How is moss removed from a slate roof?

A well-kept slate roof can last a very long time. The CORC trade body describes slate as lasting 100 years or more when it is well maintained, and that long life depends on the slates staying whole and fixed. Each one is a thin, hard sheet held by two nails or a hook, overlapping the ones below. The weak points are the nail holes and the lower edge, both of which can crack under a careless foot or tool.

Moss makes the picture worse by sitting in the overlaps, holding damp against the nails and working between the layers. Left alone, it can speed up the very failures a slate roof is meant to avoid. Heavy growth along the lower edge is where this tends to start, because that is where the damp collects. Taking it off is worthwhile for the roof's sake, but the method has to respect how fragile the covering is, and the care taken counts for more than the speed of the visit. An extra hour on the day is cheap against a slate that has to be replaced, and a replacement slate rarely matches the weathered ones around it.

A pressure jet is out. It drives water under the laps and into the nail holes, can strip the surface of softer slates and loosens fixings. A wire brush or metal scraper scores the face and breaks the edge of an old slate. And nobody should stand on the slates themselves if it can be avoided, because a footfall that would be nothing on a concrete tile can snap a slate in two.

Roofs that are old, or that have slipped or cracked slates, need care before any cleaning begins. Treating over defects hides them, and the leak that follows is blamed on the clean. If a slate has already slid, the water has a route in, and the right order is to repair first and clean after. Our page on the signs a slate roof is failing lists the warning signs you can spot from the garden.

We start with a survey from a safe position, photographing the surface, the ridge, the valleys and anything that has moved. Where the growth is thick and the slates are sound, loose moss is lifted by hand with soft tools, working from boards or a platform so the load is spread. The roof is then treated with a biocide chosen for the surface, applied at low pressure.

The treatment is given time to act, and the roof is rinsed gently, not blasted, so that dead growth is carried off without forcing water into the laps. The gutters are cleared as well, because the debris ends up there. Our guide to removing moss from a roof shows the sequence. Afterwards the waste leaves with us, so no moss is left in the borders.

Fixings matter on slate. Cupa Pizarras, a slate supplier, says hook fixings must be avoided on any pitch below 25 degrees, and that BS 5534 gives a 20 degree minimum pitch for 500 mm natural slate, in moderate exposure. A roof near those limits has less margin for water to be forced back up, so wetting it harder is the wrong response.

We also look for missing or slipped slates, nail sickness (where old nails have corroded and the slate drops) and loose ridge pieces, because moss tends to gather where they have moved. If a cause is found, we repair it as a separate decision for you to agree, rather than slipping it into a clean. Our answer on repairing a slate roof explains how those jobs are done.

Bognor's seafront terraces and villas include slate roofs, and Arun's conservation area guidance for The Steyne talks about reinstating natural slate or plain clay tiles where modern materials have replaced them. Older slate can be thinner and more brittle than new, so the survey looks harder at it, and some cleaning methods are ruled out before anyone reaches the ridge. Where a slate is too weak to be touched, we tell you before the treatment goes on.

If the house is listed, or sits in a conservation area, a word with the council's planning team is wise before any change to the look of the roof. Cleaning alone does not usually do that, but each case differs, and a short call to the council settles it before work starts. We cover the older-roof side in our answer on cleaning historic roofs, and the whole service is described on the roof cleaning and moss removal page.