How to Fibreglass a 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 a smaller roof like a bay window complete with HD Video see How to Fibreglass a Bay Roof
Fitting the new roof decking
Sterling board or OSB ( 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 form the surface of the wood ). Allow 3mm gaps between adjoining boards for expansion.
The best is tongue and groove OSB3 but this can be hard to get hold of so you may have to do with normal OSB3. Some fibreglass suppliers and timber yards will deliver T&G ( tongue & groove ) but this can make your costs spiral upwards which can be a problem if your trying to remain competitive. OSB T & G does have expansion gaps built in by design which is nice.
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 the new wooden decking underneath 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 its sterling board, tongue and groove or plywood and its 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.
A 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 OSB sterling board. Bitumen traces are a contaminant to fiberglass and must be completely covered.
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 run 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 8 by 4′s instead of 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 average measurements of the trims and the sizes provided should hold you in good stead. All fibre glass trim lengths are 3 meters long unless stated otherwise.
To see a full list of GRP fiberglass trims and plan fitting instruction diagrams click here - Fiberglass 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 ( 65mm drop ) the A200 ( 90mm drop ) and the A250 ( 140mm drop )
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 ( 105mm trim height ) the B260 ( 125mm trim height ) and the B300 ( 170mm trim height )
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 into 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 ( 100mm cover ) and the C150 ( 150mm cover ) Normally push fixed into place with P.U. Adhesive then sealed with a product like silicone, ’lead mate’ or ‘roofers mate’.
How to fix fiberglass roof trims
Personally I quite like to use 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 some one bashes into a trim. P.U. is very useful anywhere you need to allow for a little more expansion and contraction though. I advise not to use screws, as you can split the trims and large screw heads will sit proud and be seen through the laminate after fibreglassing.
When to nail GRP trims – When there are two or more persons working on site sometimes someone will put a ladder onto the trim you have just P.U.’d into place, This can move it without anyone noticing until its really inconvenient. I tend to use 20mm galvanised clout nails spaced about every 150mm ( 6″ )
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. It comes in a few different variety’s these days and a lot of trade outlets sell it, and of course fiberglass suppliers. 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. underneath the fiberglass trim then apply firm pressure with a rubbing motion. You can also spot dab or apply small beads at 150mm or 300mm intervals.
Can I something other that PU adhesive ? Yes a product like Fix-All also works and is available at nearly all DIY chains, but never silicone.
How to cut a fiberglass trim
Four good methods are tin snips, hack saw, the diamond wheel cutter ( mini angle grinder ) and Multi tool ( oscillating cutter ). My preferred method is tins snips at it creates the least dust. Do not breathe any dust, always wear a mask and work up wind if possible.
How to join a trim
When joining a Fiberglass 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 meter or more. This will double the strength of the trim and provide an impact resistant 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 topcoated.
Overlapping a fibreglass trim – Some people like to apply a fiberglass bandage over the face, but my preferred method is to bring the roofs fiberglass matting to the top of the trim, sand a feather edge onto the overlap. Lastly apply the top coat colour to get a nice smooth appearance.
Now clean all your fiberglass 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 60 to 80 grade abrasive paper.
Joining GRP trims at Corners
When jointing or joining fiberglass 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.
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
Now we’re ready to prepare the decking for the application of resin and matt. Firstly we need to apply a layer of bandage to the edges of the trims. You can buy 100mm ( 4″ ) or sometimes 75mm ( 3″ ) on a roll or cut your own, but to be honest for what it costs it 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.
Pictured here is where you would place the bandage and how you could expect it to look when it’s rolled it into place with some resin. This part is a 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. Personally I find I can do this with the mini roller alone and save using a separate consolidating roller. If you are using a consolidating roller I recommend using a aluminium fin roller or a PTFE fin roller, this will reduce any splashing to zero in careful hands as a normal horizontal fin paddle roller flicks resin very easily.
To see how to mix roofing resin click here – Roof resin and topcoat ( opens in new window )
If you have never done this before, mix 1 litre of resin with the correct amount of catalyst given the temperature, to give you about 30 mins of working time. This should give you plenty of time to bond down your bandages. If your short on resin just mix more. Use a mini paint scuttle marked internally in Litres. When done your bandage should be transparent and bubble free.
Bandaging OSB joints with fibreglass
If you have used 2.4m x 1.2m ( 8′ x 4′ ) OSB boards its advisable to bandage all the joints as mentioned above. OSB boards were designed for the building industry and are machine engineered for the specific purpose of resisting expansion and contraction, load bearing and in the case of OSB3 humidity resistance. Having said that leave a gap of 3mm between adjoining boards for expansion.
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.
Laying the fiberglass matting laminate
Ok, with all the roof trims in position and bandaged, the boards bandaged if necessary, we’re ready to fibre glass 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 clean from debris with a soft bristle brush.
Preparation is king for a nice easy experience, make it easy on yourself. We are now ready to Laminate the fibreglass roof.
To see the process of laying to main roof laminate click here How to laminate fibreglass
Fibre glassing open ends of fibreglass GRP trims
How to fibreglass the open ends on fiberglass GRP trims
When the roof resin has cured enough to walk on, I cover any holes in the edge of raised edge trims or others. Make sure your feet are clean before walking on your new laminate fiber glass roof, its not a good time to contaminate the surface before topcoating. Cut a piece of matting roughly 25 to 50mm ( 1 – 2″ ) larger than the hole you are covering then soak it both sides with a fast drying mix of resin. Place the wet matting so that even areas of excess mat 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.
B – Sand and rough spots when dry.
C – How it will look after topcoating.
Sanding a fiberglass roof
It’s at this point I like to sand off any little rough edges, high spots or sharp edges. This is not necessary for the topcoat as long as your topcoating on the same day.
My preference is a re-chargeable corner sander with 60 grade aluminium oxide paper, but it can be done by hand if its a one off but you will get a slicker faster finish with a power tool. Do not breathe the dust, wear a mask and work up wind.
Topcoat a fiberglass roof
Again measure the temperature of your new roof and get your tools and kit ready and on hand as before. A 100mm ( 4″ ) resin roller is excellent for details like the face of edge trims and will leave a superior finish to the detail brush where possible. In fact a small resin roller can topcoat a roof up to 4m2 if necessary. Bear in mind when mixing top coat that a large roller may absorb up to 1 litre of colour before it becomes usable for moving the colour about.
To see how to mix and apply Topcoat click here - Mixing roof resin and topcoat
As mentioned above, it is not necessary to sand the surface of your roof within 24 hours as the Topcoat will chemically bond to the resin within this time frame. To be on the safe side I like to topcoat within a 12 hour time frame to account for hot weather and possible fast curing.
Finishing with fibreglass flashings
When finishing with fibreglass simulated lead flashing’s C100 or C150 simply cut with tin snips where required. Any overlapping joints should be 150mm ( 6″ ). On an internal corner place some lead behind the two abutting trims and into the mortar chase.
GRP Simulated lead flashings
Best fixed in place with P.U. adhesive and the chase sealed with mastic or a product like ‘Lead mate’ or ‘Roofers mate’
Finishing a fibreglass roof with lead flashings instead of GRP fibreglass
Some people prefer to finish their flat roof with lead flashings instead of GRP flashings, as a fibreglass roof installer I prefer the look of traditional lead flashings also. One of the benefits is that you can finish with sand and cement and remain the look of a period property for example. When fitting lead flashings I tend to use about 1 meter or 3 feet lengths to allow for expansion and contraction up to a maximum of 1.5 meters then overlap any joints by 100mm minimum ( 4″ )
I know some branded GRP system’s guarantee their products for 20+ years etc. but here’s the problem, that’s exactly what they are guaranteeing, the product. They know there is very little chance of product fault, and a much higher probability of poor installation. Do you think they will be putting their hand in their pocket without a thorough and possibly biased investigation? Even then if you manage a claim check the small print, its probably just for GRP materials and not boards, labour etc. Keep this in mind before installing yourself, or be educated as a paying customer. To this day I have never had a problem with any roofing resin.
Just to be clear, I’m not bad mouthing or trying to stop anyone from buying branded fiberglass roofing systems, but just because a company slaps a nice rosette on the tin it doesn’t mean its time for triple brandy’s all round. You have to be a responsible installer first and foremost.
- How to Mix Resin and Topcoat - How its done the easy way
- How to Fibreglass a Bay Roof - A nice starter project
- How to Lay the Fiberglass Laminate - Laying techniques small and large
- How to Fit Fibreglass Roof Trims - Fibreglass trims and sizes
- How to fit EPDM - Have you considered a rubber roof instead?
- Fibreglass Problems and FAQ’s - Questions and answers
- How to fit lead flashings – Finish any roof with classic lead