How to install a fibreglass roof

August 2, 2018

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.