How to repair an asbestos cement roof
Instapatch can seal small cracks, chips and holes in asbestos cement roof sheets without cutting, drilling or removing anything. This guide is for minor damage only. Asbestos needs care, so the safety steps matter as much as the repair itself.
Is this repair right for you? It suits small cracks and holes in a sheet that is otherwise sound. It doesn't suit sheets that are badly damaged, broken or crumbling. HSE's guidance is that badly damaged asbestos cement should be removed rather than repaired, so use a contractor experienced with asbestos for that.
What you'll need
- An Instapatch Standard patch big enough to cover the damage with room to spare. The order wizard will suggest a size.
- UV Curing Primer and a brush. Always prime asbestos cement.
- Scissors or a sharp knife
- Disposable overalls with a hood, and gloves
- A bucket of water and rags
- Polythene sheeting, duct tape and heavy-duty polythene bags
- A safe way to reach the damage without standing on the roof
Before you start
- Never stand or kneel on the roof. Asbestos cement roofs are always fragile and can't take your weight. Work from a ladder or platform at the edge, or use proper access equipment.
- Don't make dust. No sanding, scraping, wire brushing, drilling or power tools. Clean with damp rags only.
- Keep other people and pets away while you work.
- Wear disposable overalls with a hood, and gloves.
- Pick a dry day. The surface has to be completely dry before you prime and patch.
- Only open the patch when you're ready to use it. It can start to cure within minutes in daylight.
Steps
If debris could fall onto anything below, cover it with polythene sheeting, taped to surfaces that aren't asbestos.
Gently wipe the damaged area with damp rags to lift loose dust, moss and debris. Don't brush or scrape. Put used rags straight into a polythene bag.
Let the area dry completely.
Brush UV Curing Primer over the damaged area and let it dry fully.
Shape the patch with scissors while it's still in its packet.
Peel the release film from one side, press the patch onto the primed area straight away, and mould it to the shape of the sheet. Apply firm pressure all over.
If you want to paint the repair later, peel off the top film now for a matt finish. Leave it on for a gloss finish that can't be painted.
Leave the patch to cure in daylight. Keep pressing the edges down until it hardens, so the corners don't lift.
Put the rags, polythene and any debris into heavy-duty polythene bags, seal them, and ask your local council how to dispose of asbestos waste.
Cure timing for this surface
Daylight cures the patch in 15 to 45 minutes, depending on how bright it is. On a dull day, allow longer. It cures in air temperatures from -15°C to 70°C.
After the repair
Once it has hardened, the patch is waterproof and highly resistant to weather and UV. If you removed the top film, you can paint it. Don't sand or drill the repair on an asbestos roof, because that would disturb the sheet underneath.
Common questions
HSE treats repairing damaged asbestos cement as non-licensed work, as long as the precautions in this guide are followed. If the sheet is badly damaged, or you can't reach it without standing on the roof, use a contractor.
Yes. Asbestos cement is porous, so always prime it first.
Yes. Follow the same safety steps, and see our gutter guide for fitting it to the curve.
Common mistakes to avoid
- Standing on the roof to reach the damage
- Brushing, scraping or sanding the sheet to clean it
- Patching over a damp surface or primer that hasn't dried
- Opening the patch before you're ready to use it
Safety steps based on HSE Asbestos essentials sheet A13, Repairing damaged asbestos cement (AC): hse.gov.uk/pubns/guidance/a13.pdf
Safety
Wear gloves and protective clothing, and only use Instapatch in a well-ventilated area. It contains polyester compound and styrene, which can irritate the eyes, skin and lungs. If it gets in your eyes, rinse them straight away with plenty of water and get medical advice. If it gets on your skin, wash it off with soap and water.
Storing spare patches
Keep unused patches on end in their original wrapping, somewhere cool, dry, dark and frost free. If a patch is very cold, warm it before use.