The short answer
A cracked asbestos cement sheet can release fibres, but the risk is low while the damage is left undisturbed and the sheet stays out of reach. It becomes a real hazard when a broken sheet is scraped, swept, drilled, pressure washed or walked on. Keep people away, do not touch it, and have the sheet identified and the repair or removal planned by someone who handles asbestos cement properly.
Why a crack matters
Asbestos cement sheet was made by pressing cement and asbestos fibre into corrugated panels, and it went onto many garages, sheds, farm buildings and car ports. So long as the fibres remain bound inside the cement, the sheet is stable. Weathering slowly roughens the surface, and moss and lichen take hold, but an intact sheet that nobody disturbs is generally considered a low risk.
A crack changes that in a small way. The broken edge exposes cement that was never meant to be open to the air, and loose particles can come away if the sheet flexes, is knocked or is cleaned. The harm from asbestos comes from breathing in fine fibres over time, so the aim is always to avoid making dust. That is why a cracked sheet is treated with more care than a sound one, even though a single hairline crack is not an emergency.
How the sheet cracked also tells you something. A sheet struck by a fallen branch or a ladder has a clean fracture. A sheet that has sagged between its purlins (the cross members that carry the sheets) may be tired and brittle across its whole length. One cracked sheet on a garage roof is often a sign that its neighbours are not far behind.
What to leave alone
The usual damage to an asbestos roof is done by well-meant clean-ups. Nobody should do the following to a cracked sheet.
- Do not sweep or brush it. Dry brushing breaks the surface and lifts fibres into the air.
- Do not pressure wash it. Jet washing drives fibre-bearing water and debris into the gutter, the ground and the air.
- Do not drill, cut or screw into it. Fixing a new bracket, aerial or solar clip through a sheet creates fresh broken edges.
- Do not walk on the roof. Old sheets are fragile and a person can fall through them. A fall through a garage roof is a serious risk in its own right.
- Do not use a standard vacuum cleaner. It will pass fine dust straight through the filter.
If pieces have already fallen inside the garage or onto the ground, do not pick them up bare-handed or sweep them away. Stop anyone using the space under the damaged sheet, and shut the garage door if that can be done without brushing against the roof.
Is it definitely asbestos?
A corrugated grey sheet on a garage is a strong suspect, but looks alone do not settle it. Asbestos cement sheets from the post-war decades onward are common on outbuildings and were often used for car ports and lean-tos. Newer sheets made without asbestos can look very similar, and some old sheets have been painted or covered in growth.
The only way to know is a sample analysed by a laboratory, or an inspection by someone who works with asbestos materials. Taking the sample is itself work that disturbs the material, so it is better done by a person who knows how to take it with the right precautions than by a homeowner with a screwdriver. Our guide to car port roof sheets explains how the newer sheet materials differ in colour and feel from the old ones.
Until the answer is known, treat the sheet as asbestos. That costs nothing and removes the guesswork.
How a broken sheet is dealt with
What happens next depends on how much damage there is. The Health and Safety Executive separates careful removal from heavy breakage. Lifting asbestos cement roof sheets off in one piece falls on the lower-risk side of that line, whereas work that will smash the sheets and throw up a lot of dust and rubble has to be notified first. The detail is on the HSE page on work that needs no licence, and it is why a shattered sheet is handled differently from a single crack.
In practice there are three routes for a garage or outbuilding.
- A single cracked sheet on an otherwise sound roof. The sheet may be removed whole and replaced, with the neighbouring sheets checked for movement.
- Several tired or cracked sheets. This usually points to replacing the whole covering rather than patching, because the whole covering has aged out.
- A badly broken roof. Where sheets have collapsed or fragmented, the clear-up is specialist work and the notification route applies.
The broken sheets themselves become hazardous waste. They are sealed in heavy plastic and delivered to a tip that takes asbestos, never put out with household rubbish or loaded into an ordinary skip.
What goes back on the roof
Once the old sheets are out, the garage still needs a roof. The usual choices are a lightweight metal box profile sheet, a fibre cement sheet made without asbestos, or on a flat-roofed garage a new membrane over a fresh deck. Which is right depends on the purlin spacing, the roof pitch and whether the timbers beneath have been damaged by years of drips.
Many old asbestos garage roofs sit on a thin, widely spaced frame designed for brittle cement sheet. A heavier covering can overload it, and a lighter one may need extra fixings. Where the old roof was nearly flat, the garage may be a candidate for a pitched roof instead, and our page on flat to pitched garage roofs sets out what that involves. If the timbers or boards are also damaged, our page on the garage roof deck explains what is checked underneath.
Whichever covering is chosen, the new sheets are fixed mechanically with the right fasteners and the ridge and edges are sealed, so the roof sheds water and stands up to wind.
What we do when we are asked to look
When someone tells us they have a cracked sheet on a garage roof, the first step is a survey taken from the ground and from inside the building, with nobody stepping on the roof. We note how far the damage runs, how the neighbouring sheets sit, what shape the frame is in and whether any fragments have already dropped. Photographs of your roof come back to you so you can see the same things.
From the survey we set out the safest route for that particular roof, either swapping the one sheet or re-covering the lot, and how the removal and disposal would be handled. After the survey you get one price on paper, and the finished work carries our written ten-year workmanship cover.
If you are not ready to arrange anything yet, the interim step is simple: leave the sheet alone, keep the area clear and resist the urge to patch it. When you want it looked at, the number is 01225 285181, and WhatsApp or the web form work too. The main garage roof replacement page covers the whole service.
