Is Moss Bad for a Slate Roof?

Slates are hung in overlapping rows, each one covering the nail holes of the row below, and that lap is what keeps rain out. The stone itself takes up very little water, so a clean slope dries fast after a shower.

Why is moss a problem at the laps?

Moss behaves like a sponge. A cushion packed between two slates draws rain in by capillary action, the way water climbs a tissue. That held water can travel higher up the lap than the rain would have driven it, and in cold weather frost works at the joint. Thick moss also adds weight and can lift the tail of a slate a fraction, so wind catches it.

Well-kept slate can last 100 years or more according to CORC, the Confederation of Roofing Contractors. Keeping the laps free of heavy growth is part of that upkeep. A light, dry green dusting on a north slope is a cosmetic matter. Wet moss crammed into the laps or a valley, where two slopes meet in a channel, is the version to deal with.

Growth does not spread evenly. It collects in places that stay damp and shaded, and those are also the places where water is already working hardest.

Which parts of a slate roof need watching?

Valleys come first. Leaves, grit and moss pile up in the channel, slow the flow and push water sideways under the slates beside it. Next come the junctions with chimneys and walls, where a lead flashing seals the joint against the slate. Moss packed behind flashing keeps the timber and mortar there wet. Then comes the lowest course above the gutter, where run-off from a mossy slope drops into the gutter and clogs it.

Our answer on moss in a roof valley goes further into that channel, and moss in gutters covers the other end of the slope. Shaded slopes grow it fastest, as moss on the north side of a roof explains.

From the ground, binoculars tell you whether growth is thin or thick. Only a close survey shows what lies beneath it, such as a cracked tail or a nail that has let go, and the photographs show it too.

Slate is hard but brittle at its edges, and the nail holes near the head are the weak point. A boot on the wrong spot or a blade pushed under a slate can crack one.

Can slate take a powerful wash?

Marley, the tile maker, does not recommend powerful jet washing for slate or tile, since it can scar the face and shorten the life of the covering. A strong jet also drives water up under the laps and strips the face of old slate. Instead we loosen moss with a soft brush and plain hand tools, one small section after another, working down the slope in the direction the water runs so growth is never pushed under the slate above.

Roof ladders or crawl boards spread our weight, and treatment goes on within the limits of its authorisation. A slipped or cracked slate found along the way is noted and photographed, not quietly patched, so you decide what to repair. Our page on cleaning a slate roof sets out the method, and the roof cleaning and moss removal hub covers the other coverings you may have.

Moss comes off in sections, bagged as it goes, so the gutters at the foot of the slope stay clear.

Moss sometimes hides a problem. With a slope clear you see the slates properly: nail sickness, the corrosion of the fixing nails that lets slates slide out of line, cracked tails, or slates that have dropped a few millimetres.

Is the moss the cause or the symptom?

Often it is the symptom. Moss thrives where slates have slipped, because the gap holds water and dust. Removing it exposes the gap. Where a few slates are loose, repair is sensible. Where the same fault runs right across a slope, a fresh slope of slate may prove the sounder purchase, and our survey photographs give you the evidence for weighing it up.

Heritage homes need extra thought. Slate stays slate, and a change of covering goes to Somerset Council's planning team first. A clean and an inspection give the facts for that conversation, which saves guessing later.

Want the slope checked? Send a photo of the moss. After the survey you receive one written total, covering the clean and any slate repairs you choose to add.