The short answer
Black streaks on a roof are usually Gloeocapsa magma, a blue-green algae widely reported to feed on the limestone filler in asphalt shingles. It is alive, it arrives on the wind, and it holds moisture that lets moss and lichen follow. Pressure washing strips shingle granules. A bleach-based soft wash, as ARMA describes, kills the organism.
It Is Not Dirt, and It Is Not a Stain
The dark streaks running down the north-facing slope of your roof are a colony of living organisms. The species usually responsible is Gloeocapsa magma, a cyanobacterium, which is the group commonly called blue-green algae. Once you know what it is, the pattern makes sense.
It appears on the north side because that slope gets the least direct sun and stays damp longest. It runs in vertical streaks because rainfall carries the colony downward. It is worst under overhanging trees and around chimneys and dormers, because those areas hold moisture. And it shows up on your neighbors' roofs around the same time as yours, because, as the Asphalt Roofing Manufacturers Association (ARMA) notes, roof algae is transported through the air and settles on roofs and other outdoor surfaces.
Why It Is Suddenly Everywhere
Homeowners across Macomb County ask why nobody's roof looked like this thirty years ago. The explanation most often given is that shingle makers began using limestone as a filler, and Gloeocapsa magma is widely reported to feed on calcium carbonate, which is what limestone is. ARMA's own algae bulletin does not discuss limestone, so treat this as the common industry explanation rather than settled science.
What It Is Doing to Your Roof
The cosmetic damage is obvious from the street. ARMA treats the streaks themselves as discoloration. The lasting risk comes from what the algae makes room for.
- It holds moisture against the shingle. The colony forms a film that stays damp long after the roof surface would otherwise have dried.
- It invites worse tenants. That damp film is the environment moss and lichen need to establish. Moss holds water against the shingle and can lift shingle edges, and Michigan's dozens of freeze-thaw cycles each winter work on any edge that has been lifted. Lichen attaches tightly and can pit the shingle surface.
- It is easier to treat early. A colony of algae alone comes off with one treatment. Established moss and lichen take longer and may need a second application.
- Check your warranty. Shingle warranty terms on maintenance and cleaning vary by manufacturer, so read your brand's wording before anyone cleans the roof. Our post on roof cleaning and shingle warranties covers what to look for.
The compounding problem: algae holds moisture, moisture lets moss and lichen in, and those hold more moisture and lift or pit the shingle. Every year it goes untreated, the job gets harder.
Why You Should Not Pressure Wash a Roof
An asphalt shingle is a mat saturated with asphalt and surfaced with embedded ceramic granules. Those granules protect the asphalt from UV, and high-pressure water strips them off. The roof looks cleaner that afternoon, but service life went with the granules. Check the downspout discharge after someone pressure washes a roof: the pile of granules at the bottom is the roof's UV protection. High pressure aimed upward can also drive water underneath the shingle courses and into the decking, which is how a cleaning job becomes an interior leak.
ARMA's guidance is direct: do not use a power washer, brush or broom to clean algae from a roof, because high-pressure systems can damage asphalt roofing.
The Correct Method
ARMA's algae bulletin describes applying a solution of household bleach and water gently to the roof, leaving it for 15 to 20 minutes, and then rinsing it off by gently spraying the surface with water. ARMA also notes the roof becomes slippery and recommends that only trained professionals do this work.
A proper professional roof cleaning looks like this:
- All landscaping around the drip line is saturated with plain water first
- Downspouts are managed so runoff is controlled rather than dumped into beds
- The solution is applied at low pressure from the ground or from a ladder at the eave wherever possible, minimizing foot traffic on the shingles
- The mix dwells long enough to kill the colony, 15 to 20 minutes or longer on heavy growth
- The roof is rinsed gently, along with siding, windows and landscaping
Because the organism is killed rather than displaced, results typically hold for three to five years in Southeast Michigan rather than a single season, and the treatment repeats on that cycle.
What About Zinc or Copper Strips?
Zinc and copper strips near the ridge release trace metal into rainwater that suppresses growth on the slope below them. Adding strips to an existing roof is not recommended: fastening them means either exposed nails, which can leak, or breaking the sealant bond between shingles, which raises the risk of wind damage. They also do nothing about an existing colony. The time to plan for algae resistance is at reroofing, when strips can be installed properly or you can choose shingles made with algae-resistant copper granules, which ARMA notes are commercially available.
How Fast Does This Get Worse?
Progression depends on shade, nearby trees and slope direction, so treat this as a rough pattern rather than a schedule:
| Stage | What you see | What it takes to address |
|---|---|---|
| Early | Faint gray shadowing on the north slope | Standard soft wash |
| Established | Defined black streaks visible from the street | Standard soft wash |
| Spreading | Streaking across multiple slopes, first moss or lichen spots | Soft wash, sometimes a second application |
| Long neglected | Established lichen and moss, visible granule loss, lifted or pitted shingles | Cleaning helps appearance; any granule loss or lifted edges remain |
Roofs that have gone many years without treatment often cannot be returned to their original appearance. Cleaning still works on the growth, but granule loss and damage left by moss and lichen are permanent.
The Bottom Line
Black streaks are a living organism, and the moss and lichen that follow them get harder to deal with every year. Treatment is straightforward and inexpensive relative to a roof. The costly mistakes are leaving it for another five years or having the roof pressure washed.
Sources
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