What are the black streaks on my roof?

What causes black streaks on a roof?

Black streaks are usually a type of algae, not dirt or soot. Its spores are carried in the air and settle on the roof, and it grows wherever the tiles stay damp after rain. That’s why shady, north-facing slopes suit it.

The algae darkens as it builds up, which is why it looks black rather than green. It tends to form streaks because it follows the way rainwater runs down the roof, so the marks usually run from the ridge or a high point down towards the gutter.

Why are the streaks worse on some roofs?

The algae grows where a roof is wet most often and stays damp the longest. Streaks are most common:

• On north-facing slopes, which get little direct sun and dry out slowly.
• Under or near trees, which shade the roof and keep it damp.
• Where water doesn’t run off cleanly, such as on low-pitched roofs.
• On roofs that take the brunt of driving rain.

They show up most on lighter-coloured tiles, simply because dark streaks stand out against a light background. On a darker roof, the same growth is less obvious.

If the dark marks are only right under a gutter, a downpipe or an overflow, check that it isn’t leaking: algae often grows where water keeps dripping onto the same spot.

Is it algae, moss, lichen or something else?

A roof often has more than one kind of growth on it. From the ground, you can often tell them apart:

A rough guide from the ground; growths can look alike from a distance.

All three grow in the same damp, shady conditions, so a roof with black streaks often has moss or lichen too. You can read more in our guide to whether moss damages roof tiles.

Do black streaks damage roof tiles?

On their own, black streaks are mostly a cosmetic problem. Growths like this are unlikely to cause major structural damage, and low to moderate growth isn’t harmful to roof tiles or slates under normal circumstances.

They are still a sign worth noticing. Streaks show that the roof stays damp for long periods, which are the same conditions moss and lichen like. Thick moss is the bigger worry, because it holds water against the tiles and washes into the gutters and valleys.

If the streaks are the only thing on the roof and you can live with the look, there’s no rush. If moss is building up as well, or you’re getting ready to sell, a clean deals with all of it at once.

How are black streaks removed from a roof?

With a soft wash and a biocide, not a pressure washer:

• A soft wash uses a cleaning solution at low pressure, so the tiles aren’t blasted.
• A biocide kills the algae, moss and lichen on the tiles, including the spores, so growth is slower to come back.
• No pressure washing. A high-pressure jet can strip the surface of the tiles, crack older ones and force water into the roof space, and it doesn’t kill what’s left on the tile. See whether pressure washing damages a roof.

At Washmates, every roof is cleaned with a soft wash, then treated with a biocide, so streaks, moss and lichen are dealt with in the same clean. The work is done from roof ladders, scaffold towers or a cherry picker, normally by a team of two, and a three-bedroom semi usually takes a full day. Every roof clean includes the gutters, fascias, soffits and any solar panels on the roof.

How long do black streaks take to fade?

It isn’t instant. A biocide usually takes a few days to work, and the dark staining then fades over the following weeks as the rain washes the dead growth away. Crusty lichen is slower: it can take months to break down and wash off fully. Dying lichen can turn grey or black before it fades.

How quickly the roof clears depends on the weather and on how much growth there was. That’s also why the biocide needs a dry spell after it goes on: heavy rain straight away can wash it off before it has worked. Washmates cleans roofs in dry or mild weather and in light rain, never in heavy rain, frost, ice or high winds. See the best time of year to clean a roof.

How can you stop black streaks coming back?

You can’t stop the spores landing, but you can make the roof less welcoming:

• Cut back overhanging trees, so the roof gets more sun and dries out faster.
• Keep the gutters and downpipes clear and in good repair, so water drains away instead of sitting on the roof edge.
• Think about copper or zinc strips. A roofer can fit them along the ridge to slow regrowth on the roof below them.

A roof cleaned and treated with a biocide usually stays clear for two to four years. Shaded, north-facing roofs come round sooner. See how often you should clean your roof.

Can you remove black streaks from a roof yourself?

Not from the roof itself. Falls from height are the most common cause of fatal accidents at work in Great Britain, and that’s among people trained and equipped for it. Wet tiles are slippery, and older tiles crack underfoot.

A professional works from properly supported roof ladders, crawling boards, scaffold towers or a cherry picker, not by walking on the tiles. Cleaning chemicals also need care: keep them away from plants, animals and drains. If the streaks bother you, you can see typical roof cleaning prices by type of house in our cost guide, or tell us about your roof and we’ll send you a fixed price, usually without a site visit.