The short answer
Black streaks on a roof are almost always a dark algae growing on damp, north-facing or shaded slopes, not dirt or soot. The staining is mostly cosmetic at first, but it holds moisture against the surface. It is treated with a low-pressure soft wash and a biocide that is left to work, and the roof is looked over for damage before anything is sprayed.
What the dark stripes are made of
Stand in the road and look at the roofs opposite. On many of them you will see grey-black bands that start near the ridge and fade towards the gutter. That is algae, a single-celled growth that lives on the surface of the covering. It produces a dark pigment as protection from sunlight, which is why it looks like soot or tar rather than the green of moss.
Algae is easy to confuse with other things. Moss is raised and furry, and lichen forms flat, crusty rosettes in grey, orange or yellow. Algae is thin and flat, with no texture you can pick at from a ladder. A roof often carries all three at once, and each needs the same approach: kill the growth, let it die back, then rinse it off gently. Our page on lichen on roofs covers the crusty kind.
Soot from a chimney, bird droppings and run-off from a rusting vent can also stain a roof, but those leave a patch or a single trail, not even stripes across a whole slope.
Why it runs in streaks and why it picks one side
Algae spreads wherever moisture lingers on the covering. Rain carries spores down the slope, so a patch that starts near the ridge leaves a trail below it. That is the stripe you can see. Spores also travel on the wind and settle anywhere, but they only take hold where moisture lingers.
That is why one slope is often clean and the other is dark. A north-facing pitch gets little direct sun, so it dries slowly. Shade from trees, a taller neighbour or a chimney stack does the same job on a smaller patch. Lower-pitched roofs hold water for longer than steep ones, and rough surfaces such as older concrete tiles give the growth something to grip.
Here the streaking is a clue as much as a nuisance. If one stripe is far darker than the rest, check what sits above it. A leaking vent pipe, an overflowing gutter or a cracked valley can keep one strip of tiles wet all year.
Does it damage the tiles?
Left alone for a season or two, black algae is mainly a matter of looks. Over longer periods it behaves like any growth that keeps a surface wet. It stays damp underneath, and in cold weather that damp water can freeze in the pores of the tile and flake the face. Concrete tiles lose their surface coating over time, and once that is gone the tile soaks up water faster and grows more algae.
The bigger risk is not the algae itself. It is what people do about it. A hard pressure wash strips the surface from concrete and clay, forces water up under laps, and can crack old slate. We would sooner leave a stained roof alone than have it blasted. The black will not make a roof leak, but a damaged tile might.
Staining also tends to be the first sign that moss will follow. Algae builds a thin film that holds moisture, and moss spores settle in that film. Treating the film early is easier than scraping a thick cushion later.
How algae is treated, and what it costs
The standard method is a soft wash. A biocide is sprayed on gently, given time to work, and then rinsed lightly or left to wash away in the rain. The chemical does the cleaning, not the force. The growth dies back over the following weeks, and the roof lightens gradually rather than all at once. Do not expect a bright result on the same afternoon.
MyBuilder's roof cleaning guide, updated on 21 August 2026, puts soft washing at £15 to £20 per m², against £10 to £15 per m² for pressure washing, which it warns may damage delicate tiles. Its overall bands for a pitched roof are £450 to £850, and for a tile roof £400 to £750. Treat those as national ballparks. Access, roof area and how much growth there is all shift the price, so a firm figure only comes after the survey visit.
For a deeper look at the cleaning options, see our guide to how long roof cleaning takes, which explains why the work is done in stages.
Keeping the streaks from coming back
You cannot stop spores landing, but you can make the roof a poorer place to live. Cut back branches that shade a slope or drop leaves into the gutters. Keep the gutters flowing so the bottom courses of tiles are not sitting in overflow. Fix any dripping vent or pipe above a stripe.
Some owners choose a protective coating once the roof is clean, and we cover that in roof coatings for homes. It is a decision for after the survey, because a coating on a damaged or poorly cleaned tile only seals the problem in. Whatever you choose, expect a clean roof to darken slowly again on its shady side, and plan a look every year or so.
What we check before we treat anything
Before any biocide goes on, we inspect the slopes from street level and then closer up from a tower or ladder. We are looking for slipped or cracked tiles, failed pointing, lifted ridge tiles and blocked valleys, because cleaning a roof with a hidden fault only makes the fault harder to see afterwards. The quote arrives with photographs of what we found.
If the tiles are soft and failing, we will say so and talk through repair or replacement, not a clean. If a clean is the right job, that is all we recommend. Whatever we do on your roof is covered by a ten-year guarantee on our workmanship, given in writing.
To book a survey, ring 01793 626096, or send photos through WhatsApp or the form on this site. The whole service sits under roof cleaning and moss removal.