How to Fibreglass a Roof

Fibreglassing man

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.

How to fibreglass a grp roof

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 roof. It opens in a new window so you can refer back to the website at any time.


Fitting the new roof decking

OSB 3 Sterling Board OSB Ordinary Strand Board

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 form the surface of the wood ). With standard square edge boards allow 3mm gaps between adjoining boards for expansion.


t and g osb sterling board

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.

Roof stipped of old felt for overboarding

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.



t and g osb 3 sterling board gapWall gap osb picGaps – Always leave a 3mm gap between all edges of any new 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.

Old chipboard roof being strippedAn 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.

Install GRP trimsGRP 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…

Drip trim size options in grp fiberglassThe 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.




Raised edge trim size options in grp fiberglass 2

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.




D260 wall fillet abutting a wall in grp fiberglassThe 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.



C150 C100 simulated lead flashing in grp fiberglassSimulated 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 some one knocks into a trim. P.U. can very useful anywhere you need to allow for a little more expansion and contraction though and for stopping trim rattle on trim support battens.

fibreglass trim fixed with 20mm clout nailsFixing 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. It comes in a few different varieties these days and a lot of trade outlets sell it, and of course fibreglass 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 )

fiberglass trims fixed with pu adhesiveP.U. Sealant – Squeeze out a bead or small lines of P.U. 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.




For an alternative to PU adhesive a product like Fix-All also works and is available at nearly all DIY chains, but never silicone.

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.

Hacksaw and tin snipsmultitool and daimond cutter

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.

fiberglass trim overlap ladder access overlapOverlapping 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-grp-trim-raised-edgeOverlapping 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.

How to join joint a fiberglass corner

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.

Laminate csm bandages YT

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.

Bandage GRP fiberglass trim

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 )

5 Litre paint pot resin bucket

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.

Bandaged joints on 8x4 osb boards

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.

fiberglass mat on hollow end corner trimA – 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 topcoating.

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 clean from debris with a soft bristle brush.

Man in deck chairPreparation is king for a nice easy experience, make it easy on yourself. You are now ready to Laminate the fibreglass roof.

To see the process of laying to main roof laminate where you will find part 4 of the video series, click here How to laminate fibreglass

Sanding a fibreglass roof

It’s at this point I like to sand off any little rough edges, high spots or sharp edges, purely for aesthetic reasons. This is not necessary for applying the topcoat as long as you are topcoating on the same day.

Sanding fiberglass deckingEasy sanding – 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. The cordless Ryobi sander pictured is a absolute godsend, see it in the roofing tools section below.


If for some reason you have had to wait more than 24 hours to topcoat the roof laminate you will have to sand very thoroughly first with 40-60 grade aluminium oxide grit paper. Try not to do this.

Topcoat a fibreglass 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, including the final part 5 of the video series, click here – Mixing roof resin and topcoat

Topcoat pic

Timing – 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 flashings 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.

C150 simulated lead flashing fibreglass trim sizes size dimensions roofingC100 simulated lead flashing fibreglass trim sizes size dimensions roofing

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 retain the look of a period property for example. When fitting lead flashings I tend to use about 1 metre or 3 foot lengths to allow for expansion and contraction up to a maximum of 1.5 metres then overlap any joints by 100mm minimum ( 4″ ) See how to create and fix lead flashings in the link at the bottom of the page.

Fibreglass guarantees

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 fibreglass 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.

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