The short answer
Black streaks on a roof are normally a dark, algae-type growth that settles on damp, shaded or north-facing slopes and spreads in the direction the rain runs. They are not soot, and they are not a sign that the roof is leaking. A gentle clean with a suitable biocide removes them, and the cause, usually damp and shade, decides how fast they return.
What the streaks actually are
The dark bands that appear below the ridge, down the slope and under a chimney are a living film. It is a microscopic, algae-type growth that arrives as airborne spores and settles wherever the surface stays damp for long periods. It darkens as it matures, which is why a patch that looked grey last year can look black this year.
The streaks are rarely random. Rain washes the growth downhill, so a colony that starts near the ridge draws a long dark line towards the eaves. The same growth often clings to the stone-like grit on concrete tiles, to the rough face of clay, and to natural slate and stone slates where the surface holds a little moisture.
It helps to know what the streaks are not. They are not soot from a chimney, they are not rust, and they are not a stain from the roof itself. Brown or orange runs are a different matter: those can come from iron in some slate or from a corroding fixing, and we would look at those separately. If the colour is a clean dark grey or black and it follows the fall of the water, you are almost certainly looking at a growth.
Why they appear on some slopes and not others
Three things decide where streaks start: moisture, shade and the texture of the covering. A slope that dries slowly gives the growth time to take hold. That is why the side of a roof facing north, or one sitting under a tree, is usually worse than a sunny slope on the same house. Our page on why moss is worse on one side of a roof explains the same pattern for moss.
Valleys, the V-shaped channels where two slopes join, take runoff from both sides and stay wet longer. Areas below a chimney, around a dormer and beneath overhanging branches also hold dampness. A flat patch of lichen or moss can sit among the streaks, because the same conditions suit all of them.
The covering matters too. A slightly rough surface gives spores a place to lodge and holds water between showers. Older tiles and slates have usually roughened with age, so a roof that was clean for years can darken quickly once the surface has weathered. Newer roofs are not exempt, however. A damp, shaded position will get streaks on almost any covering.
Do black streaks harm the roof?
On their own, the streaks are mostly a cosmetic problem. They darken the roof, make it look neglected and can mark out a house to a buyer or surveyor. The growth does keep the surface wet for longer, though, and a roof that stays wet is a roof that invites moss to follow. Moss is the more serious neighbour, because a mat of it sits wet on the covering, can lift edges, clog valleys and fill gutters with debris. We cover that in our guide on whether moss damages a roof.
Streaks do not by themselves cause leaks. If you can see staining on a ceiling, look for a separate cause, such as a displaced slate or a failed joint in the leadwork. Cleaning will not fix a leak, and we would not clean over one. It is one reason a survey comes before a clean: we want to see the roof properly first.
How black streaks are removed
The method that suits most coverings is a soft wash. A biocide, which is a treatment that kills the growth, is sprayed gently over the surface and left to do its job. The dead growth then loosens over the following weeks and is rinsed away by rain, or it can be rinsed at low pressure during the visit. Moss, being heavier, is lifted off by hand first.
This is slower than blasting the roof with a jet, but it is kinder to the covering. A hard jet can strip the protective face from a concrete tile or push water up behind slates, which is why we do not rely on it. You can read more on the difference on our page about pressure washing a roof.
Before any treatment, we look at the covering, because a soft or flaking tile or slate needs a gentler approach than a sound one. Once it is clean, a roof that was streaked often looks several shades lighter, though it will weather in as the treatment settles. For how long a clean takes, see how long roof cleaning takes.
Keeping them away afterwards
No cleaning method makes a roof immune. If the slope is shaded and damp, the growth will come back in time, and the slower you let it spread, the more work it is to remove. The practical steps are modest. Trim any branches hanging over the slope, keep the gutters clear so the eaves can drain, and have the roof looked at from time to time instead of waiting for it to look bad.
For roofs where cleaning has been done several times, it is worth asking whether the covering is also reaching the end of its useful life. For slate in particular, cleaning slate roofs covers what to look for, and the same thinking applies to tile. A cleaned roof should be sound underneath, and the survey will tell you up front if it is not.
Should the streaks on your own roof need attention, the survey is free, and the quote you get afterwards is a set figure on paper. Our roof cleaning service page explains the whole process, and you can reach us on 01225 285181 or WhatsApp.
