Question answered
Black streaks on the roof: what are they?
The short answer
Black streaks on a roof are usually a growth of algae, sometimes with lichen, feeding on the damp surface of the tile. They run down the slope because rain carries the growth and its nutrients along. They are mostly a cosmetic problem at first, but they show that the roof stays wet, so they are worth treating and watching.
Dark streaks running down a tiled roof look alarming from the pavement, but they are usually a surface growth rather than a sign of failing tiles. Knowing the cause tells you what to do about them.
What the dark stain actually is
The stain is alive. A film of algae settles on a damp tile and darkens as it matures, forming a grey, brown or black coating that looks like soot. It spreads when spores land on a surface that stays wet for long periods and has a little dust or mineral content to feed on.
It is easy to mistake for dirt, and washing with plain water barely shifts it. That is because the growth grips the surface of the tile. Rubbing it makes the tile look no better, whereas a biocide that kills the growth at its root does the job properly.
Lichen can sit alongside it, forming crusty rings that cling to the tile, and moss is thicker and greener, usually beginning along edges and laps. Algae is the thinnest layer of all, which is why it is often the first sign that a slope has stopped drying out between showers.
Why the streaks run in lines
Streaks follow the water. Rain running down a slope carries spores and fine nutrients with it, and the growth takes hold along those paths, leaving long dark lines from ridge to eaves. A stain that starts beneath a chimney, a vent or a lead flashing is often fed by whatever washes off that fitting.
The pattern also shows where the roof stays damp. A shaded slope, a north-facing roof, a roof under overhanging trees, or the lower section of a slope where water slows will stain first. Sun and wind dry the other slopes, and they stay lighter for longer.
This is useful when we survey a roof. The line of the stain tells us where the water is running and where it lingers, and we check those areas for blocked valleys, sagging gutters and slipped tiles, because a stain that always starts at the same spot usually has a drainage reason behind it. Photographs taken from the same place each year make the spread easy to track.
Does it harm the roof?
On its own, a thin layer of algae does little structural harm. It makes the roof look older than it is, and it can hold a little moisture against the tile. The concern is that it signals conditions in which moss, which does more damage, will follow.
Moss lifts the edges of tiles and holds large amounts of water, and the full picture is in our guide on moss blocking gutters. Streaks left for years can also turn into a mat of mixed growth that is harder and slower to clear.
Run-off matters as well. Stained, wet tiles shed debris into the gutter, and over time that adds to blockages. A stain is therefore best read as an early warning: not an emergency, but a prompt to look at drainage and at what is growing beside it, before the growth thickens and starts to lift edges.
How the streaks are treated
The method is gentle and slow. Any moss is removed by hand first. A biocide is then applied across the stained slopes, and it kills the algae so that the dead film loosens and washes away with the rain. Colour improves over weeks rather than hours. The first showers after treatment bring dead growth down, so a dirty gutter in the following weeks is normal.
Hard washing is rarely the answer. It can strip the surface from concrete tiles and push water under the laps, and the stain comes back if living growth remains. To weigh the options, read cleaning concrete roof tiles for how to protect the tile.
Access is part of the plan too. Some roofs are reachable from a tower or platform, while others need scaffold, and the choice is made on safety and pitch rather than on how bad the stain looks from the road. We explain the choice when we survey.
Insurance, appearance and when to act
Black streaks are normally treated as weathering, which is gradual. Policies often exclude gradual damage, so a stained roof is not something to expect an insurer to pay to clean. Our page on whether insurance covers moss damage goes through how insurers treat it.
There is no urgency in the first season of staining. What counts is whether the roof is sound beneath it. If tiles are cracked, flashings are lifting or valleys are choked, the stain is a side issue and the repair comes first.
If the look bothers you, or the house is about to be sold, treating the stain is a modest job. We photograph the roof, explain what we find, and send a written quote. The wider picture is in roof cleaning and moss removal.
Dark stains, two questions
Are black streaks a sign my roof needs replacing?
No. Streaks show surface growth, not that the tile has failed. A roof can be stained and perfectly sound. The things that suggest replacement are cracked or crumbling tiles, sagging battens and repeated leaks, and those are what the survey looks for.
Will the streaks come back after treatment?
They can, if the slope stays damp and shaded. Treatment slows regrowth rather than stopping it for good. Trimming branches, keeping gutters flowing and a fresh treatment when the colour starts to return will keep the roof looking tidy.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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