Does Moss Damage Slate Roofs?

Why moss grows on slate at all

Moss needs moisture, shade and something to grip. Slate is smooth, which makes it harder to colonise than rough concrete tiles, but any roof that stays damp will grow moss. North-facing slopes, slopes under trees and roofs next to higher walls dry slowly, and so the growth starts there. Along the lower edges of slates, in the overlaps and along the edges of the roof where debris collects, moss finds enough grit and moisture to start. Once a patch gets going, it traps more dust and leaves, and the growth thickens each year.

This answer is about slate; concrete and clay tiles behave differently, as explained in our answer on moss on roof tiles.

Lichen is different from moss. It forms flat crusty patches, usually grey, orange or yellow, and sits on the surface. It is slow-growing and generally harmless to slate, though it can mark it. Algae gives the green-black staining that many roofs show. These organisms tell you the roof is damp more than they tell you it is failing.

When moss does harm

The damage comes from what a thick mat does, not from the plant itself. A cushion of moss holds water against the roof long after the rain has stopped, and where it bridges the gap between one slate and the next it can draw water sideways and up the lap by capillary action, past the head of the slate below. That water reaches the battens and nails, which rust and rot. In winter, the water in the moss freezes and expands, forcing the layers of an already weak slate apart. Moss also pushes into the joints, lifts the edges of slates and builds up in valleys and gutters, where it blocks the flow and sends water over the edge.

Heavy clumps of moss that fall into the gutter block downpipes, and the overflow then runs down the wall or into the eaves. If the roof leaks at one corner after rain, look at the gutter first. Our page on slate roof maintenance explains what to check each year.

Removing it safely

The wrong way is a pressure washer. A jet can crack slates, strip the surface, blast out the bedding and push water under the laps into the loft. It also leaves the roots, so the moss comes back. The usual method is to loosen the growth by hand with a soft brush or scraper, working downwards from the ridge and being careful not to lift slates, and then apply a biocide made for roofs to kill what remains. The dead growth falls away over the following weeks. Treat the gutters afterwards and clear what has dropped into them.

Working on the slope needs the right access, which is why this is usually a job for a roofer using a tower or roof ladder. Our guide to walking on a slate roof explains why. Typical UK ranges reported by one cost guide for hand removal with biocide are about £748 for a small roof, £863 for an average one and £978 for a large one, against £553 to £723 for pressure washing, which we do not use on slate. Your fixed quote follows the survey.

Slowing the return

Reduce shade by cutting back overhanging branches, and keep gutters clear so that the lower edge of the roof can dry. Zinc or copper strips near the ridge release metal ions in rain that discourage growth below them, though they take time to work and do not replace cleaning. Some roofers apply a protective treatment after cleaning. Do not expect moss never to return on a damp, shaded roof, but a clean roof is much easier to keep clean.

Also look at what lies under the moss. Once it is gone you may find slipped, cracked or delaminated slates that were hidden. Our page on slipped and cracked slates covers what to do.

Moss on slate compared with tiles

Concrete and clay tiles have a rougher, more porous face, so moss roots into them and the surface can break up as the growth is removed. Slate is much denser and absorbs very little water, which is why moss on slate usually lifts away more cleanly and does less harm to the slate itself. The real risks on a slate roof are at the joints and the fixings, where trapped water speeds the rusting of nails, and at the gutters. A slate roof with a mossy surface but sound fixings is often in better condition than a clean-looking one with rusted nails underneath.

We clean slate roofs by hand and report what the clean surface shows, so any repair is based on what we can see. Our other slate work, from repairs to full re-slating, sits within our slate roofing service.