A slate roof is cleaned safely with a low-pressure soft wash and a biocide, with any moss lifted off by hand first. Slate itself is hard, but the nails and hooks holding each slate are not, so high pressure or walking on the roof can crack or dislodge slates. Gentle cleaning leaves them undisturbed.
What grows on slate roofs?
Mostly algae and lichen, plus moss along the ridge, in valleys and wherever debris collects. Slate is smooth and dense, so moss finds it harder to root on the face of the slate than on rough concrete. What you usually see is green or black streaking down the slope and growth along the edges and joints.
Which Essex homes have slate roofs?
Victorian and Edwardian terraces and villas, which are common in Southend, Westcliff and Leigh, Chelmsford’s Moulsham, Colchester’s older streets and the town centre of Grays. Slate was carried all over the country by rail in the 19th century, so it turns up on older homes in almost every Essex town.
Why is pressure washing risky on slate?
The slate is hard; its fixings aren’t. Slates are held by nails or by hooks, and on an older roof those nails may be corroded. A high-pressure jet can lift a slate, snap a weak nail and leave a gap. It can also drive water up between the slates, and wash mortar out of the ridge.
Why shouldn’t anyone walk on a slate roof?
Slates crack easily under a foot, and a cracked slate lets water in. Roofers use roof ladders and crawl boards to spread their weight; a cleaner should plan access so that standing on the slates is avoided wherever possible. Ask anyone quoting how they’ll reach your roof.
How do we clean a slate roof?
- Check the roof. We look for slipped, cracked or missing slates, signs of nail failure and loose ridges. If we see damage, we tell you before anything else.
- Clear by hand. Moss along the ridge, valleys and edges is lifted off by hand.
- Soft wash. A cleaning solution goes on at low pressure and kills algae and lichen across the slope.
- Biocide finish. It keeps working after we leave.
- Tidy. Gutters and ground cleared of what comes off.
What does a clean slate roof look like?
Dark and even again. Algae streaks lift on the day; lichen fades over the following weeks. Slate doesn’t lose its colour the way concrete does, so a clean slate roof usually looks very close to how it did when it was laid.
What is “nail sickness”?
It’s the name roofers give to slate roofs where the nails have corroded and slates start slipping. You’ll see slates sliding out of line, or pieces on the ground. Cleaning won’t fix it and shouldn’t be done over it. If we see signs of it, we’ll tell you to get a roofer in first.
Are man-made slates different?
Fibre cement and other man-made slates are lighter and more brittle than natural slate, and some older ones need extra care. We clean them the same gentle way. If your house was built or re-roofed from the 1970s to 1990s, mention it when you ask for a quote; a photo helps us check what you have.
How often does a slate roof need cleaning?
Less often than concrete, usually. A slate roof in the open may go many years without needing anything. One under trees or near the coast will streak sooner. A biocide treatment when growth first reappears keeps it looking right.
Can slates crack from frost after cleaning?
Good natural slate is very resistant to frost, and a soft wash doesn’t change that. Cracking from frost is more of a risk on poor-quality or very old slates that are already flaking. We look at the condition of the slates before we start and tell you if we have concerns.
What about the ridge on a slate roof?
Slate roofs often have clay or concrete ridge tiles bedded in mortar. Ridge growth is common, and the mortar is easily washed out by high pressure. We hand-clear and soft-wash the ridge and tell you if the mortar is cracked or missing.
Why are the lead valleys on my slate roof green?
Valleys collect water and debris, so moss and algae grow there first. Lead itself doesn’t grow anything, but the silt and leaves sitting on it do. Clearing valleys by hand is part of every slate clean.
What about slate on bay roofs and porches?
Victorian and Edwardian bays and porches are often slated separately from the main roof, with their own lead work. They’re low and on show, so growth on them is very visible from the street. We clean them the same way, by hand and with low pressure, and take care around the lead.
Can you clean a slate roof on a terrace with no rear access?
Often, yes. Access is planned roof by roof. If a slope can’t be reached safely, we’ll tell you rather than take the job.
In short
- Slate is hard, its nails are not.
- Low pressure and no walking on the slates.
- Report damage first; clean second.
- Ridges, valleys and bay roofs get hand clearing.
- Natural slate rarely loses colour, so a clean slate roof looks close to new.
Next step
Send us a photo of your slate roof from the garden or the street. We’ll tell you what it needs and give you a free quote. More on our method on the roof soft wash page.
