The short answer
Moss on slate is mostly an appearance problem, but a thick mat holds water against the surface. Marley says to avoid powerful jet washing, which can harm slate or tile surfaces and cut the covering's expected life greatly. Gentle hand removal is the safer method, and a roof with heavy moss deserves a check for cracked or slipped slates.
Why slate needs a gentler hand
Slate is a dense stone that does not soak up much water, so moss grows on it less readily than on porous concrete. It still appears, mainly on shaded or north-facing slopes, in the lower courses and along the laps where water lingers. A roof with trees overhead or a damp garden tends to gather it.
The question is how to remove it. A pressure jet seems quick, and many people reach for one. Marley, a roof tile maker, does not recommend high-powered jets, because the surface can be harmed and the expected life shortened a great deal. A jet can also drive water under the laps into the roof, flake the surface of a weak slate, and blast out the pointing and bedding of ridge and hip tiles.
Slate is brittle as well as thin. Walking on it breaks it, and cleaning from a ladder leant on the eaves can crack the slates beneath. A cleaner who uses the wrong method can cause more damage than the moss ever did.
Our answer on cracked slates explains how foot traffic and impact damage slate. The cleaning hub covers moss on roofs generally, and this page deals with the slate point.
Hand removal and what else is used
The gentlest way to clear moss is by hand. A roofer works from a roof ladder, a platform or scaffold that spreads the weight, and lifts the moss with a soft brush or a plastic scraper, working from the ridge down so debris does not clog the laps. It takes longer than a jet and leaves the surface intact. Loose moss is cleared from the gutters afterward, because it blocks them.
Treatments are sometimes applied after removal to slow regrowth. Their use is regulated. Biocide rules apply. Article 17 of the Biocidal Products Regulation requires use within the terms of the product's authorisation, and HSE tells ordinary householders to steer clear of any product meant for professionals only, and the control rules make selling or using an unapproved one an offence. A homeowner should read the label and stay with consumer products.
Treatment also reaches beyond the roof, because run-off ends up in gutters, drains and the planting below, and the product instructions say how to protect them.
If moss is clearly returning, the cause is often shade and damp, and trimming a tree or clearing a gutter helps more than repeated cleaning. Roof size and access set the price of any moss job, so the survey comes first.
What moss can hide or cause
A mat of moss holds moisture against the slates and keeps the laps wet. On sound slate that does little harm, but on a slate with a hairline crack or a laminating edge it keeps water in the flaw, where frost can work it wider. Moss also collects debris, which blocks the gutters at the eaves and sends water over the edge.
It can also cover problems. Under a thick carpet of moss, slipped slates, cracked slates and rusting nails are hard to see. A cleaning visit that scrapes off moss can knock loose a slate held only by a failing nail, and a roofer who cleans a slate roof should look for these at the same time.
Heavy moss on one slope, with damp patches on the loft side below it, is worth a closer look. The cause may be a blocked gutter, a leak or poor ventilation, and cleaning the surface does not cure any of them. Our answer on a leaking slate roof explains how to trace the source.
Moss and lichen are not the same. Lichen is a flat, crusty growth that bonds to the surface, and it is harder to remove without marking the slate. Often it can be left alone, as it does little harm.
What we do on a slate roof
On a slate roof, we begin by looking. We photograph the slopes from the ground and, where safe, from a platform, and note moss, lichen, slipped slates, cracks and the state of the gutters. Then we agree what is worth doing.
Cleaning, where it is done, is by hand and with care. We avoid jetting and avoid walking on the slate. If chemical treatment is suggested, the product must suit the purpose, and a professional-only product is never one for a homeowner to apply.
If the survey finds damage under the moss, we tell you what we found with photographs. A few cracked slates are a repair, as covered in our guide to slate roof repair. A roof in poor condition across a slope may be better re-slated than cleaned.
If a roof needs nothing more than a cleared gutter and a trimmed tree, that is what you will hear. A single written price follows the survey. See roof cleaning and moss removal for the general service, and slate roofing for the wider slate work.
