Safe cleaning
Is roof cleaning safe for your tiles?
The short answer
Yes, when the method suits the covering. Gentle treatment and careful hand work are safe for most tiles and slates. The damage comes from the wrong method: a hard jet that strips the surface or forces water under the laps, or a heavy foot on fragile material. Some coverings, such as fibre cement that might contain asbestos, need identifying before anyone touches them.
Safe depends on how the clean is done and what the roof is made of. Here is what to look for, and what to ask before work begins.
Where roof cleaning goes wrong
Almost every case of damage traces back to force. A pressure washer used close to a tile can strip the protective surface and leave it rough, pitted and quicker to re-grow. It can also push water beneath the laps, the overlaps between courses, and on into the roof space. Boots on brittle tile snap it. Scraping with a hard tool takes the surface off together with the moss.
These are not rare accidents. They are the natural result of treating a roof like a patio. A cleaner who has worked on roofs before knows that gentle and patient is the safe way. Our page on pressure washing roof tiles explains the risks of the jet in more detail, and why we choose another method.
Why a gentle approach suits tiles and slates
The usual alternative is a low-pressure application of a biocide, with the bulk of any thick moss lifted off by hand beforehand. The product does the work, so the surface is not scrubbed. The growth dies back and weathers off over time, which is slower, and much kinder to the covering.
Soft treatment is not a licence to be careless. Product choice, mixing and rinsing all matter, and so does keeping the spray off the lead flashings, the painted fascias and the glass. The approach is covered in roof soft washing. What counts for the owner is that the method is named in the quote, and that the person who gives it to you can say why it suits your tiles.
Safety for the people on and around the roof
Roof work is a fall risk, and a damp, moss-covered roof is slippery at the best of times. A cleaner should be working from proper access, using ladders that are tied off or a scaffold or tower where the height calls for one, and should not be walking directly on weak or old tiles. If someone offers to do a two-storey roof from a single ladder with no edge protection, that is a warning.
You have a part to play too. Children and pets should stay clear of the working area and the ground beneath, and cars and washing need moving. Ask about the product, and keep windows closed for the time the cleaner suggests. Anyone working at your home can explain how they keep people safe. If they cannot, find someone who can.
Plants, pets and the ground below
Run-off from a treated roof goes to the gutters, the drains and the borders below. Good practice is to protect plants and ponds, direct the water away from them, and avoid treating before heavy rain. Ask how the cleaner deals with this, and which product they use, so you can read the label.
The law protects bats and nesting birds, and a roof can shelter either in the right season. One sentence is enough here: if you know or suspect either is present, tell your cleaner before work begins and the work can be planned around them. Gardens, ponds and pets are easily protected with sheeting and planning, which is part of the setup before any treatment is applied.
Roofs that need checking before a clean
Some roofs should not be cleaned until they have been properly identified. Corrugated fibre cement sheet on a garage or outbuilding can contain asbestos, and HSE guidance says to treat a building from before 2000 as though asbestos is in it. Such roofs are fragile as well, and must not be walked on or jetted. Thatch, stone slates and very old clay tiles also call for specialist handling.
If your covering is in doubt, say so at the start. The survey will identify it, and if testing is needed we will say so before any work. For tile types and the methods that suit them, see cleaning concrete roof tiles, and the main roof cleaning and moss removal page.
Questions that show a cleaner knows roofs
You do not need to be a roofer to test a cleaner. Ask what method they will use on your particular covering and why. Ask how they get to the roof, and whether anyone will walk on the tiles. Ask what product goes on and where its label can be read. A cleaner who knows their trade answers these without fuss and in terms you can follow.
Be wary of the answers that raise flags: a jet wash on tiles at close range, a promise of a spotless roof by the weekend, no mention of the gutters or the plants below, and a reluctance to look at the roof before naming a price. A written quote that names the method and the access is a good sign. Our survey comes with photographs of your roof, so you see the same view we do. For a wider view of the job, the roof cleaning and moss removal page is the place to start.
Related questions
Before the work starts
Do I have to stay in while the roof is treated?
You do not have to be, but it helps on the first visit so you can agree where the equipment goes and which windows to close. Pets and children should stay clear of the working area until we say the ground is safe.
Do I need to move the car and bring in the washing?
Yes, as a sensible precaution. Dislodged moss, spray and drips can fall on parked cars and clothes. Clearing the drive and line before we arrive keeps everything clean and keeps the working area clear.