Black streaks on a roof: what are they?

On most pitched roofs the dark staining is a thin growth of algae. It arrives as airborne spores, settles on a surface that stays damp, and darkens as it matures. A pale concrete tile shows it first, because the contrast is strongest, but clay, slate and stone are all affected given enough time.

The growth feeds on the damp surface and on the dust and mineral traces that wash down onto it. That is why a roof can look clean one summer and visibly striped a few seasons later. Nothing has been done to the roof in between; the film has simply built up in the places that never dry out properly.

If you want to compare it with the paler, greener growth that appears in sheltered corners, our answer on green algae on a roof covers that side of the same problem. Dark streaking is the stage after the colour has deepened and the film has spread in lines.

The stripes follow water. Rain runs from the ridge towards the gutter, carrying spores and nutrients with it, and the growth takes hold wherever that flow lingers. Streaks therefore tend to sit beneath ridge tiles, below chimneys and along the lines where one course overlaps another.

Aspect makes a difference too. A slope that gets little direct sun stays wet for longer, so a north-facing pitch often carries heavier staining than the one facing south. Overhanging branches add shade and drop leaf litter into the valleys, which gives the growth more to feed on.

Look at where the lines start. A streak that begins at one fixed point, such as a vent pipe or the base of a stack, points to something being washed out of that fitting. A general wash of dark colour across a whole slope points to algae spreading evenly through the damp.

Not every dark mark is a living growth, and the cause changes the treatment. These are the other things we meet when we look closely.

Soot and flue deposits

A dark smear fanning out beneath a chimney can come from a flue that is no longer drawing well, or from deposits washed off the top of the stack. The marks usually stay close to the chimney rather than running the full length of the slope.

Bitumen and felt run-off

On a roof with an older felt or bitumen section, such as a flat dormer or porch, heat can soften the surface and let dark material drip onto the tiles below. That stain does not respond to a soft wash in the way algae does.

Metal and rust

Rusting nails, old clips or ferrous fittings can leave brown or black trails. These point to a fixing that is failing, and wiping off the stain while the fitting stays as it is just means the trail comes back.

When we survey, we take pictures of each streak at ground level and along the eaves so you can see exactly where each one starts.

In its early stages the film is mostly about appearance. The concern is what it does over time. A dark, damp surface absorbs more heat and stays wet for longer, and a layer that holds moisture against the covering gives moss and lichen a place to anchor. On porous tiles, repeated wetting and frost can then loosen the surface.

It also masks what is underneath. A heavily stained slope makes it harder to spot a split tile, a dropped slate or crumbling pointing from below, which is one reason we like to clean before a detailed inspection rather than after.

None of this means a striped roof is failing. It does mean the staining deserves a decision rather than a shrug, particularly if the growth is spreading or moss has begun to appear among the streaks.

The method matters more than the force. MyBuilder's 2026 roof cleaning guide lists soft washing at about £15 to £20 per m², pressure washing at about £10 to £15 per m² and manual scraping at about £5 to £10 per m². The same guide cautions that jet washing risks harm to fragile tiles, and on older clay, slate or stone we avoid high pressure entirely. It strips the protective surface and can force water under the laps.

A soft wash uses a low-pressure application of a cleaning solution that is left to break down the growth, then rinsed away gently. It treats the film rather than blasting it off, which is kinder to the covering and tends to leave the surface cleaner for longer. Whole-roof cleaning is typically quoted from about £300 on MyBuilder's figures, with the final price depending on roof size, pitch and access. Our guide to access for roof cleaning explains how scaffolding or a tower changes that.

Conservatory glazing needs different handling again, covered in cleaning a conservatory roof.

No clean lasts for ever, because the spores are in the air. What you can control is how long the surface stays damp. Trim back overhanging branches, keep gutters flowing so water does not back up at the eaves, and clear debris from valleys. Those jobs slow the return of the film more than any treatment applied once and forgotten.

After a clean, we photograph the finished slopes and list any tile, slate or mortar repairs the cleaning revealed, so you can decide on those once you have seen the photographs. Any repair we make to a stained slope is covered by a 10-year workmanship guarantee that you get on paper.

A couple of phone photographs of the stripes are enough for a first opinion: send them over WhatsApp, ring 01453 933613 or use the enquiry form, and we will tell you what the pattern points to. Back to the main guide on roof cleaning and moss removal in Stroud for the full picture.