Why are there black streaks running down my roof shingles?
Those dark streaks are almost always a hardy roof algae that feeds on the limestone filler in asphalt shingles and spreads as a dark, sooty stain. It is a living growth, not dirt, soot, or granules washing away, which is why it keeps coming back after rain and why scrubbing or a garden hose barely touches it.
The one thing that changes the answer is pattern. Algae streaks run down the slope in wide, soft-edged bands and show up worst on north-facing and shaded sections. If the dark areas are sharp-edged, follow a nail line, or appear directly under a vent, chimney or valley, you may be looking at granule loss, rust runoff or a flashing problem instead — that is a roofing issue, not a cleaning one.
Why it happens
Algae spores travel on the wind and land on every roof in a neighborhood, which is why streaking often shows up on several houses on the same street at once. They take hold where the shingle stays damp longest: shaded slopes, areas under overhanging trees, and the lower portions of a roof where runoff concentrates.
The streaking runs downward because rain carries the growth down the slope as it spreads. Where a metal component sits above the stain — a chimney cap, flashing, or a vent with zinc or copper content — you will often see a clean strip beneath it, because trace metal in the runoff suppresses the algae. That clean band is a good confirmation that you are dealing with algae rather than shingle wear.
Beyond appearance, algae and the lichen or moss that often follow it hold moisture against the roof surface. Lichen in particular grips the granule layer and can pull granules loose when it is scraped or blasted off, so the removal method matters as much as the removal itself.
What you can check yourself
Walk the perimeter of the house with binoculars or your phone camera zoomed in. Asphalt shingles with algae on them are slick, and a roof is not a safe place to inspect a cosmetic problem.
If the streaking is heaviest on shaded and north-facing sections and lighter where the sun hits, that fits algae. Even staining on every slope regardless of sun points more toward age-related granule loss.
A streak-free band running down from flashing, a chimney cap or a vent is a strong sign the stain is algae. That strip is where metal runoff keeps growth from taking hold.
Scoop a handful of gutter debris and look for a heavy layer of sand-like granules. Significant granule loss means the shingles are wearing, and the roof may need a roofer's opinion before any cleaning is scheduled.
Branches that shade and drip onto the roof keep it damp and feed the problem. Trimming them back is the single most useful thing a homeowner can do to slow regrowth.
When to call a pro
Call someone when the streaks cover a large area, when moss or crusty lichen patches have formed, or when you are tempted to climb up with a pressure washer. High pressure on asphalt shingles strips the granule layer that protects the mat underneath, can force water under the shingle courses, and commonly voids shingle warranties — manufacturers generally specify low-pressure cleaning, so verify the terms of your own warranty. A pro handling roofs uses a soft wash approach instead: low pressure and a cleaning solution that kills the growth rather than blasting it off.
Also call if you found heavy granules in the gutters, see curling or lifted shingles, or find any sign of a leak at the ceiling below — those are roofing questions that need answering before a cleaning crew gets on the roof. Call USA Home Repairs at (888) 996-7488 and we match you with a vetted local pro near you — free estimate, and you approve the price before anything starts.
What a pro will do
- Identify the stain first
A pro confirms whether you are looking at algae, lichen, rust runoff or granule loss. Cleaning only solves the first two.
- Soft wash the roof at low pressure
Shingles are treated with a cleaning solution applied at low pressure, left to work, then rinsed gently. No surface cleaner, no high-pressure wand, no scrubbing on the shingle face.
- Protect plants and runoff paths
Landscaping, foundation beds and anything downslope get pre-wet and rinsed, since roof runoff carries the solution straight into the garden.
- Clean the connected surfaces
The same visit often covers soft washing the siding, brightening the gutter faces, and cleaning walkways or a patio where algae has made the surface slick.
- Advise on regrowth
Expect guidance on trimming overhanging limbs and keeping gutters flowing, the two things that most affect how fast the streaks come back.
Related questions
Will the black streaks come back after cleaning?
Eventually, yes — spores keep landing on the roof. How fast depends on shade, tree cover and humidity, so a roof under heavy canopy will restreak sooner than one in full sun. Trimming branches and keeping gutters clear slows it down.
Can I just pressure wash the roof myself?
Not on asphalt shingles. High pressure strips the protective granules, can drive water under the shingle courses, and commonly voids the shingle warranty — check your own warranty terms. Low-pressure soft washing is the method made for this surface.
Do the streaks mean my roof is failing?
Usually not on their own. Algae stains the surface but does not eat through shingles. Heavy granules in the gutters, curling edges or an interior stain are the signs that point to actual roof wear and deserve a roofer's look.
Is moss the same problem as the black streaks?
They often appear together but are not the same. Moss is a thick green mat that holds far more water and lifts shingle edges as it grows, and lichen bonds into the granule layer. Both need gentle treatment rather than scraping or blasting.
What else usually gets cleaned at the same time?
Roof runoff carries the staining down, so siding, gutter faces and the walkways below are often streaked too. Many homeowners pair the roof with soft washing the siding and a surface cleaner pass on the driveway, patio or pool deck.
Want a pro to look at it?
Leave your number and we call you right back to set a time. Every visit is a free estimate, with no obligation, and we'll match you with a vetted local pro near you.
Rather talk to someone? Call (888) 996-7488.