Is moss on your roof a problem? When to leave it and when to act

What the tile makers say

Marley's guidance is unusually direct for a company that sells roof tiles. It describes low to moderate moss and lichen as "a natural part of the ageing process on many pitched roofs" and "not harmful to roof tiles or slates" in normal circumstances. Its default advice is to leave the growth where it is, unless the roof is leaking or the gutters and outlets are blocked.

Heavy growth is different. Marley says a heavy build-up "will inhibit the flow of water off the tiles" and can "accelerate the deterioration of the roof tiles". Alex Morrisey of Best Roofers, speaking to Homebuilding & Renovating, compares moss to a sponge holding rainwater against the tiles, and warns that "when tiles absorb water repeatedly, they weaken".

Light, moderate or heavy: how to tell

Why moss grows so well in the Chilterns

Moss needs shade and damp, and this part of Buckinghamshire has plenty of both. The Chilterns is over one-fifth woodland. Buckinghamshire Council's townscape study notes Beaconsfield's "high degree of tree cover, including mature trees in rear gardens", with ancient woodland at Walk Wood, Throshers Wood and Hog Back Wood. Amersham Old Town sits low on the floor of the Misbourne valley, Chalfont St Giles climbs both sides of the same valley, and Chalfont St Peter has deciduous woodland on its Common.

That's why two houses on the same road can look so different. A north-facing slope under a big tree may never dry out between autumn and spring, while the sunny side of the same roof stays clean.

Why a roof should never be pressure washed

• Marley warns that high-pressure washing "can damage roof tiles, particularly older or more delicate ones", and can flood gutters and outlets with water, chemicals and debris.
• Spraying from the ground pushes water upwards, and Marley says that upward force can lift and damage tiles.
• Paull Newsome of Vanguard Waterproofing Systems told Homebuilding & Renovating: "Avoid pressure washing in most cases; it can damage finishes and drive water where it shouldn't."
• For the old clay roofs of the Chilterns, the Chilterns Conservation Board prefers moss brushed off by hand to power jets or chemicals.

Do moss treatments work?

A moss treatment can slow regrowth, but it isn't a permanent fix, and it isn't harmless. Historic England's research on biological growth on historic masonry found that common biocides are "proven to be harmful to the environment and wildlife", that their longer-term effectiveness "has been observed to be extremely limited", and that some products appeared to stop working after about three months. Marley also warns that chemical washes can harm plants and animals, and that gutters should be protected while they're used.

So we take the moss off by hand first. A treatment only goes on where you want regrowth slowed and the roof suits it. It's a professional product approved for use in Great Britain, applied exactly as its label directs and kept away from plants, ponds and drains. On old handmade clay, hand removal alone is often the better choice.

Should you do it yourself?

Marley's view is that getting onto a roof to remove moss "should not be attempted due to the inherent risks of working on roofs", and that foot traffic can damage tiles. If you only want to know whether your moss matters, you don't need to go up at all. A few clear photos from the garden are usually enough for a roofer to tell you.

When to have moss removed

Winter is best, from November to February. The leaves are down, so the roof stays clear for longer, and the work is done before nesting season. The RSPCA advises clearing moss from roofs and gutters in winter for exactly that reason, because an active nest is protected by law. Treatment needs a dry spell afterwards, so the day is picked by the forecast.

How we remove roof moss

We scrape and brush the moss off by hand, working from roof ladders so we don't stand on the tiles or slates. Where it suits the roof, we follow up with a treatment sprayed at low pressure. We clear the moss that falls into the gutters, photograph every slope, and tell you honestly if your roof can wait another year.

Written by Hawkes Roofing, roofers based in St Albans who remove roof moss by hand across Amersham, Beaconsfield and the Chalfonts. Last reviewed 26 October 2026.