Slate roofs
How should a slate roof be cleaned?
Slate lasts a long time if treated gently, and a hard wash is the quickest way to harm it. Below: the method, the checks and what it shows.
The short answer
A slate roof should be cleaned by hand first, with soft brushes and scrapers, then treated with a low-pressure spray of biocide to kill what remains. A high-powered jet is not part of it: Marley names slate among the surfaces a jet wash can damage, and it says the roof's expected life falls considerably as a result. Every slate is checked as it is uncovered, since moss can hide slipped and cracked ones.
5 min read ยท Part of Roof cleaning
Why slate needs a gentler hand
Slate is a thin natural stone laid in overlapping rows, each slate held by nails or clips, and the roof depends on every one staying whole and in place. HomeBuilding gives approximate lifespans of 100+ years for Welsh slate, but a long life on paper is only reached if the surface and the fixings survive the maintenance they get.
Hard washing threatens that, which is why a jet is left off the roof entirely. Brushing also stays light, working with the direction of the slope so that a slate is never levered up at its tail.
Walking is the other risk. A slate can snap under a boot, so we work from roof ladders and crawling boards that spread the load, rather than stepping across the slope. That adds time to the job. It also means the roof you get back has no fresh breakages in it, and no cracked slate hiding under a scuff of dried moss for the next storm to find.
The same care applies to the nails. A nail that has worked loose lets a slate slip, so every slate that moves under the brush is looked at before it is left.
The order the work follows
Each stage on a slate roof is kept small and checked before the next begins.
- A look from the ground and a ladder, to see the pitch, the fixings and any slipped slates before anyone climbs
- Thick moss lifted by hand, working down from the top of the slope
- Lichen and thin growth brushed lightly, without scraping hard enough to scar the face
- A low-pressure spray of biocide, put on in the weather its label asks for
- Gutters cleared, where the loosened growth has landed
Between the second and fourth steps, any slate that moves under the brush is marked rather than forced. Slipped or cracked slates are shown to you in photos once the surface is clear, with a note on whether each is a repair, and nothing extra goes onto the bill without your say-so.
That order keeps the roof under control. Nothing is forced, and the roof is never left part-done and part-wet, because a half-finished clean on a slate roof is a hazard for whoever climbs up next. If the weather turns, the work stops at the end of a stage, the loose growth is cleared from the gutters, and we return when the roof is dry.
What a cleaned slate roof shows
Slate that has sat under growth for years may have faults nobody has seen. A cleaned roof is the first time every slate is visible, and we report what turns up, so you can decide what to do about each find in your own time.
| What we find | What it calls for |
|---|---|
| One slate slipped | Refix with a clip |
| A cracked slate | Swap for a match |
| Slates loose across a slope | A closer look first |
The fixings matter as much as the slate. Cupa, a slate maker, says hook fixings should not be used on any pitch below 25 degrees, which is one of the things the survey checks. Our slate roofing page covers repairs and re-slating when a clean shows that a slope needs more than that.
Photos of every find go to you, with the position marked on the slope, so a roofer you ask for a second opinion can see exactly where to look. A slate that has slipped by a few millimetres is a small job when caught early, and a leak into the loft when it is not. That is the value of seeing the surface bare.
Clean first, or repair first
On slate, the usual order is a clean to uncover the faults, then repairs to the slates that need them. Cleaning a roof with known slipped slates can shift them further, so where we can already see trouble from the ground, we agree a repair first and come back to the clean once the roof is sound.
Where the roof is sound, cleaning is straightforward, and the gutters are cleared at the end so the debris from the slope is gone with it. You receive one written total price after the survey, covering the clean and anything you have asked us to do alongside it.
If you are weighing whether to clean at all, our post on whether roof cleaning is worth it looks at the question from the other side. The main roof cleaning and moss removal page sets out what a clean includes on every roof, slate or not.
Either way, the roof is left in better order than it was found: gutters clear, loose slates recorded, and any work you have agreed done to the slope. A cleaned slope also makes it easier to judge, later, whether a re-slate is on the horizon or years away.
More questions
Can I walk on a slate roof to clean it myself?
We would advise against it. Slates can snap under weight, a wet slope is slippery, and one broken slate becomes a leak. If the roof looks overgrown from the ground, send photos, and we will say whether it needs a clean and how the work would be reached without stepping on it.
Will cleaning make my slate roof last longer?
We cannot promise that. Published lifespans for slate come from the stone and its fixings, and none links a clean to a longer life. What a clean does is let us see every slate and fix what has slipped, which protects the roof you already have.
Tell us what's wrong with the roof.
You'll get one written total price.