How to get rid of roof moss permanently

Why moss keeps coming back

If you've scraped or jet-washed your roof before and watched the moss return within months, this is why: you removed what you could see, but not the spores.

Moss, lichen and algae all spread by microscopic spores carried on the wind and left behind in the surface of the tiles. Strip off the visible growth without treating those spores and they simply regrow — often faster than before, especially if the tile surface was roughened or damaged by high-pressure washing. A more porous, damaged tile holds more moisture, and moisture is exactly what moss needs.

Add Gloucestershire's damp climate, plus the usual suspects — overhanging trees, a shaded or north-facing pitch, nearby fields — and a roof that's only been scraped will green up again before you know it.

The two-step method that actually lasts

The closest thing to "permanent" is a two-part job, and both parts matter:

1. Physically remove the existing growth

We do this one of two ways. A low-pressure clean gently lifts the moss off there and then, so you get an instant, same-day clear roof. Or we do a moss scrape — hand-scraping the moss off — and the roof keeps clearing over the following weeks to months as the biocide works through what's left. Same clean end result either way; one is instant, one is gradual. Crucially, both are done without high pressure, so the tile surface stays intact rather than being roughened up for the next lot of moss to grab onto.

2. Treat it with a biocide

This is the step that makes the difference. After the visible moss is gone, we apply a biocide that keeps killing the spores left in and around the tile surface for months afterwards. It's the biocide — not the scraping — that stops the rapid regrowth. A clean without a biocide is cosmetic; a clean with one is what keeps a roof clear for around two years.

That's the realistic version of "permanent": physically clear it, chemically suppress the regrowth, and top the treatment up every couple of years. No honest roof cleaner will promise a roof that never needs looking at again — but this gets you as close as it's possible to get.

Moss vs lichen vs algae — what's actually on your roof?

People use "moss" as a catch-all, but there are three different things growing up there, and it helps to know which you've got:

• Algae — the thin black or green staining and streaks, especially the dark vertical streaks you see running down a roof. Flat, not raised.
• Lichen — the crusty patches, often pale grey, yellow or orange, that grip tightly to the tile like a scab. The most stubborn of the three.
• Moss — the thick, green, spongy cushions that hold water and lift off in clumps. The one that does the most physical damage by trapping moisture and lifting tiles.

The good news: all three respond to the same clean-plus-biocide approach. The difference is in the effort. Thick moss often needs a hand scrape; algae usually lifts with a low-pressure clean alone; lichen is the clingy one and sometimes needs a second biocide pass to fully break its grip.

Why jet washing and DIY don't give you "permanent"

Two things people try that rarely last:

Jet washing. A high-pressure washer blasts off the visible moss in a satisfying way — but it leaves the spores, and it can strip the protective coating off concrete tiles. The result is a roughened, more porous surface that grows moss back faster. It also risks cracking slates and forcing water under the tiles. It looks great for a few months and then it's back, sometimes worse.

DIY. We understand the temptation, but be careful. Walking on a roof is genuinely dangerous and easy to damage; scraping from a ladder only reaches a small strip; and shop-bought moss killers, applied without the right dilution and coverage, rarely stop the regrowth. For a light touch-up it might be fine — but for a properly mossed roof, a professional low-pressure clean or moss scrape with a biocide is safer and lasts far longer.

How to keep moss away between cleans

A few things genuinely help slow regrowth once the roof is clean:

• Keep the biocide topped up — a re-treatment every couple of years is the single most effective thing.
• Cut back overhanging branches — less shade and less leaf-fall means a drier roof.
• Keep gutters clear — standing debris and damp at the roof edge is a moss breeding ground (our roof cleans include a free gutter clear for exactly this reason).

Do those and a clean roof stays clean for a good while. Skip the biocide and you'll be back to scraping next year.