How to Fix a Leak with a Fibreglass Flat Roof Repair
A leaking flat roof looks like a big job, but plenty of leaks come from one small split, crack or failed joint. If the rest of the roof is in decent shape, a fibreglass patch is a straight forward flat roof repair that you can do yourself in an afternoon, for a fraction of the cost of replacing the roof.
This guide covers how to work out whether your roof is worth repairing, how to find where the water is getting in, and how to make a proper patch repair, step by step. It is written for fibreglass (GRP) roofs. If your roof is EPDM rubber or traditional felt, the materials are different, so skip to the section on other roof types.
Is your flat roof worth repairing?
A repair makes sense when the damage is local and the roof underneath is still sound. Think of a single crack, a small blister, a split at a joint or a damaged patch where something has been dropped or dragged across the surface.
Replacement is usually the better option when:
The leaks keep coming back in different places
The deck underneath is soft, spongy or rotten
The surface is crazed or breaking up across most of the roof
Water sits for days after rain and the roof has clearly lost its fall
The roof is very old and has already been repaired several times
If you are not sure, press gently on the deck from underneath if you can reach it, and look at the ceiling below for staining. A repair on rotten boards will not hold, so deal with the timber first.
What causes a flat roof to leak?
Most flat roof leaks come from a short list of causes:
Ponding water. Water that sits on the surface puts constant pressure on any weak spot and speeds up wear. Poor falls and blocked outlets are the usual culprits.
Cracks and splits. Movement in the deck, age and sun exposure can all crack the surface, especially along joints and at up stands.
Damage from foot traffic or debris. Dragging ladders, dropping tools or letting branches and stones build up can all cut or wear through the surface.
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Failed edges and details. Trims, drip edges, outlets and the junctions where the roof meets a wall are the weak points on most roofs. If you can’t find the leak anywhere obvious, look here first.
Age and UV. Every roof covering wears out eventually. A tired roof with several failures is usually past the point of patching.
How to find the leak
Water rarely drips from the spot where it gets in. It can run along the deck and show up on the ceiling some distance away, so work out the source before you mix any resin.
Check the ceiling stain inside, then look at the roof directly above it and in a wide circle around it.
Walk the edges, outlets and wall junctions carefully, looking for splits, lifted trims and gaps.
Look for dark patches, standing water or areas where the surface has cracked.
If you still can’t see it, pick a dry day, wet the roof section by section with a hose andhave someone watch the ceiling below.
Mark the damaged area once you have found it. Check the rest of the roof while you areup there, so you can fix everything in one go.
What you need for a fibreglass roof repair
The easiest way is a ready-made fibreglass flat roof repair kit, which gives you the resin,catalyst and matting together. Whatever you use, you will need
Fibreglass matting
Resin and catalyst, with a mixing stick and container
Acetone for cleaning and preparing the surface
Topcoat and catalyst for the finish
A fluffy roller, a laminating brush and a paddle roller for removing air bubbles
Sandpaper, a marker pen and scissors
Gloves and eye protection, and good ventilation
Resin and acetone are strong chemicals, so wear protective gear and keep naked flames well away. Follow the mixing ratios on your pack exactly, since too little catalyst and the resin stays soft, while too much makes it cure too fast.
Pick a dry day with a roof that is dry, and avoid working in damp or very cold conditions.The instructions with your product will give the temperature limits.
How to repair a flat roof with fibreglass, step by step
1. Cut the matting to size. Measure the damaged area and cut the fibreglass matting so it extends 50mm beyond the damage on every side. The overlap onto sound roof is what makes the repair watertight.
2. Mark the repair area. Lay the matting centrally over the damage. Draw a line around it, 20mm out from the edge of the matting on every side. Everything you do from now on stays inside this line.
3. Sand the surface. Rub over the area with sandpaper, working onto the marked lines.This roughens the surface so the resin can grip. Stay within your markings so you don’t damage the roof further.
4. Clean with acetone. Soak a cloth in acetone and wipe the sanded area inside the lines.This removes dust and grease and helps the new resin bond to the old surface. Let it evaporate before you carry on.
5. Mix a small batch of resin. Add catalyst to a small amount of resin and stir it well with the mixing stick. Only mix what you can use quickly, since it starts to set once the catalyst is in. Use the fluffy roller to coat the marked area with resin.
6. Lay the patch. Place the fibreglass matting centrally over the repair and roll more resin over it until it is completely soaked through, with no dry white patches. Keep within the lines. Then use the paddle roller to press the patch down and push out air bubbles. Pay attention to the edges, because working them down flat makes the repair far less noticeable.
7. Wait for the jelly stage. Leave the resin for about 10 to 20 minutes, until it has gone jelly-like. It should no longer be liquid, but not fully hard.
8. Apply the topcoat. Mix the topcoat with its catalyst and brush it over the repair with the laminating brush. The topcoat can cover your pencil marks but shouldn’t spread past them. It sets exactly as you apply it, so take your time and aim for a smooth, even finish.
That’s it, your first fibreglass flat roof repair is done. Leave it to cure fully before walking on it or exposing it to rain, and check the product instructions for the cure time.
Common mistakes to avoid
Repairing a damp roof. Resin won’t bond to a wet surface. Let the roof dry fully first.
Skipping the sanding and acetone. A smooth, dirty surface is the main reasonpatches lift.
Mixing too much resin at once. It will start to set in the pot and waste material.
Leaving dry matting. Any white, unsoaked fibre is a weak spot where water can get in.
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Ignoring the cause. If water is ponding or an outlet is blocked, the same problem willreturn.
What if my roof is EPDM or felt?
This method is for fibreglass roofs only. EPDM rubber roofs and felt roofs need different repair materials, because resin doesn’t bond to them in the same way. Check what your roof is made from before you buy anything. If you are not sure, send us a photo and we’ll help you identify it.
What customers say
“I did my research in depth before attempting this project, I had a friend of mine on hand and ready to get stuck in. I read through the many articles available on theRoofing Kits Direct website under the Help & Info section. Ordered my products and cautiously worked through the steps mentioned. The roof looks great and as we weren’t in a position to replace it right now our repair will certainly suffice for sometime. The one thing worth mentioning if you have any reservations, ask. The RKD team couldn’t have been more helpful, especially Neil who gave on-the-go advice from start to finish.”
Flat roof repair FAQs
How long does a fibreglass flat roof repair last? It depends mainly on the condition of the rest of the roof. A properly prepared patch on a sound roof can last for years, but if the roof is old and failing elsewhere, you may only be buying time.
Can I repair a flat roof in winter? Resin cures slowly in the cold and won’t bond properly to a damp surface, so wait for a dry spell and follow the temperature guidance with your product. If a leak can’t wait, a temporary fix may get you through until the weather improves.
Can I repair an EPDM or felt roof with fibreglass? No. Use repair materials made for theroof you have. Fibreglass resin is for GRP roofs.
Do I need planning permission to repair a flat roof? Repairs don’t normally need planning permission. Replacing a roof can involve building regulations, especially where insulation is concerned, so check with your local council if you’re replacing rather than repairing.
How do I know if I need a new roof instead? Repeated leaks, soft or rotten decking,widespread cracking and permanent ponding all point to replacement rather than repair.
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Can I do this myself? Most small repairs are well within DIY reach if you prepare the surface properly and follow the instructions. For large areas, damaged decking or work at height, a professional may be a better choice.
For more educational and information Roofing Kits Direct has a whole hub of articles and blog posts available to browse here : https://www.roofingkitsdirect.co.uk/news-blog/
Ready to repair your roof?
Our fibreglass roofing repair kit has what you need for a patch repair. If you’d rather askfirst, get in touch and tell us about your roof, and we’ll point you to the right materials.
