Moss removal on a slate roof
By the Toiture Wash team · Updated on 20 October 2026 · 3 min read
Short answer. Moss is removed from a slate roof in the same way as from a tiled roof, with two precautions: first find out whether the slates are natural or artificial, because older artificial slates may contain asbestos, and set the cleaning so that the slates are neither moved nor cracked. A sound natural slate is not very porous: the moss settles mostly at the overlaps.
Natural or artificial: the first question
The word "slate" covers two very different materials:
| Natural slate | Fibre-cement slate (artificial) | |
|---|---|---|
| Material | split stone (schist) | cement reinforced with fibres |
| Appearance | irregular edges, variable thickness, glints | clean edges, regular thickness and colour |
| Asbestos | no | possible for slates laid before 1 October 1998, the date asbestos cement was banned |
| Before the moss removal | check for cracked slates and hooks | analysis if the date of installation is unknown |
Any doubt about asbestos is cleared up before we quote. A roof that contains asbestos is not cleaned. Eternit slates without asbestos usually carry the NT marking, and the 60 x 40, 60 x 32 and 45 x 32 cm sizes never contained any; a laboratory analysis settles it. It is all explained in Does my roof contain asbestos?
How moss is removed from a slate roof
- Check for cracked, slipped or missing slates, and the hooks that hold them.
- Walk on the roof as little as possible: a slate can crack under a foot.
- Clean in the direction of the water flow, without aiming the jet under the slates, which are often held only by a hook. On its US site, the natural slate manufacturer Cupa Pizarras accepts a pressure washer in most cases, with precautions (spraying the water from top to bottom, never upwards), but first recommends a thick brush and vinegar.
- Rinse and clean the gutters.
- Treat: at Toiture Wash, every moss removal ends with a treatment included in the price: depending on your roof, a colourless water repellent or an algicide, fungicide and bactericide product; on a hard-wearing roof, it is generally the algicide. On slates without asbestos, painting can also follow, always on a different day from the moss removal, as in Forchies.
Our jobs on slate roofs
- Woluwe-Saint-Pierre, May 2024: moss removal on a slate roof without asbestos, colourless water-repellent treatment, cleaning of the gutters and the clay pavers. See the job
- Forchies, May 2024: moss removal on a slate roof without asbestos, gutters, then roof painting. See the job
Frequently asked questions
Can slates be cleaned with a high-pressure washer?
For natural slates, the manufacturer Cupa Pizarras accepts a pressure washer in most cases, spraying the water from top to bottom, never upwards, but it first recommends a thick brush and vinegar. Never on a roof that may contain asbestos.
My roof is "Eternit": what should I do?
"Eternit" is a brand name that has become the everyday word for fibre cement. If the slates or sheets date from before the asbestos ban, or if you do not know their date, have them analysed before any cleaning.
Do natural slates need a water repellent?
The treatment is chosen according to the roof: a colourless water repellent, which waterproofs it, or an algicide, fungicide and bactericide product, which slows down moss regrowth; on a hard-wearing roof, it is generally the algicide. With us, the question does not change the price: the treatment is included in every moss removal.
Sources
Pages consulted on 2 October 2026:
- Royal Decree of 3 February 1998 restricting the placing on the market, manufacture and use of certain dangerous substances and preparations (OpenJustice reproduction) (in French).
- Finding out about the health risks linked to asbestos exposure (Wallonia) (in French).
- The history of asbestos (Eternit) (in French).
- How do you clean a slate roof? (Cupa Pizarras), the manufacturer's US site (in English).