Ask Hamish: how to fix a roof box in 7 easy steps


Technical guru Hamish Cook is always on hand to answer your epoxy-related questions. Amongst the flurry of emails this month was a query about how to fix a broken roof box. Here, Hamish shares his know-how to help you mend and save money.

I recently heard from someone who’d broken the lid of their roof box near the hinges. They’d tried to put a larger-than-normal item into the box and it had cracked.

Roof boxes are expensive, costing anywhere from £100 to more than £200. So it’s good to know that you can make an ultra-strong repair for around the £20 mark – with the help of some epoxy resin.

I recommend using the WEST SYSTEM G/flex® epoxy range, along with some fibreglass cloth. G/flex is ideal for roof boxes, as it makes structural bonds that absorb the stresses of expansion, contraction, shock and vibration.

G/flex is available in two viscosities: 650 Epoxy, a versatile, easily modified liquid epoxy; and 655 Epoxy Adhesive, a convenient pre-thickened epoxy. A number of handy kits are also available to suit different uses and projects.

Below is the suggested approach.

Step 1: sand the area

Working on the inside of the box, start by sanding the area around the break. The coarser the grit of the sandpaper (around 80 grit should do it), the better the adhesion to the epoxy.

Step 2: clean the area

Wipe the area clean using alcohol wipes. A clean surface is vital as it will ensure a strong bond.

Step 3: flame-treat the plastic

Pass the flame of a propane torch across the surface quickly. Allow the flame to touch the surface, but keep it moving – about 30 to 40 cm per second. No obvious change takes place, but the flame oxidises the surface and dramatically improves adhesion with adhesives and coatings applied over it.

Step 4: warm everything up

Heating the resin and hardener before mixing them will lower the epoxy’s viscosity, allowing it to penetrate deeper into cracks.

Also gently heat the area to be repaired with a heat gun. Even a hair dryer will do it!  Do not over heat or you may cause further damage. 

Step 5: mix your epoxy

Mix a small batch of WEST SYSTEM G/flex epoxy, making sure you mix it thoroughly. G/flex is very easy to use, as it just needs a simple 1:1 ratio by volume of resin and hardener. It has a pot life of 45 minutes, so you’ll have plenty of time to make the repair carefully.

Step 6: apply the epoxy

Fix the roof box into a position where gravity will pull epoxy into the crack. Then work the epoxy into the crack with a mixing stick or a small brush, or inject the epoxy using a syringe. Use a fine blade or stick to push the epoxy as far into the crack as possible. Wait a few minutes for it to soak into the surface and then clamp the crack closed.

Allow it to cure for 7-10 hours before removing the clamps and sanding away any excess epoxy.

Step 7: add extra strength

For extra strength, thicken G/flex epoxy and use it as a filler, filing in any holes or chips. It’s also a good idea to apply some lightweight fibreglass fabric over the top for extra reinforcement.

To do this, cut several sections of cloth, making sure it goes at least 5cm over the crack all around. Then clean the roof box surface as above and mix the epoxy.

Coat the fixed area of the roof box with G/flex 650 epoxy and lay the fibreglass fabric in position on the wet adhesive, spreading it with a plastic spreader. Use the spreader to smooth and remove excess epoxy/bubbles. Then repeat the process with additional layers.

Leave it to cure for another 7-10 hours. If you like, varnish or paint it to finish.


Thanks very much to Hamish Cook for his contribution.

For more information on the full WEST SYSTEM G/flex range, visit the West System International website.

Got something else that needs fixing? Read How WEST SYSTEM epoxy can help you with household repairs (Part I).

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