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How to install a fibreglass roof

How to install a Fibreglass a flat roof

Below is a guide for Fibreglass roofing installation. As each project is different this guide cannot be a definitive but provides good basic direction to install your project.

As everyone is aware, but don't always action, the key to any successful project lies in good foundations and preparation. No matter how good the fibreglass roof installation is, if the timber work supporting it is suspect this will always cause issues down the line.

RKD recommend the use of OSB (Oriented Strand Board) decking for laying

our new fibreglass roof onto. We lean towards the T&G option. OSB 3 comes in 2 types - 8’ x 4’ (2400mm x 1200mm) sheets and T&G in 8’ x 2’ (2400mm x 600 mm)sheets.

Tip with OSB 3 sheets allow 2 -3mm gaps between boards to cope with expansion. – T&G decking incorporates expansion within the system.

The most likely decking works you will encounter are – the need to overboard and complete replacement.


The first question here is, having removed the old roof covering, is the decking fit for purpose?

1.Is it 18mm thick or above

Is it rot free

3.Is it firm

4.Is it sterling board, plywood or T&G

A good example here would be where removal of a bitumen based covering leaves traces of bitumen on the decking. The deck is sound but bitumen will contaminate the fibreglass. So overboarding is a good solution.

Tip – leave a 3mm gap between OSB 3 sheets and a 25 mm gap where the decking meets a wall or upstand feature. (T&G OSB boards have expansion built in)

Decking Replacement

Commonly flat roof replacement comes because the roof has been leaking. This can leave the decking weak and flaky and with water damage you can’t always see all the damage at first inspection.

Sometimes the roof fall or slope is poor and ponding may have occurred so this is worth checking. You may need to replace with correct firing strips applied to the rafters.

When removing an old deck remember, this is not a demolition job, be firm but careful when levering up the old deck so as not to damage the roof rafters or any insulation beneath.

Clean and remove any debris and nails that have pulled through (don’t sweep this onto the insulation or plasterboard underneath)

Replace the deck with OSB 3 or OSB 3 T&G and screw down.

Fibreglass Trims

Trims commonly come in 3m lengths. The most commonly used trims are -

The Drip trim -
used at the roofs lowest point directing the water into the gutter.

The Raised Edge trim –

used to prevent water dripping off the roof edges and guides waterflow to the gutter edge.

The Wall Fillet trim -
used to create a waterproof upstand against a wall or upstand (Fits over the 25mm gap between decking and wall)

Simulated lead flashings - 
fits over the wall fillet trim and slotted into a groove in the wall and fixed with PU adhesive. Finished off with silicone bead to groove to waterproof

Fixing edge trims - use ring shank nails/clout/felt nails (screws can split the trim) You can also use PU adhesive where you think expansion needs to be considered.

Below are diagrams of how to fix various trims within the installation of a fiberglass roof. As above these are nailed (flush) to the deck. Diagrams show where bandaging is applied.

Cutting trims - RKD recommend that you invest in a good pair of snips. Old tech but they work well -You can also use a hacksaw or grinder but remember that here dust will arise - a mask is mandatory.

Joining trims - simply overlap by 50 - 75mm applying a bead of PU adhesive between the trim surfaces. Using a small amount of manual persuasion they will clip together nicely.

Corners - There are two options here

Mitre join - keep one trim straight to the edge and mitre the other at 45 degrees overlapping this to the inner edge of the straight trim. You will have an open end evident - do not be concerned. For visual neatness and strength bandaging is applied later. RKD kits provide bandaging rolls for your convenience.

We also provide a range of preformed corners to enable certainty of visual corner installation these can be found on our website here –

Before you start - clean your trims down with acetone and light sand smooth areas to create good bond capabilities.

Tip - The application of bandaging provides excellent practice in mixing and laying before the main event. Here you can test your mix, working and curing times.

Check the temperature and your catalyst addition chart, use 1 litre of resin to start with, you can always mix more. Add the catalyst and mix thoroughly.

Bandaging the trims -

Apply the mixed resin to the trim and adjacent deck with a small resin roller. Roll the bandage on top of the resin and then apply more resin. Give it a minute or so to settle then work with a bubble buster roller - this will break down the binding agents and as its name suggest remove any bubbles or trapped air. When complete your bandage should be transparent and no bubbles within it evident.

Bandaging the expansion gap you left in the OSB 3 decking boards -

Lay your bandage over the 3mm gap, apply mixed resin and bubble bust as above. You may like to feather the edges of the bandage pre resin this help provide a smooth final finish.

Bandaging corners and open ends -

Corners (using bandage) - rip off an appropriate length of bandage, wet the bandage on one half in the bucket and then apply over the corner join holding the dry half. Add resin to the bandage and smooth over the corner join. Obviously a bubble buster roller cannot be used here - the same effect is achieved by adding and working the resin with a stippling motion of the brush

Open ends - Cut a piece of matting which will overlap all surfaces by about 50mm and face up to the corner - Snip the bandage where folds are needed. Apply resin at the bucket to the bottom half of the bandage and press to the corner. Add more resin to the bandage to work the folds. Again use stippling motions with the brush to remove bubbles and be careful to smooth the bandage on all surfaces away from the open end to give a flat finish over the gap.

Laminating your fibreglass roof is, providing you get the mix right actually a straightforward and repetitive job.

Much of the work is in preparation -

  1. Wear mask, gloves and safety goggles throughout the installation process 

  2. Lightly sand the bandaged areas to enable a good bond here – remove dust 
diligently make sure the entire roof is clean and dust/debris free 

  3. Take your matting and measure out each length required – physically lay it 
out for certainty. Make sure it's square and true overlapping any trims properly. Roll the length of matting up. Stack it at the beginning of where it will be applied. Repeat the process allowing for a 50-75mm overlap of each sheet until you have enough to cover the roof. You should then have a set of rolls stacked in order of usage ready to go. Matting rolls have one straight edge and one feathered edge this allows blending between sheets for aesthetic smoothness of finish. (you can feather an edge yourself by pulling firmly on the edge of a sheet and thus feathered edges overlapping for a totally smooth result.) 

  4. Plan your process and leave yourself an escape route - you'd be amazed how often even experienced installers can 'paint themselves into a corner' 

  5. Get your tools ready to go and to hand where you will need them. If you are nervous about time lay out more rollers, brushes, bubble busters than you need - better to be safe than sorry. 

Tip - Now you are ready to go - If you are new to this you will be worried about laying out and can you do this to meet curing time and what if? Don't - this is a methodical process, Step by step and repeat.

If you can have a second member of the team to hand use them

Don't mix too much resin, 4 or 5 litres is enough to start, you can always mix more. If the mix starts to cure quicker than you thought and brushes/rollers are getting hard - well that's why you laid out extra and you can lessen the % catalyst added on the next mix.

  1. Mix your resin and catalyst – take note of the temperature and if you are in direct sunlight - use your chart to calculate the right amount of catalyser for the number of litres of resin in the mix. User a proper catalyst dispenser - Mix thoroughly! 

  2. Your going to use about 2litres per sq.m on a 450 gsm laminate 

  3. Get your first roll ready and correctly aligned. 

  4. Start in manageable sections – using a roller place resin out on the section of 
deck - roll out the matting onto the resined section and roll in. repeat with the next section then go back to section 1 adding more resin on top of the matting to wet it out. Don't skimp on the resin it makes consolidating hard work (don't go mad either) 

  5. Repeat 

  6. Consolidating the matting and resin - here's where the second person comes 
into play. 

  7. By the time you've got to section 3 or 4 sections one and two will be ready to 
consolidate using a bubble buster roller. This entails smooth rolling up and down to help break down and remove bubbles. Take care over joints. If you come across dry areas add more resin, if areas remain opaque consolidate until gone. 

  8. Continue as a team one wetting out one consolidating – move from row to row of matting ensuring smooth overlap and keep going ( you can't stop for a break unless there's subs to take over.) 
Tips - make sure you overlap trims and bandaged sections neatly with enough to ensure watertightness. On wall trims overlap as high on the upstand as possible. ( this will be covered by the flashing when finished) 

  9. Clean up - discard brushes and roller sleeves environmentally - clean frames and handles in acetone. - DON"T clean your hands in acetone use proper wipes and cleansing agents. 

Topcoat - 
If you apply topcoat same day or within circa 12 hours there's no need to sand the laminate for bonding purposes. Outside this you will need to. However many installers like to give a light sand to check for any bumps and smooth trim edges etc. (obviously clean off properly) 

Again for topcoating always wear mask, goggles and gloves from A-Z 
As with laminating get your tools ready and to hand. Mask off with masking tape any edges to insitu fascia boards etc 

Start with a small amount of topcoat say half a litre - Check the temperature and apply catalyst from dispenser in the correct ratio - mix thoroughly. Use this for cutting in corners and trim edging with a paint brush. 
You will use about 1 litre of topcoat for about 2 square metres of roof. ~In winter this would reduce a little to about 1.5 square metres.

OK you're good to go with the full topcoat - use a normal size roller for the main section and a small one to cover edge trims as this will provide a more pleasing finish.

Installers often use a pour and spread technique to apply the topcoat tipping small amounts straight onto the laminate and spreading the liquid evenly around the deck. Again, as with resin, don't skimp as you will need to have a thickness to accommodate good UV protection. Use your small roller to neatly finish off trims and edges.

There you go - done - your roof will appear glossy initially but this will cure to a more satin appearance in a few days.

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