Black Streaks on Your Roof: What They Are and How to Remove Them

They’re alive, they spread, and rain won’t shift them. Here’s what does.

The short answer

Black streaks on a roof are almost always living growth, not dirt or soot. The usual cause is algae (often named as Gloeocapsa magma), sometimes mixed with lichen and mould. It spreads down the roof with rainwater, which is why it shows as vertical streaks. It won’t wash off with rain, and it won’t go away on its own.

Why it happens

• Shade and damp. North-facing slopes and roofs under trees stay wet for longer, and algae needs moisture.
• Tile type. Concrete tiles are porous and hold water, so they streak sooner than glazed or smooth surfaces. See concrete tile roof cleaning.
• Age. As the tile surface weathers it gets rougher and holds more water and more spores.
• Airborne spores. Algae travels on the wind, so once one roof in a street has it, the neighbours usually follow.

Black streaks, moss or lichen: how to tell

• Black or dark grey streaks running down the slope: algae.
• Green cushions along tile edges and laps: moss. See moss removal.
• Flat white, grey or orange circles stuck hard to the tile: lichen. This is the slowest to shift.
• Green film: young algae, the stage before streaks.

Is it harmful?

On its own, algae is mainly a cosmetic problem. It makes a sound roof look old and neglected, which matters if you’re selling. See does roof cleaning add value?

The practical issue is what comes next. A roof that stays damp enough for algae is a roof that moss and lichen will move onto, and thick moss does cause problems: blocked gutters, lifted tile edges and water held against the tile through winter. Streaks are the early warning. See 5 signs your roof needs a clean.

How black streaks are removed

Because the streaks are alive, the fix is to kill the growth, not blast it off.

• Soft washing applies a biocide at low pressure. It kills the algae in the tile surface, and the staining fades over the following weeks as the dead growth weathers away. See soft washing.
• Pressure washing removes the colour quickly but takes some of the tile surface with it and leaves spores behind, so the streaks tend to return faster. See soft wash vs jet wash.
• Bleach lightens streaks fast but is hard on plants, metalwork and pond life, and doesn’t last as long. See biocide vs bleach.

Don’t go up to scrub them yourself. See should you scrape moss off your roof yourself?

Stopping them coming back

• Cut back overhanging branches so the roof dries faster.
• Keep gutters clear so water leaves the roof. See gutter cleaning.
• Ask about a repeat biocide treatment every few years rather than waiting for a full clean.

What to do next

Hampshire Roof Care is a matching service: tell us about the roof and our vetted local specialist partner quotes it directly. WhatsApp 023 8129 0232 or send an enquiry with a photo or two. Local pages: Southampton · Portsmouth · Winchester · Eastleigh · Chandler’s Ford · Fareham · Romsey · Alton · Emsworth · Waterlooville · Havant.

Quick answers

What causes black streaks on roof tiles?

Black streaks are usually algae growing on the tile surface, sometimes with lichen and mould. Rain carries it down the slope, which is why it forms vertical streaks, most often on shaded or north-facing roofs.

Are black streaks on a roof harmful?

Algae itself is mostly cosmetic, but it shows the roof stays damp, and moss and lichen usually follow. Thick moss can block gutters and lift tile edges, so streaks are worth treating early.

How do you get rid of black streaks on a roof?

A soft wash applies biocide at low pressure to kill the algae, and the staining fades over the following weeks. Pressure washing removes the colour faster but damages the tile surface and the streaks tend to return sooner.