What Are the Black Streaks on My Roof?

Dark streaks running down a slope, often on the shadier side, are usually algae. It starts as a dusty film, spreads in bands in the direction water runs and darkens as it grows. It feeds on moisture and the dust that collects on tile surfaces, which is why it likes roofs that stay damp: north-facing slopes, roofs under trees and tiles with rough surfaces that hold water.

Algae is a surface problem at first, and it does not eat through a tile overnight. But a roof that grows it is a roof that stays wet, and wet roofs tend to attract moss and lichen next. The streaks also hold moisture against the tile, and over many winters that can add to frost damage on tiles that are already porous.

From the ground, algae tends to cover broad, uneven areas with soft edges. It is often joined by green patches, which is the earlier stage; see our answer on green algae on a roof. A binocular look, or a photo with the zoom on your phone, helps you compare the streaks with the pattern of the roof and where water runs. Do not climb up to check.

Not every dark mark is growth. Where a streak starts at a metal detail and runs straight down, the metal itself may be the source. Lead flashings, which seal the gap between roof and chimney or wall, shed small amounts of material when rain runs over them, and that can leave a mark on the tiles below. The mark has a clean start, close to the lead, and follows the water's path.

Chimneys add another cause. Soot and smoke residue from a flue can stain the stack and the tiles near it, particularly if a flue has been used for solid fuel. Staining like that sits close to the chimney, and it often collects on the windward side. A sooty streak will not come away with an algae treatment, because it is not living growth.

Tell the roofer where the marks begin and finish. A line that begins at the chimney points to flashing or soot, while broad streaks across the middle of a slope point to algae. Photographs taken from a safe place in the garden are enough to show it. If the flashing has been leaking too, your lead work may also need a look.

For algae, the usual route is a treatment made and authorised for roof use, applied as its label says, and then allowed to work. Dead growth is brushed or rinsed away gently. Tile maker Marley advises against strong jetting, which it says wears the surface and shortens the roof's expected life. A low-pressure wash, described in our answer on roof soft washing, is gentler on the surface.

Lead run-off is dealt with at the source. If a flashing is shedding, the fix may be to renew it, or to fit a sealed or patinated finish so that less washes down. Cleaning the tiles alone, without addressing the lead, means the marks return. Soot staining may fade with a gentle clean, but the better answer is to have the chimney and its flue checked.

Whatever the cause, what you want is a clear explanation and photos. We survey the roof, show you where each mark starts and what the cause appears to be, and set out one written total for the work. Whichever roofer you choose, check that VAT and access equipment are accounted for in the price.

Usually not. Streaks are cosmetic in the first instance and often disappear after a proper clean. A roof stained with algae but otherwise sound, with intact tiles, tight fixings and clear gutters, needs cleaning, not replacement.

The streaks can still be a warning. They show where water lingers, so the survey should look for cracked tiles, slipped tiles, blocked valleys and failing flashings in the same areas. If the tiles are crumbling or flaking, or if the battens beneath are soft, cleaning will not help and a repair or partial relay may be the better spend. Our answer on how often you should clean your roof shows how to plan ahead.

Some owners consider a coating after a clean, to slow regrowth. That is a separate decision, with its own pros and cons, covered in our answer on whether to coat your roof after cleaning. For the full service, see roof cleaning and moss removal. Send us ground-level pictures of the streaks and mark where each one begins, and we work out the cause on the survey.