How Do You Get Rid of Green Algae on a Roof?

Both like damp, shaded roofs, so they often share a slope, but they behave differently. Algae is a thin film, green, dark or sometimes streaked, that sits flat on the surface of the tile. It rubs off on a finger as a smear. Moss is a raised cushion with tiny stems, it holds water like a sponge, and it pushes into the overlaps between tiles.

The difference decides the work. Algae mostly spoils the look and keeps the tile damp for longer. Moss does more harm because it lifts tiles, fills valleys and sends water into places it should not go. If your roof has dark streaks running down the slope, our page on black streaks on roof tiles covers that look in more detail.

Stand in the garden and look at the colour and texture. A flat green or grey wash is likely algae, a puffy green or brown tuft is moss, and a crusty, rounded patch in grey, yellow or orange is lichen. A roofer can tell on the survey, close up, with photos you can keep. Mixed growth is common, with algae on the lower courses, moss in the overlaps and lichen along the ridge, and each part is dealt with in its own way.

Biocides are chemicals formulated to kill off living growth. Put onto a dry roof in settled weather, the liquid soaks into the film and keeps working for days and weeks, so nobody has to scrub the tiles bare. Dead algae then rinses away with the rain. Where moss is mixed in, the thick growth is lifted off by hand first, and the surface is treated afterwards.

Not every product suits every user. The HSE warns that some biocides are sold for professionals only and householders should leave those alone. The product we choose is authorised for roof use and applied exactly as its label directs, on a sheltered day with a dry spell ahead.

Pressure washing is the method to avoid. Marley advises against powerful jet washing of tiles and slates, because it wears the surface and shortens the life of the covering. A worn, rougher face also holds moisture and gives algae a better grip next time round.

You may see little change on the day. The roof looks cleaner gradually, and the survey photos show where we started from.

Algae lives on moisture, shade and a surface that stays wet. A slope facing away from the sun, a roof beneath branches or a tile with a coarse face all hold water longer, so growth returns in those spots first. Leaves and debris sitting in valleys and behind chimneys add to it.

Practical steps help. Cut back branches that shade the roof, clear gutters so water does not back up under the lowest course, and sweep away leaf litter after autumn. Keep an eye on the north slope especially, because that is where a clean roof turns green again first.

A repeat treatment is the only thing that deals with a return, and how soon it is needed depends on the roof, not the product. Our page on why moss grows on the north side explains the same pattern for moss.

Zinc or copper strips along the ridge are sometimes suggested as prevention. They give a thin trickle of metal ions with the rain, but how well they work depends on how much of the slope they reach. Cleaning does not harm a tile. Leaving growth to build, over many winters, can.

Algae can hide faults. Beneath the film there may be cracked or porous tiles, failing pointing on the ridge, or a blocked valley, and these show up once the roof is clean. We check for them on the survey so the price is not changed by surprises halfway through the job.

You see pictures of your own tiles, and the price arrives as a single written total. It reflects the roof's area, how it is reached and what the survey turns up. As a guide only, Checkatrade's April 2026 roof cleaning table puts typical UK prices at £8.50 to £14 per m² for a clean, with scaffold, where needed, about £18 per m² on top.

Check that VAT and scaffold are accounted for in any price you are given, because the two can move the figure more than the cleaning does. Our roof cleaning and moss removal page covers the full service, and the page on how a biocide roof treatment works goes through the treatment step by step. A snapshot of the greenest slope is enough to start a conversation about what to check first.