What the HSE guidance says
HSE's sheet A12, "Cleaning weathered asbestos cement (AC) roofing and cladding" (09/17 edition), sets out good practice for cleaning AC cladding and roofing, either to improve its appearance or to prepare it for a surface coating. Its position on pressure is blunt: "Only in exceptional circumstances is high-pressure jetting appropriate. This requires a specialist contractor."
The method it describes instead
For cladding you can reach from ground level, the sheet describes preparing a biocide to the label instructions and applying it with a low-pressure sprayer. You then allow time for the biocide to work, keep the surface wet and remove growths by gentle scraping. Dead plant roots are hard to remove and the sheet says to leave them in place. Debris is scooped into an asbestos waste bag.
The alternative it covers is a specialist roof cleaning contractor with a cleaning machine. HSE notes this method creates a lot of slurry that must be collected for disposal, so the contractor should divert it through a collection and filtration system and keep solid waste wet and bagged. Our guide to where pressure washing wastewater goes covers the ordinary-surface side of that question.
Fragile, and not for standing on
The sheet states that AC roofs are always fragile and cannot bear weight. It also says to prevent slurry entering the building by sealing gaps with polythene sheeting and duct tape, and lists protective equipment for the manual and filtration work: disposable hooded overalls, boots without laces, single-use gloves and a disposable particulate respirator (for example FF P3).
What the sheet does not cover
HSE says the sheet is not appropriate for cleaning asbestos insulating board, which needs an HSE-licensed contractor. It also notes that following HSE guidance is not compulsory unless specifically stated, but that doing so will normally be enough to comply with the law.
Where the slurry goes
The HSE sheet is clear that the specialist machine method creates a lot of slurry that must be collected for disposal, through a collection and filtration system, with solid waste kept wet and bagged. For a Dover property that is the practical test of a specialist quote: ask how the slurry is collected, rather than assuming it runs off into the nearest drain.
Sources: HSE, "Cleaning weathered asbestos cement (AC) roofing and cladding" (sheet A12); HSE, Asbestos essentials. This is a summary of published guidance, not a survey or a substitute for advice on your own property.