The short answer
It can be, but only with great care and never by the usual methods. Scraping, wire brushing, power tools and pressure washing can damage asbestos cement sheets and release fibres, and the run-off can carry contamination onto the ground. If the roof is sound, gentle low-pressure treatment may be possible, but first confirm what the material is, assess its condition and follow the Health and Safety Executive guidance. Do not do it yourself.
What an asbestos cement roof is
Asbestos cement was a common roofing material on garages, outbuildings, sheds, industrial units and some houses for much of the twentieth century. It is made of cement reinforced with asbestos fibres and appears as flat or corrugated grey sheets. Left intact and undisturbed, it releases few fibres. The danger comes when it is broken, drilled, abraded or weathered to the point of flaking. Many older garage roofs are covered in moss and lichen, which makes people want to clean them, but that is exactly where care is needed.
Why normal cleaning is unsafe
Scrapers and stiff brushes abrade the surface. Pressure washers drive water and debris against it, can break sheets, and spread contaminated slurry across the ground, gutters and drains. Power tools make dust. The HSE advises against using these methods on asbestos cement, and the page on non-licensed asbestos work at hse.gov.uk explains the duties around it. Dry moss that is scraped or brushed off can carry fibres with it. Our guide to whether pressure washing damages roof tiles shows how forceful water harms even sound tile, and the risk is greater on a brittle sheet.
First, confirm what it is
You cannot tell asbestos cement from a modern fibre cement sheet by eye. Age is a clue, as is a pale grey, weathered, slightly rough surface, but only a sample analysed by a laboratory, or a survey by a competent person, can confirm it. Do not take a sample yourself. If the roof is on a commercial or rented building, the duty-holder has legal obligations to manage asbestos. We do not give legal advice, so check with the HSE or your own adviser.
What gentle methods exist
Where a sheet is in good condition, a low-pressure application of a biocide with a soft tool, followed by letting the growth die and weather away, is sometimes used, because it does not abrade the surface. It is slow, and the dead moss must not be brushed off. Run-off needs to be controlled so it does not carry fibres into a drain, and risk assessments and controls are always needed. Sheets that are cracked, crumbling or heavily weathered should not be treated at all. A survey decides which case you have.
Alternatives to cleaning
Often the sensible answer is not to clean. The HSE treats careful removal of whole sheets as non-licensed work, with a step up to notifiable work where the sheets would end up in many pieces. Overlaying the old roof with a new covering is sometimes an alternative to removal. Our page on roof cleaning, run-off and wildlife explains why run-off needs thought on any roof.
Why moss grows so thickly on these roofs
Asbestos cement is porous and slightly alkaline at first, but with age the surface weathers and roughens, which gives moss, algae and lichen a good grip. Corrugated sheets have troughs that hold water and debris, and low-pitched garage roofs drain slowly. The growth looks untidy and can block the gutter, but on a sound sheet it is usually a cosmetic issue before it is a structural one. The main practical concern is that heavy moss and wet debris make the roof slippery and hide cracks, so nobody should go onto it.
Gutters, downpipes and run-off
Asbestos cement gutters and downpipes were common too, and the same care applies to them. Fragments and dust can wash into the gutter, so clearing a blocked asbestos gutter by hand is a job for someone who has assessed the risk. Water from the roof should not be directed to a place where it can pick up debris and carry it onto a path or into a drain. If you see sheets that have cracked, slipped or been patched, mention it when you call, since it changes what is safe.
What to do now
Leave the roof alone for now, and do not let anyone talk you into a quick jet wash. Keep children and pets away from any damaged sheet. Do not walk on it, because asbestos cement is brittle and a fall through it is a serious risk. Photograph the roof from the ground and send the pictures. We can survey the structure, advise on the safe options and, where overlaying or careful removal is sensible, plan it. If the building is commercial, see our related guide on asbestos roofs on commercial buildings.
See also the main roof cleaning page for what we do on ordinary tiled roofs.
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