Natural slate is one of the longest-lived roof coverings there is. HomeBuilding gives Welsh slate 100 years or more, Spanish natural slate 75 to 100 years and reclaimed Welsh slate 50 to 80 years (Sep 2025). Those are typical ranges, not promises, but they show why people expect a slate roof to outlast most things around it.
That reputation hides a weakness. Slate is hard, but thin and brittle. It cracks under a boot or a ladder foot. It flakes along its layers when it has aged. And it is held on a roof by small nails that can rust through while the slate itself looks perfect. A moss-covered slate roof therefore needs more care to clean than a concrete tile one, not less, and the care starts before anyone climbs a ladder.
This guide describes why slate is easily damaged, the checks that come before any cleaning, a gentle method that suits old slate, the things never to do and what to expect afterwards. It applies to slate roofs of every age on houses around Andover, and the wider roof cleaning and moss removal page covers how we approach other coverings.
Why slate is easily damaged
The qualities that make slate durable also make it unforgiving to work on.
Thin and layered
A slate is a thin plate split from stone along natural layers. The same layering that lets it be split makes it prone to flaking and delamination as it ages, and a slate that has begun to lift in layers can break at a touch.
Held by a nail or two
Natural slates are nailed through holes near the head, or in some roofs held by hooks. They hang rather than sit. Each slate depends on one or two small nails, and when those fail the slate slides. A sideways push from a ladder or a foot can drag a slate out of place.
Brittle under load
Slate cracks under point loads. Standing on the middle of a slate, where it is unsupported between battens, is often enough to break it. Old slates, softened by decades of weathering, are the most fragile of all.
Pitch and fixings
For natural slate, BS 5534 gives 20° as the lowest workable pitch, and slate supplier Cupa limits hook fixings to pitches of 25° and above. A roof at the lower end of its range has less margin, so rain reaches weaknesses sooner.
What this means for cleaning
Every step of cleaning a slate roof is planned to keep weight and force off the slates. Our guide to roof cleaning methods sets out the range of approaches, and slate takes only the gentlest of them.
Nail sickness and the checks before cleaning
Before any brush or product touches the roof, the slates are checked for the fault that matters most: nail sickness.
What nail sickness is
Nail sickness is the corrosion of the fixings that hold slates in place. Iron nails rust away, or are eaten through where moisture sits against them. The slate above the nail then has nothing holding it, and it slips, either on the day or in the next gale. A roof can look sound and be hanging on very little.
How it is found
Looking up from the garden, you can pick out slates that have sagged by a few millimetres and left a gap at the tail, and for lines of slipped slates in the same course. On the roof, a surveyor lifts a few slates at the eaves and at sample points and examines the nails. In the loft, fallen slate fragments, daylight and rust staining on the underside show where nails are failing.
Why this comes first
If the nails have gone, a clean will loosen slates that were hanging by habit. Removing moss from a roof with sick nails is a poor bargain: the roof looks tidy for a short time and then sheds slates. The survey decides whether the roof should be cleaned, repaired first or re-slated.
What the survey records
We photograph slipped, cracked and flaking slates, note the condition of nails where we can see them, and send the pictures with our advice. Our hub on slate roofing covers repair and re-slating.
A gentle method for cleaning old slate
The method for a slate roof is slow and mostly manual, because force is the enemy.
Access without weight
Work is planned from a scaffold or from roof ladders and boards that spread a person's weight across several slates, never from the slates themselves. Where the roof is steep or high, edge protection goes in first.
Remove the bulk by hand
Thick moss is lifted off by hand or with a soft brush, working with the lap of the slates, from the ridge down, and not against them. Prising under the slate edge with a scraper can crack or lift it, so scraping is light and flat.
Treat what is left
A biocide is then applied to kill the remaining moss, algae and lichen. Using one is a regulated activity. Article 17(5) of the Biocidal Products Regulation ties use of these products to the terms of their authorisation, and HSE adds that biocides made for professional users are never for the public. The label sets the dry spell needed. Our guide to roof biocide treatment explains what the products do.
Let the weather finish the job
The dead growth loosens over time and washes off in rain. That is slower than a blast, and it is much safer for the slate.
Clean the gutters and the ground
Moss that comes off lands in the gutters and on the ground, and slate roofs often drain into cast-iron or older plastic gutters that block easily. It is cleared as work proceeds so that it does not block downpipes or stain paths.
What never to do to a slate roof
Several common cleaning methods do lasting harm to slate, and each is worth naming.
Jet washing
Marley does not recommend high-powered jet washing of roof coverings, giving surface damage and a shorter life as the reasons. On slate, a hard jet can drive water up under the laps, strip the surface, open hairline cracks and dislodge slates already held by weak nails. The damage may not be obvious on the day, and it shortens the life of the roof.
Wire brushes and stiff scrapers
Hard tools scratch the surface and can lift the layered edge of an old slate. A scratched slate holds moisture and grows moss again faster.
Walking on the slates
Standing or kneeling directly on slate invites cracks. Even a careful tread on a dry day can break an old slate. Crawling boards spread the load, and a ladder with a ridge hook gives a firm line of travel without bearing on the slates beneath it. Anyone offering to clean a slate roof by simply walking over it should be asked how they plan to avoid breakages.
Homemade chemical treatments
Some household products sold for patios are not approved for use on roofs, and the biocides rules treat storing or using a non-approved product as an offence. Spilt run-off can also harm the plants below.
Cleaning without checking
The most expensive mistake is cleaning a roof with failing nails. The cost shows up weeks later as slipped slates, and the cleaner is not around to put them back.
If a slate roof has been cleaned badly, tell us what was used, and we survey for damage before any repair.
Problems that show up once the moss is off
Moss can hold slates in place, and it can hide faults. When it comes off, the true condition of the roof appears.
What commonly appears
Cracked slates that the moss had been bridging, slipped slates that show as gaps, flaking along the layers and nails that have sheared. Lead flashings at chimneys and abutments become visible too, and so do failed mortar bedding on the ridge and hips.
Why that is useful
It is far better to find faults now than in the first heavy rain. The clean gives a chance to repair before the roof leaks. The pictures go to you slope by slope, so the extent of each fault is clear.
How repairs are done
A single slipped or cracked slate can be replaced, but the work is delicate. The slate above is lifted gently, the old slate and nail drawn out, and a new slate fixed with a copper or other durable nail, or held with a hook where the pitch allows. Reused slates can be matched in size and thickness. Our guide to roof problems found during cleaning lists what a clean can uncover, and the answer on repair is agreed with you before any work starts.
When to re-slate
If nails are failing across whole slopes, patching becomes expensive, and re-slating on new battens and underlay may be the better buy. The survey weighs the two.
Aftercare, frequency and booking
A clean slate roof is not a finished job unless the conditions that grew the moss change.
Keep it dry
Moss thrives where roofs stay damp and shaded. Trimming overhanging branches, clearing gutters and checking that the roof can dry between showers all slow its return. Regrowth tends to start on the shaded slopes.
How often
There is no schedule that suits every roof. Look from the ground each season. If moss is gathering at the laps or a green film is spreading, it is time to have the roof looked at. Cleaning only when needed is gentler on slate than a routine every year or two.
Coatings
Some roofs are given a protective coat after cleaning. Whether that suits slate depends on the product and the slate, so our guide to cleaning methods and the data sheet are the starting points. A coat goes on only after the product has been checked as compatible with the slate.
Booking
The price comes after the survey, since it depends on the roof's height, pitch, area, how far the growth has spread and the access needed. Scaffold, where required, is arranged with the scaffolder and a licence is obtained from the highway authority if it stands on the road. You receive it as a single written total.
Next step
Send a photograph of your slate roof, or book a survey, and we will check the nails first. If the roof has been cleaned before, mention when and how: knowing whether a hard blast was ever used helps us read the condition of the slates, and an old record of treatment helps us judge how quickly growth returns on your particular slopes.
