Black Streaks on Your Roof: Causes and Fixes

Why the streaks run in lines

Rainwater runs down a roof in the same channels every time it rains, following the shape of the tiles. Airborne algae spores settle where that water lingers and feed on moisture and minerals in the surface. Over months, colonies grow along the routes the water takes, which is why the marks look like vertical stripes rather than random patches.

Lighter roofs show the effect most clearly. Pale concrete or grey tiles can look heavily stained while a darker roof nearby carries just as much growth but hides it.

Which roofs are most affected

North-facing slopes, roofs shaded by trees and houses in damp, sheltered spots stay wet for longer after rain, giving algae more time to grow. Slopes that catch the sun dry quickly and usually stay cleaner. Below chimneys and dormers, where water is channelled into narrow paths, streaks are often darkest.

If one side of your roof is streaked and the other is not, orientation and shade are almost certainly the reason.

Do black streaks damage the roof?

Algae is mostly a cosmetic problem. It has little bulk and does not lift tiles the way thick moss can. Its main practical effect is to keep the surface damp, which makes it easier for moss and lichen to take hold later.

That said, many people find heavily streaked roofs spoil the look of an otherwise smart house, and a stained roof can make a property seem neglected when it is put on the market.

How streaks are treated

Because algae is a thin film, it is not something to scrape. Where suitable for the roof, a soft wash and biocide treatment is applied to kill the growth. The streaks usually fade over the following weeks as rain washes the dead algae away, rather than disappearing on the day.

Our biocide treatment page explains how this works and why it is only applied where it suits the roof covering.

Why not just jet wash them off?

A pressure washer will lift the stain quickly, but it does so by blasting the surface of the tile. That can erode concrete, strip coatings, chip edges and push water under the laps. A roughened tile holds more moisture, so the streaks tend to come back sooner. We never pressure wash roof tiles.

Our guide on whether roof cleaning damages tiles covers this in more detail.

When streaks come with moss

Many roofs have streaks and moss together. In that case moss is removed by hand first, working with the tiles, then a soft wash and biocide where suitable deals with the remaining algae and lichen. Gutters are cleared afterwards because moss inevitably falls into them.

Our moss removal service explains the hand-removal part of the job.

Streaks below copper, lead and chimneys

Sometimes a clean stripe appears below a lead flashing or a chimney, with streaks either side. Run-off from some metals discourages growth, so the roof directly beneath stays cleaner. That contrast can make the surrounding streaks look worse than they are.

Where streaks are darkest below a chimney stack, it is often because water is being channelled into a narrow path down the slope. It is worth looking at the flashing at the same time, since that area also collects moss.

Streaks on render and walls below

Algae on the roof is not the only staining you may notice. Water dripping from an overflowing gutter can carry growth down onto render, leaving green or black streaks on the wall. If you see marks on the roof and the walls together, the gutters may be part of the story.

Our render cleaning guide explains staining on rendered walls, and gutter cleaning deals with overflow at source.

What a realistic result looks like

On the day of treatment, the roof may look only slightly different. Over the next few weeks, as rain falls, treated streaks lighten and fade. Heavily stained pale tiles may take longer. Tiles themselves can also have weathered unevenly underneath, which treatment will not change.

We explain this before any work so you know what to expect, rather than promising an instant transformation.

Getting a quote for streaked tiles

Photographs of each slope from the ground, ideally from both the garden and the road, show where the staining is heaviest. Note any moss, cracked tiles or a conservatory or extension roof below. That is usually enough for a free quote.

Send photos through the form on this page, by WhatsApp or call us, and we will explain whether treatment is likely to help your roof.

Gloeocapsa magma: the algae behind the streaks

The dark streaks on many roofs are caused by a blue-green alga often referred to as Gloeocapsa magma. It produces a dark outer layer that protects it from sunlight, which is why the streaks look black rather than green. Spores travel on the wind, so once one roof in a street is affected, others nearby often follow. Like other algae, it is treated with a low-pressure soft wash and biocide where suitable, then fades over the following weeks.