Why does my roof have black streaks?
Updated August 2026
Those dark stripes running down your roof are a blue-green algae called Gloeocapsa magma, not dirt or soot. It feeds on the limestone filler in asphalt shingles and shows up first on the shaded, north-facing slopes. A professional soft-wash cleaning runs $250 to $600 and lifts the streaks, and zinc or copper strips installed near the ridge keep them from returning. What you should never do is pressure-wash the shingles, which blasts off the protective granules and takes years off the roof.
Safety first: roof cleaning means working at height on a slick surface, so most homeowners are safer hiring a soft-wash crew than climbing up. If you do clean it, work from a ladder or use a roof-safe wand from the ground, never a pressure washer, and keep the cleaning solution off plants and out of gutters that drain to a garden. A wet, algae-covered roof is extremely slippery, and a fall from a roof is a life-changing injury.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Gloeocapsa magma algae | Most common | Soft-wash the roof with a low-pressure cleaner and a manufacturer-approved solution; the algae feeds on shingle limestone and streaks the shaded slopes. | $250–600 clean |
| Moss growth | Common | Green, clumpy moss on damp, shaded roofs holds water and lifts shingles; kill and gently remove it, then trim overhanging branches. | $300–700 |
| Mildew or mold in shade | Occasional | Persistent shade and trapped moisture grow mildew; open up the tree canopy and improve airflow so the roof dries after rain. | Tree trimming |
| No zinc or copper protection | Occasional | Without metal at the ridge, rain cannot carry algae-killing ions down the slope; add zinc or copper strips near the peak. | $100–300 |
Check these first
- Confirm it is algae, not a shadow or shingle discoloration: algae runs in vertical black streaks, worst on the north and shaded slopes.
- Look for green, three-dimensional clumps, which are moss rather than flat algae and need different handling.
- Check for overhanging branches that keep the roof shaded and damp, feeding the growth.
- Look at the ridge for existing zinc or copper flashing; roofs with metal up top rarely streak below it.
- Note the roof's age and granule condition before cleaning, so you know whether cleaning or replacement is the better spend.
Prevent it next time
Once algae is cleaned off, zinc or copper strips near the ridge are what keep it away: every rain washes a trace of metal ions down the slope that the algae cannot tolerate. Pair that with trimming back branches so the roof dries faster, and fold a roof look into your asphalt shingle roof maintenance checklist. Catching growth early also protects the roof's lifespan, which our guide on how long an asphalt shingle roof lasts covers.
When to call a pro
Hire a roof-cleaning pro when the roof is steep, tall, or slick, or when moss has taken hold and needs careful removal. A professional soft-wash runs $250 to $600, moss treatment and removal $300 to $700, and zinc or copper strip installation $100 to $300. If the shingles are already thin and shedding granules, cleaning may not be worth it, so read how often to replace an asphalt shingle roof before spending on a roof near the end of its life.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
Will bleach or a pressure washer remove roof algae?
A diluted, roof-safe cleaning solution applied with low pressure (soft washing) removes algae without harming the shingles. A pressure washer does the opposite: the high-pressure stream strips away the granules that protect the asphalt from UV, taking years off the roof and often voiding the warranty. Straight chlorine bleach can kill the algae but harms plants and gutters and corrodes metal if it is not controlled. Stick with a manufacturer-approved solution and gentle pressure, or hire a soft-wash crew who does it safely.
Do the black streaks actually damage my roof?
Over time, yes, though slowly. The algae feeds on the limestone filler in the shingles and holds moisture against the surface, which can speed granule loss and shorten the roof's life, and dark streaks also absorb more heat. It is not an emergency the way a leak is, but leaving it for years does more than look bad. Cleaning it and adding zinc or copper strips protects both the appearance and the shingles' remaining lifespan.
Why is only one side of my roof streaked?
Because algae favors shade and moisture. The north-facing slope, or any side shaded by trees, stays damp longer after rain and dew, giving the algae the conditions it needs, while the sunny south slope dries fast and stays cleaner. That is also why streaks often start under overhanging branches. Trimming the canopy to let more sun and air reach the roof, and adding metal strips at the ridge, evens things out and slows regrowth on the shaded side.