How to laminate a fibreglass roof with CSM
This section deals with understanding CSM & weight ( chopped strand matting ) and the correct technique for laminating the CSM to the OSB decking. If you have never fibreglassed a roof before I strongly advise practising first on a small project like a bay window as shown in my article and video on fibreglassing a bay roof or a spare OSB sterling board, so you understand the principle and concept.
Choosing the right weight of CSM fibreglass matting
You may find all sorts of suggestions from fibreglass suppliers of what weight of fibreglass CSM mat to use on your roof. Some don’t say, some say 450g, some 600g or others say double laminate 300g or 450g. In the end it turns into a “mines bigger then yours” contest devoid of all logic and reason. The simple truth is there are guessing because they want the best for you but they haven’t seen your roof, it’s size, it’s shape or how well it’s constructed. Some fibreglass suppliers are just suppliers and only that, and some have a history of fibreglassing roofs and can speak from real experience. Here are my suggestions, feel free to use them, ignore them, or improve on them.
For estimating the actual amount of roof resin and matting see the resin and topcoat page linked at the bottom.
- Small roofs – small garages, porches and bays 450g ( 1.5oz)
- Small garage sized roof’s – with foot traffic or medium to large roof 600g ( 2oz )
- Large roofs – i.e. > 50m2 single 600g or possible 450g double laminate for walkways.
- Large roofs – i.e. > 100m2 double 450g laminate
These figures are on the assumption the roof is well constructed and in good sound condition, any type of roof covering fitted to a poorly built roof will fail prematurely. If you have a funny shaped roof or a large roof and are concerned about laminate thickness or expansion and contraction please read our Fibreglass FAQ page ( opens in new page )
Overlapping fibreglass matting and joints
CSM matting – Overlap the matting 50 to 75mm ( 2-3″ ) , lay the feathered edge of the matting on top of your last run if possible, this will help the look of visible edges on the joints when the roof is completed. Feathering edges is purely for aesthetics and will not alter the strength.
Feathering a CSM edge – If you haven’t got a feathered edge but you want one, just pull firmly at the edge of the matting and it will break away in your hand, or rip the matting carefully on a cut line.
Aligning the matting and laying the laminate
Have all your matting pre cut and ready to go, roll out a run of fibreglass mat and make sure it’s square and aligned how you want it. When you know its all OK, roll it back up again and store it on the roof or floor so its easily accessible later.
Planning your escape – Always think about where you will exit the roof, you don’t want to laminate yourself into a dead end…
Preparation is everything
Prepare your tools ready for use, If its a larger project say over 10m2 or you’re new to fibreglassing I would recommend you double up on resin rollers, consolidation rollers and 50mm ( 2″ ) brushes. Better to have them and not need them than to be faffing about.
If it’s warm weather or the catalyst cures faster than you can work your rollers or brushes can harden on you, this can cause stress or a feeling of panic. If your backup gear is on hand it allows you to carry on without the panic and to adjust your next mix accordingly.
Wear full wrap around safety glasses before touching any resin or catalyst. Seriously!
Never mix more roofing resin than you know you can work with, it’s easy to mix more if you need it. There’s nothing worse than stressing and rushing because you’ve mixed too much resin to work with before it hardens. I recommend 4 or 5 litres maximum at a time if you haven’t done this before.
Laying the GRP laminate
With the preparation complete, we are now ready to laminate the CSM to the roof decking, if you have never seen this done before the video below should help.
This video is part 4 of 5, and shows the technique for laminating CSM to the roof deck on a small demo roof. This can then be simply up scaled for larger projects.
On a larger roof, roll out and align your pre-cut matting, when the fibreglass matting is in the correct position roll it back up without moving it, so that when you roll it back out into the resin it’s in exactly the right place. Pictured below.
Laminate CSM – Dunk the roller into your resin bucket and wet out just over a square metre of roof deck, now roll the CSM into the resin. Wet more roof deck and continue rolling out. Apply more resin on top of the CSM you rolled out, and wet out the CSM. Then repeat.
Laying fibreglass matting is often compared to wallpapering except that you put the paste on the wall, roll out the paper and paint more paste on top of the paper.
If laying 450g fibreglass mat, the correct coverage is about 2 litres per square metre in my opinion ( or just under ). If you follow this advice you should find it to be a smooth operation. Do not skimp on resin, this makes it harder work as you roll backwards and forwards many times trying to spread the resin out, skimping will also increase the chances of pinholes and dry spots. To see the suggested resin ratios for other weight matting see How to Mix Resin and Topcoat
Consolidate the CSM – After a couple of minutes you will notice the matting becoming see through, this is the resin breaking down the binding agent that holds the fibreglass strands together, this is exactly what you are looking for. Its time to consolidate.
Using a consolidating roller
Preferably have a second person on hand to consolidate the matting and resin for you on a large job, this will allow you to concentrate on laying and wetting out, a second person is not necessary but it helps. Look for bubbles or opaque areas, if you see any, consolidate again until they are gone. If you see dry patches apply more resin with the roller, if you see pin holes use the consolidating roller and apply resin as necessary.
Roll up and down the matting a few times with the consolidating roller taking particular care with any joints or seams. My preference is finned roller on small roofs or a V-notch roller on a larger roof, splashes are greatly reduced as less resin flicking occurs than with a plain horizontal finned paddle roller.
Fibreglass matting on top of trims
I like to lay my fibreglass laminate so the matting finishes on top of any raised edge trims like this, again this isn’t necessary but it looks nice. You will need to allow the resin time to break down the binding agent in the fibreglass mat so it goes soft, before rolling it with the edge of your resin roller, mini roller or using a detail brush to press it down.
Details – Here during the sanding process you can see how the matting sits on top of the trim if you want it to. It also has the effect of raising the deck laminate joint higher, away from potential running water.
Fibreglass matting against upstand or fillet trims
Try to allow enough fibreglass matting to enable it to finish at the top edge of your upstands or D260 Fillet trim. This strengthens it, and stops the possibility of driving rain finding a pin hole in the joint between your fibreglass and trim.
Brushes and roller sleeves will obviously have to be disposed of when used but consolidation rollers and roller frames etc. can be cleaned in a shallow tray of acetone ready for re-use.
Care for your skin – Do not clean your hands or skin in acetone as it strips out the oil in your skin and can lead to skin problems. Clean you hands with industrial cleaning wipes instead, some brands are better than others at cleaning resin and topcoat so you may need to experiment.
Tips on removing small spillages of resin or top coat are in the FAQ section at the bottom of the page.
Is there an easy way to laminate fibreglass ?
Contrary to the very good advice given by your GRP supplier, you can push resin through CSM matting and get a perfect waterproof finish, and what’s more I would say there are times when this is a preferable method. For me this is on small bay roofs with intricate detail like a multipart bay window, the benefits are…
- Less chance if spillages, flicking resin and run off onto surrounding glazing, UPVC or brickwork. ( use a fin roller to consolidate for the same reason )
- Matting can be adjusted for accurate fit first rather than placement onto a wet deck, sometimes in a gusty wind.
- Less chance of catching the underside of overhanging soffits, guttering, or tiles. These are sometimes contaminated with dirt or bitumen, which can then drop onto the wet roof deck if disturbed.
Resin through matt – To see how to apply roofing resin the easy way on a small roof like this 4m2 bay roof window, click the picture for video link – Applying roofing resin
Also shows cold cure resin for cold weather situations.
I would not condone this technique for large jobs though as you risk dry spots, air pockets and pinholes from resin starvation should the resin not make it through the CSM and saturate the roof deck effectively. This is the very good reason suppliers do not approve this method, be warned.
- How to Fibreglass a Flat Roof – The starting point for fibreglassing a flat roof
- Fibreglass Roof Trims – Trims and sizes
- How to Mix Resin and Topcoat – Catalysts, resins and top coat
- Fibreglass Roofing Supplies – Buy GRP roof supplies Trade or DIY
- Fibreglass Problems and FAQ’s – Questions and answers
- How to Fibreglass a Bay Roof – A nice starter project
- Roofing tools – Some of the tools I use daily