How to stop moss coming back, and how long a roof clean lasts

Why moss comes back

NFRC, the roofers' trade body, explains that moss grows on rough tile surfaces that catch dirt, decaying leaves and airborne spores and seeds. Removing the moss doesn't change any of that. The tiles are still rough, the trees are still there, and spores are in the air everywhere. So moss returns, sooner on some roofs than others.

The biggest factors are shade and damp. A north-facing slope, or one under a big tree, may stay wet from autumn to spring. The sunny side of the same roof may stay clean far longer.

What actually slows it down

Trees and light

This is the change that makes the biggest difference, and it's often free or cheap. Taking back branches that hang over the roof lets light in, helps the slope dry out, and stops leaves settling on the tiles. The RHS gives similar advice for paths and patios: prune overhanging plants to improve airflow.

Check before you cut. Trees in conservation areas need six weeks' notice to the council, and trees with a Tree Preservation Order need consent. Our guide to trees over gutters covers the rules.

Treatments

A treatment applied after the moss has been removed can slow regrowth, but not for long. Historic England's research found that common biocides have very limited longer-term effectiveness, and that some appeared to stop working after about three months. Even Wet & Forget, which is designed to work gradually with the rain, says results on heavily affected surfaces can take up to six months.

So a treatment buys time. It doesn't change the conditions that let moss grow.

Zinc and copper ridge strips

Metal strips fixed along the ridge are sold as a way to stop moss. The idea is that rain washes traces of the metal down the roof. A Minnesota home inspector, Reuben Saltzman, tested a zinc strip on a moss-covered garage roof, and after about sixteen months concluded that "zinc strips are effective at killing moss". A UK roofing supplier, Roofing Superstore, recommends copper instead.

Two cautions. Most of the evidence comes from American asphalt-shingle roofs, not British clay and concrete tiles. And fixing anything to a ridge means working on the roof and making holes or fixings, so get a roofer's view first, and check for consent if your house is listed.

What not to do

Don't pressure wash to "reset" the roof. Marley advises against high-powered jet washing because it can damage the tile surface. On Mumsnet, one homeowner warned that it "takes off the surface". A rougher, damaged surface is exactly where NFRC says moss takes hold.

So how long does a roof clean last?

Honestly, it depends on the roof, and anyone who gives you a fixed number for every house is guessing. On an open, sunny roof with no trees nearby, a clean can last a long time. On a north-facing slope under an oak, it will be back much sooner. What you can control is the debris, the light and the gutters.

The best early warning is the gutters. If moss starts turning up in them again, it's growing back on the roof above.

How we help keep it away

When we remove moss, we do it by hand, so we're not stripping the tile surface, and treat where it suits the roof. We'll tell you if a tree is the real cause, and which slopes are likely to regrow first. Each time we clear your gutters, we photograph your roof slopes, so you can see regrowth early, long before it's a problem.

Written by Hawkes Roofing, roofers based in St Albans who remove roof moss by hand across Hertfordshire, Buckinghamshire and north-west London. Last reviewed 26 October 2026.