Should moss be removed from a stone roof?

Stone and moss go together. Sandstone slabs are rough on the face, so spores settle and take hold, and a roof that has been up for generations will usually carry some green, orange or grey growth. Building Conservation, writing about clay tile roofs, makes a point that carries across to stone: lichens and moss should not automatically be seen as harmful, and they are unlikely to cause much damage. Lichen even contributes to the colouring that makes an old roof look right.

So the first answer is not to rush. A light covering on a sound roof is a matter of taste, and many owners of period houses like it. Scrubbing every slab bare can cost more than it gains, and the clean stone may look wrong beside the old walls it sits above.

What we look at is how much there is, where it sits and what the roof is doing under it. Dry, sound slabs with a film of growth need watching, not treating. Damp, soft or lifting slabs under heavy growth need a different answer.

Building Conservation sets out the cases where moss is a problem. Moss holds moisture, and that can lead to frost damage. Significant growth adds weight to the roof structure, which matters more on stone, since a stone covering is already a heavy load. When moss dies and rolls into the gutter it can cause quite serious blockages. Checkatrade makes a related point, that heavy moss traps moisture and may contribute to deterioration over time.

On a stone roof the places to watch are the valleys, the lower courses near the gutter and any slope that stays damp because trees shade it. A thick pad there keeps the stone wet for longer, and wet stone, on a cold night, is open to frost. That is why a mossy stone roof near a stain on the ceiling below deserves a closer look.

It is also where moss and leaks meet. Our answer on whether moss can cause roof leaks covers that link across all roof types, and our guide to stone roof frost damage shows what wet stone can do.

Building Conservation is plain on this point: simply pulling moss from the roof surface is more likely to cause damage than letting it die naturally or using an appropriate chemical treatment. On stone, that damage means chipped edges, slabs shifted from their bed and old fixings disturbed. A heavy slab that has moved even slightly can let water through where none came before.

Walking about on the slopes to scrape at it carries the same risk. A foot on an old slab can crack it, and a cracked slab is a repair that did not exist an hour before. Hiring someone cheap to blast the whole roof clean is a poor bargain on a covering that cannot be replaced from a builder's merchant.

Chemical treatment has a rule of its own. The same source says the chemicals must not run down into the gutter and the surface water system. Any treatment should be chosen with that in mind, and applied where it cannot drain away into the garden or the drain.

The safest approach is a measured one. Thick growth in the valleys and low courses is eased off carefully, once it has loosened, with soft tools and no force on the stone. The rest is treated with a suitable biocide and left to die back, then brushed away lightly when the roof is dry. Debris is bagged and taken off site so it does not end up in the gutter.

The work is best done from a safe platform at the eaves, not from the slabs themselves. Scaffolding is often the sensible way to get at a stone roof, and our answer on stone roof scaffolding sets out why. Roof cleaning and moss removal is its own service, and our roof cleaning and moss removal page explains how it is carried out.

Once cleared, we look at what was underneath. Slipped, cracked or soft slabs are noted with photographs, and the repair is sized to the fault. You do not get a new roof where a clean and a few slab replacements will hold.

Clearing moss from a stone roof does not carry a set price. The size of the roof, the thickness of the growth, how steep the slopes are and whether scaffolding is needed all move the figure, and only a look at your own roof settles it. Whoever quotes, ask whether VAT and the scaffold are inside the total. Our written quote gives a single figure after the survey, so there is nothing to add up yourself.

If the roof is on a listed building, ask the council before any treatment that might alter how the stone looks or weathers. Cleaning that is plainly maintenance rarely raises a question, but a chemical wash on old stone is the sort of thing a conservation officer will want a word about, and a quick call costs nothing. Our Horsham stone roofing page covers the wider work on these roofs.