How to Fibreglass a Flat Roof
How to install a Fibreglass flat roof in easy steps…
To learn how to fibreglass a flat roof please read the article and it will take you through the process from start to finish, clicking on links along the way will open that page in a new window so it does not hinder the continuity. If you want to see the principle of fibreglassing on a smaller roof like a bay window complete with HD Video see How to Fibreglass a Bay Roof
For larger GRP roof projects please use these pages and the video series shown below, all the relevant details and more are contained on the website pages linked from here. Please don’t just rely on the video alone, the FAQ page is also worth a read so you can avoid the problems mentioned.
Roof deck and trim battens – This video is part 1 of a 5 part series that takes you through the process of How to fibreglass a flat roof.
Fitting the new roof decking
OSB3 – Oriented Strand Board is the roof deck of choice for laying a new fibreglass roof onto. 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 from the surface of the wood). With standard square edge boards allow 3mm gaps between adjoining boards for expansion.
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.
Check local stockists of cheap OSB and OSB T and G here – Local OSB stockists
The chances are you will be doing either one of two things when fitting your nice new fibreglass roof. Either over boarding or fitting a brand new full replacement roof deck.
- Over boarding – Fitting new 18mm OSB3 sterling board on top of the existing roof deck.
- Complete replacement – Removing all traces of the old roof down to the rafters and completely replacing with new 18mm OSB3.
To see information on over boarding of a felt roof see our Fibreglass FAQ page
When and how to overboard a flat roof
With the felt roof or other roof covering removed inspect the wood underneath, if it’s sterling board, tongue and groove or plywood and it’s 18mm or above AND in a good rot free firm condition you can overboard it. The benefit of overboading is that it saves undue stresses to the roof rafters that can occur whilst stripping a flat roof, these stresses can crack plasterboard ceilings and covings that may sit underneath.
Flat roof stripped – The plywood timber is in very good condition but is contaminated with Bitumen. This is an ideal candidate for over boarding with OSB3. Bitumen traces are a contaminant to fibreglass and must be completely covered.
Gaps – Always leave a 3mm gap between all edges of any new non T&G 2.4m x 1.2m (8 x 4) OSB3 boards and a 25mm expansion gap between the roof and any abutting up stands or walls.Click pics to enlarge
When to replace a flat roof decking
This seems to be most times unfortunately, normally the roof has been leaking onto chipboard and this has turned into what we call ‘ Weetabix ‘ a horrible crumbly weak substrate. On the other hand the decking may have a bad fall (slope) and be ponding (holding areas of water without running off) in which case it will be preferable to correct with firing strips onto the rafters first. When fibreglassing always leave a 25mm gap between any new roof and where it abuts a wall, and as mentioned above a 3mm gap between boards when using standard square edge 8 by 4’s instead of the interlocking T&G.
An old chipboard roof being stripped – Boards are gently levered up to expose the rafters underneath. The new OSB decking goes down leaving a 25mm gap between the wall and the edge of the new OSB roof.
Click Pic to enlarge
Fitting fibreglass roof trims
Fibreglass GRP roof trims from different suppliers tend to be exactly the same style, some vary in size a little and also in quality. I have taken some average measurements of the trims, and the sizes provided should hold you in good stead, all fibreglass trim lengths are 3 meters long unless stated otherwise.
GRP roof trims – This video is part 2 of 5, and shows how to fit fibreglass roof trims. Here we see common roof trims like the wall fillet, upstand kerbs, and the drip trim, as well as common methods of cutting and fixing.
To see a full list of GRP fibreglass trims and plan fitting instruction diagrams click here – Fibreglass GRP trims (opens in new window)
Here are the most common flat roof trims and where you would use them…
The Drip trim – Used at the lowest point of a flat roof to direct the flow of water from the surface of the roof itself and into the guttering. GRP Drip trims come in 3 sizes, the A170, the A200 and the A250.
The Raised edge trim – Used to stop the run off of water over the edges of a roof and to channel water towards a drip trip or outlet. GRP raised edge trims come in 3 sizes, the B230, the B260 and the B300.
The D260 wall fillet – Used to create a waterproof upstand from the roof to an abutting wall, and to provide a flexible cover for an expansion gap of 25mm between the decking and the abutting wall. Fixed to the OSB but not to the wall, corners both internal and external are also available.
Simulated lead flashings – Used as a fast and cheap alternative to the traditional lead flashings normally where a flat roof abuts a wall. They come in two sizes, the C100, and the C150. Normally pushed fixed into place with P.U. adhesive then waterproofed with a product like silicone, ‘lead mate’ or ‘roofers mate’.
How to fit fibreglass roof trims
Personally I quite like to use galvanised clout nails for fixing fibreglass trims for two main reasons. Number one being I’ve been doing so for years and they haven’t let me down yet, and number two, nails don’t move when someone knocks into a trim. P.U. adhesive or the just as good more more expensive fast drying Fix All Turbo can very useful anywhere you need to allow for a little more expansion and contraction or for stopping trim rattle on trim support battens.
Fixing trims with nails – I tend to use 15-18mm galvanised clout nails spaced about every 150mm (6″). I advise not to use screws as you can split the trims, the screw heads also will sit proud and be seen through the laminate after fibreglassing.
P.U. Adhesive or P.U. Sealant (Poly Urethane) for those who don’t know, It’s a Polyurethane based glue / sealant, the main properties are that it sticks to a lot of different materials very well. It has very good chemical resistance, can be over painted and stays flexible, very useful for situations where you need to keep a little bit of extra movement for expansion and contraction. (like simulated lead flashings amongst other places)
P.U. Sealant – Squeeze out a bead or small lines of P.U. or Fix All underneath the fibreglass trim then apply firm pressure with a rubbing motion. You can also spot dab or apply small beads at 150mm or 300mm intervals.
How to cut a fibreglass trim
The best way to cut fibreglass trims is with sharp tin snips as this creates no dust, make these your first choice. For more difficult cuts and trimming I use a cordless multi tool like the fast and versatile Ryobi, which is listed in roofing tools further down and at the bottom of the page.
Cut GRP roof trims – Low tech solutions are tin snips and hack saw, faster is the multi tool and diamond wheel cutter. The most dusty being the angle grinder, do not breathe any dust, always wear a mask and work up wind if possible.
How to join a trim
When joining a Fibreglass trim simply overlap them by 50 – 75mm and apply a bead of P.U. Adhesive to one of the two mating surfaces, with a little bit of effort one will pull over the top of the other one very nicely. If you are using good quality trims they will already be quite tough but on the occasion where it may take abuse from window cleaners etc., you may wish to overlap the trims by up to a metre or more. This will double the strength of the trim and provide an impact resistant ladder access point.
Overlapping a GRP trim – Here I have combined a overlapped joint with the customer’s preferred location of ladder access point.
Picture also shows suggested overlap and P.U. line
When overlapping a trim you can sand the front face of the trim on the exposed edge, this will help to make the joint almost invisible when top coated or bandaged if that’s important.
Overlapping a fibreglass trim – Some people like to apply a fibreglass bandage over the face of a join, but my preferred method is to bring the roofs fibreglass laminate onto the top of the trim, then sand and feather edge the overlap. Lastly apply the top coat colour to get a nice smooth appearance.
Joining GRP trims at Corners
When jointing or joining fibreglass trims at a corner the easiest method is to use a straight mite cut. I find its best to run one trim through long, and then butt up the next to it so you can mark it with a pencil ready for cutting with a hacksaw, multi tool, tin snips or angle grinder with a cutting blade. Another method of cutting a corner is like the one shown in the picture below, where the trim underneath is almost cut square and the top one over cloaks it slightly. The benefit being its slightly less prone to being knocked out of shape. I finish with some fibreglass matting and sand thoroughly when dry.
GRP trim joints – A slight alternative to a plain mitre joint, see how the second angled trim is cut. (picture in picture)
An extra piece of external bandage is applied later for additional strength and finishing.
Bandaging the joints and edge trims
Before you proceed clean all your fibreglass trims with Acetone to remove all dirt, grease and other contaminates that may be on them, if there are any smooth surfaces on your trims sand them with 40 to 60 grade abrasive paper.
GRP bandages – This video is part 3 of 5, and shows how to apply fibreglass bandages to the roof trims and the roof decking if necessary. It also covers a basic first mix, taping of joints, and laminating open GRP trim ends.
Now you are ready to prepare the decking for the application of resin and matt. Firstly you need to apply a layer of bandage to the edges of the trims. You can buy 100mm (4″) or 75mm (3″) on a roll or cut your own, but to be honest for what it costs it is easier to buy it ready done on a bandage roll. If you have used T&G decking all you have to do is bandage the edges of the GRP roof trims to ensure stress cracking near the trims doesn’t occur in the future.
Bandages – Here is where you would place the bandage, and how you could expect it to look when it’s rolled into place with some resin. This part is good practise for the main roof later to make sure your resin and hardener catalyst are giving you the desired drying and working times.
The way to apply bandage is to roll the resin onto the GRP trim and decking with a resin roller like a 100mm (4″) mini resin roller, then to lay the bandage on top and apply more resin. After a minute or so it can be consolidated with a paddle roller to break down the binding agents in the matting and remove possible trapped air or bubbles. I recommend using a aluminium fin roller or a PTFE fin roller, this will reduce any splashing to zero in careful hands as a standard paddle roller flicks resin very easily. When done your bandage should be transparent and bubble free.
To see how to mix roofing resin click here – Roof resin and topcoat (opens in new window)
First mix – If you have never done this before, mix 1 litre of resin with the correct amount of catalyst taking into account the temperature, to give you about 20-30 mins of working time. This should give you plenty of time to bond down your bandages. If you are short on resin just mix more. Use a mini paint scuttle marked internally in Litres.
Bandaging OSB joints with fibreglass
If you have used 2.4m x 1.2m (8′ x 4′) OSB boards, or WPB Plywood, it’s advisable to bandage all the joints as mentioned above. OSB boards in particular were designed for the building industry, and are machine engineered with favourable expansion and contraction, load bearing, and in the case of OSB3, humidity resistance. Always leave a gap of 3mm between adjoining boards for expansion in your OSB3.
8 x 4’s – Fibreglass bandages are cut and can be laid into place ready for wetting out. These bandages are covering the joins in the 8 x 4 sheets as described. To make bandages less visible you can rip the edges to feather them out if it bothers you.
Fibreglassing open ends of fibreglass GRP trims
How to fibreglass the open ends on fibreglass GRP trims…
Cut a piece of matting roughly 50mm (2″) larger than the hole you are covering then quickly soak it both sides with a new fast drying mix of resin, or the resin you were applying to the bandages if time will allow. Place the wet matting so that even areas of excess matting poke out the top and the sides, now its just a matter of gently stretching the matting in all three directions until it looks right with a brush.
A – Cut a piece of dry matting that wraps around all three faces, you may need to shape it a little. Practice before wetting.
B – Sand any rough spots when dry.
C – How it will look after top coating.
Laying the fibreglass matting laminate
Ok, with all the roof trims in position and bandaged, and the boards bandaged if necessary, you are ready to fibreglass the roof decking. Firstly cut all the matting ready to go, have it in order so its on hand as you work. Make sure the decking is
