (thereby making it easier to laminate the glass over difficult and sharp corners).
- As above but fill any tunnels (eg a corner between a raised edge and a drip fascia) with a piece of rolled up csm which will hold sufficiently for the laminate to set in place.
- 5 star method – raised edge to drip trim. Cut a piece of spare trim material the same size as the aperture left by the open raised edge trim.
Section 4 – resin and topcoat questions
I put on Topcoat on a hot day and expected it to go off really quickly – but it remained tacky. Why?
Are you using Standard or Tradesman Topcoat? If the temperature of the substrate is above 38°C (which is not unusual on a hot day) the solvent in our cheaper topcoats flashes off too quickly to effect a full cure and the topcoat will remain tacky for 2-3 days. It will cure eventually, but it is better to allow the roof to cool down before applying the topcoat. Premium Grade topcoat has an improved curing system and will cure at higher temperatures.
Do I have to put the topcoat on the same day ?
No but you will get a better bond if you put it on within 24hrs (when there will be no need for additional preparation). If you wish to come back later and finish the topcoat it is necessary to sand the substrate before applying topcoat the same day – use 40-60grit paper on an orbital sander (not a floor sander which is too aggressive).
I opened a second can of standard Matrix Topcoat and it was a slightly different grey to the first can. Do colours vary?
Yes. In our standard range the grey can vary slightly from batch to batch. If you want consistent colours either buy topcoat with the same batch numbers or if you have two different kegs pre-mix enough material for the job. Alternatively use Matrix Premium Grade topcoat – the colour will always be to specification (Dark Grey RAL 7011 and Light Grey BS 00 A 05 or special order.
Do you supply Premium Topcoat any other colour?
Yes we do. We have a readily available range (4-5 day delivery) which can be found in our product list on this website with colour swatches as a guide). We can in fact make Premium topcoat in any RAL or BS colour but if it’s not on the standard range it will take longer and cost more.
My resin is going off in the bucket – how can I slow it down?
- Use less catalyst (1% minimum)
- Make smaller mixes and keep them in larger buckets – less heat generation/less accelerated cure.
- Keep your resin and topcoat in the shade
- Choose a slower catalyst – M102 is our summer grade.
- Work earlier and avoid the hottest part of the day. Work faster and harder.
- Go for a cold beer until it cools down
My resin won’t go off quick enough. How can I speed it up?
- Use more catalyst (3% maximum – 4% for emergencies)
- Let the resin mix stand in the bucket for 5-10 minutes before you laminate
- Warm your resin – keep it indoors for 48hrs before you use it – put it in the sunshine
- Choose a faster catalyst – M402 is our winter grade
- Heat the boards from below with a space heater (turn it off before you laminate)
- In winter try Premium Zer0° - the cold weather resin. Use it in spring for quick detailing
Add more catalyst, up to 3% (that’s approx 30ml per kg) -4% in an emergency. Never go above 4%. If it’s still too slow, allow the resin mix to stand 5-10 minutes in the bucket before applying it. Swap over to winter catalyst but still never more than 3% for general laminating. 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 then ask yourself why are you working when it’s too cold to laminate?
It rained on my laminate before it set – it’s gone white – what can I do?
The resin has become emulsified by the water and has stopped curing. If the contamination is mild (spotting) it may be possible to sand it off. If the laminate has turned completely white it will need removing and replacing. If you’re somewhere in the middle give us a ring.
My topcoat has cracked (like a dry desert) and in some parts it is coming off in flakes but the roof doesn’t leak. Can I re-coat with more topcoat?
The cracking and flaking is caused by:
- Too thick topcoat (remember topcoat has no glass reinforcement )
- Poor adhesion to the laminate due to inadequate surface preparation (required if topcoat not applied within 24hrs)
- Contamination of the substrate –especially dew or moisture.
To refurbish the above we recommend a thorough sand (40-60 aluminium oxide) followed by a layer of 450g CSM and then a new topcoat.
My roof is old but sound. Can I rub it down and re-topcoat it?
Yes. Abrade with 40-60grit, dust off and re-coat the same day.
Why are some patches of the topcoat still sticky?
99% chance is down to poor mixing. Always use a stirring stick (we give them away free of charge) and make sure the stick has a square end. Mix and pour into another bucket; mix again before use. Pull out the left over topcoat from the bucket when it is set and see if there is a sticky edge – an indication of poor mixing.
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 on the underside of the mat which will be too dry to bond to the substrate. Some professionals are not as careful as they should be.
Section 5 - rumours and past errors
Why did early fibreglass roofs get a bad reputation?
- Wrong boards – (see OSB v Plywood v chipboard above) wrongly applied (no gapping)
- Incorrect resins. The only resins available in the 80s and 90s were yacht resins rather than roofing resins and as such were designed to be stiff and unyielding. Modern roofing resins are plasticised and move to accommodate expansion and contraction without delaminating.
- Contractors suffered from a lack of knowledge and an unwillingness to read and follow the instructions! We now offer training courses and technical support to all our customers.
- And in truth there wasn’t much good will amongst felt contractors towards GRP!
What are the most common causes of failure? What should I look out for?
- Wet boards and rain.
- Too cold or too hot
- Too timid application leading to resin starvation. Be bold, be agricultural. 5kg resin will laminate around 2.7m2 of 600g CSM. 20kg resin will laminate around 11m2 of 600g CSM.
- Inaccurate catalyst addition. No guesswork please!
- Poor mixing
- Before you start think finished GRP roof. Have a picture in your head. Look at our picture library. If you start with a clear vision of what your roof should look like it will help you achieve a good quality finish. Accept nothing less.
- Standard boards without gaps will cause splitting on the board perimeters.
- Damp beams will swell boards from below and distort the GRP laminate or trim
Section 6 - how much material do I need? Some basic guidelines
There are several ways to estimate:
- Ring us and tell us m2 and your trim requirements. We will send you a quotation or phone you back
- Use the Roofing Calculator found on this website
- Trims are supplied in 3m lengths
- Think about the following simple formulae:
M2 x weight of glass = total weight of glass. Eg. 20m2 x 600g = 12kg glass
Glass weight x 3 = resin requirement. Eg. 12kg x 3 = 36kg resin (2 x 20kg)
M2 x 0.5 = topcoat requirement. Eg 20m2 x 0.5 = 10kg topcoat.
Calculate total resin/topcoat mix at known catalyst ratio. Eg 50kg x 2% = 1kg catalyst.
Section 7 – do I need expansion joints?
Expansion Joints
All Matrix Roofing Resins and Topcoats are to some degree flexible and designed to accommodate expansion and contraction without the introduction of expansion joints. We have been advised by the trim manufacturers that their tests show that there is no need for an expansion joint on roof spans less than 100m2 (maximum allowable single side length 10m) and on valley gutters less than 10m long.
If you are considering a GRP roof greater than 100m2 you will need to use expansion joints – please contact our Technical Support Team for advice.
Section 8 – warm roofs and insulation
Can I construct a ‘warm roof’ with GRP?
Yes. From the GRP laminate downwards we recommend the following construction:
- GRP Laminate
- OSB3 18mm T&G top board
- Insulation (check with the architect and Building Inspector)
- Vapour check – Visqueen dpm or similar
- Base board onto beams – sufficient to take weight of operative during construction.
The OSB3 top board should be fixed with long screws through to the beams.
Hanging the insulation between the beams will not produce a satisfactory warm roof.
Section 9 – health warnings!
Are the GRP materials dangerous?
Yes and no. If you use these materials carefully and in accordance with the instructions, you will not be at risk. Read the H&S recommendations and download health and safety information which can be found on this site. Make sure all your operatives are familiar with the H&S recommendations. The most obvious risks are as follows:
- Resins and Topcoats. These materials give off styrene – a gas which is harmful in high concentrations and in contained spaces (but it will not bother you on a roof). Do not use is cellars or pits without breathing apparatus as styrene is heavier than air.
- Resins and topcoats are flammable. Do not smoke near open cans. Do not allow naked flames on the roof.
- Resins and topcoats cause dermatitis so use gloves and wear eye protection – splashes in the eyes are painful and can cause damage.
- Acetone is extremely flammable. No sources of ignition should be allowed. Buy a steel bucket to contain acetone in use and keep the lid on it to minimise evaporation. It also causes dermatitis so do not use it as a hand cleaner.
- Catalyst – can be very dangerous and needs to be handled with care. Read the data sheet in our safety section for full information. Keep off skin, and above all do not let catalyst come into contact with your eyes. Wash off splashes immediately with water and wear safety glasses when decanting or adding to resin.
- You should take an eye wash bottle on the job. If you are up a ladder and get catalyst in your eyes your descent will not be easy so keep the bottle close to where you are working.
- Catalyst spills should be cleared up using sand.
- My customer is worried that the smell will not go away and could be dangerous. What advice can I give to reassure him/her?
- Whilst there are cases of hyper-sensitivity to styrene they are extremely rare. Unless the customer is suffering from severe shortage of breath and disorientation the chances are that the objection will be to the smell which disappears as the resin and topcoat cures. Usually this will occur within 24hrs.
- The level of styrene created by manufacturing a thin laminate is very small indeed and is very unlikely to be problematic.
- If the smell does not disperse it may be because you have a spill of uncured resin!
Some of the materials are hazardous. Do I need signage on the van?
In the quantities you are likely to carry you will not need signage but you will need to know what to do in the event of spillage or an accident. Have a look on the website under Safety Data Sheets/transport and storage and you will find the information you need to know to be Hazardous Goods Aware.
Section 10 - the weather
Can I laminate in the rain?
No. No. No. And No again.
Can I laminate onto wet or damp boards?
No. Allow boards to dry before applying laminate. Drying them out with a flame gun is generally not successful. Better to allow air movement beneath a cover supported by timbers on edge. If you board up the night/day/week before cover the boards with dpm taped at the edges and fixed to the sides of the boards and then cover with a good tarpaulin to prevent flogging. Even new tarpaulins by themselves are rarely waterproof.
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 or Visqueen dpm on-hand and unroll it onto the wet laminate if it starts to rain. It will protect the laminate until it cures, it won’t stick and it can be peeled off later.
What is the lowest acceptable temperature for GRP roofing?
We would not recommend laminating below 10°C – especially when it is generally grim and daylight hours are short. When it’s cold and dry (winter sun helps too) you can laminate down to 0°C using Premium Zero resin and topcoat. Full details from our Technical Dept.
What is the highest acceptable temperature for GRP roofing?
As hot as you like but the hotter it gets, the less working time you have with resin and topcoat. To extend pot life mix smaller batches in larger containers; use less catalyst (min 1%) and slower catalyst; work earlier and later and avoid midday and early afternoon; keep resins indoors or in the shade. In practical terms I would not advise roofing above 28°C
Section 11 - getting a better finish
I don’t want join lines in the laminate – do I have to overlap the joins?
Yes but we can improve the joins. Some CSM is supplied with a ragged edge one end and a sharp cut at the other end - overlap the ragged edge over the sharp edge 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) for close up work such as balconies. And remember if you use T&G boards you do not need to add additional bandage on the joints –bandage that will show through when the roof is fully cured.
I would like to not have the glass mat pattern showing in the topcoat
Try a layer if glass tissue applied with a soft roller when the laminate is still wet
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 an 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.
Section 12 - premium grade warranty
Is the 20-year guarantee insurance backed?
No. The materials are covered by a 20yr manufacturer’s guarantee and the installation will be guaranteed by the Contractor. Should there be a problem with the materials they will be replaced foc. Similarly if the problem lies with the installation the remedial action will be the responsibility of the contractor. It is worth noting that defects in the materials are very rare indeed.
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?
Yes you could buy and use Premium but as the contractor you would have to provide your own 20yr installation guarantee which would pass with the property to the next owner. Without an independent contractor we are unable to offer a full guarantee but you can buy Premium Grade and take advantage of the quality product/colour options without the Warranty.
Why do some companies offer a 50 year guarantee? How do they do it?
50 years? Could be, we just don’t know as fibre glass roofs are relatively new (although I could take you to one laid in the 60s in Bristol which is still giving good service). How long will the boards last? What guarantee do you get from Sterling or Smart Ply (both BBA approved)? Most professional building insurance gives a 10-year guarantee so although we are comfortable with our 20-year materials guarantee I would seriously question the integrity of a 50-year guarantee. (Try it out – suppose we offered you a 200-year guarantee?). It’s true to say the longevity of a GRP roof is more to do with the quality and experience of the Contractor than the choice of materials; my priority would be finding the right contractor for the job.
Section 13 - approvals
Is Premium BBA approved?
No. BBA approval costs £28k for three years and when we looked at launching Premium Grade we had to consider the potential benefits and disadvantages. How much more would our customers pay for the same resin and topcoat with BBA approval? Does the approval cover the materials or materials and installation as a package? Until recently the BBA only approved the package.
After a recent discussion with the BBA it would appear that they may relent and approve materials without approving installations but this is still only a proposal. . But does using BBA approved materials give the customer and real advantage? We are still considering approval as a marketing device – perhaps “Premium Plus+” - but the materials would remain unaltered other than in price. We used to distribute and train for a BBA approved GRP roofing company and we would recommend them and their product but personally we would suggest you look for a highly recommended Installer as a priority and then choose the material on their recommendation.
Section 14 - waste disposal and old materials
How do I dispose of the empty cans and containers?
When you have emptied a resin container remove the lid and catalyse any residue on the side and base using some catalysed resin from a mix. This will convert raw resin to plastic and you can now use the can for waste collection and legitimately dispose of it as landfill. (steel and plastic).
Finish the acetone container and leave the lid off. Any residue will quickly evaporate.
Completely empty the catalyst containers. Wash out with water and pour to waste on soil or sand. Recycle or dispose as general waste.
I have some old CSM – can I still use it?
Yes if it has been stored properly- it must have been kept dry and out of direct sunlight. Mat that has become contaminated with moisture will never recover and should be scrapped. (Could you recycle it? It’s glass but the Green Elves might take some convincing). Store glass in a plastic wrapper.
I have some out of date resin and topcoat. Can I use it?
You can, but not if you are using Premium grade and want a guarantee (we have records of batch numbers on invoices and check the details). Premium grade material must be applied within the expiry date found on the can.
Other resins and topcoats will have expiry dates. Beyond these dates the characteristics of the resin and topcoat will alter (usually only the gel time) and if it’s liquid it’s worth doing a test piece to satisfy yourself that it’s useable. But please note: by making a test piece you are satisfying yourself and we do not recommend using out of date material.
How long will catalyst last?
Catalyst lasts around a year. If your catalyst is old again, do a test piece. If it works it’s ok. Sunlight degrades catalyst and diminishes its working life. We have seen water contamination in catalyst due to the dispensing bottle being left out in the rain and not being emptied before being filled afresh with catalyst!
Section 15 – I did it my way!
Can I ignore all your advice?
Of course you can!......but why re-invent the wheel? Take advantage of other people’s mistakes and save yourself a lot of trouble. Our advice is based on many years of knowledge and is regularly updated from information we receive from customers, suppliers and contractors. We are usually called in when things go wrong so we have first-hand knowledge of how things go wrong. If you intend to do something a bit unusual it might be a very good idea…but talk to us first!