Those dark streaks running down your roof are algae. Not dirt, not mold, and not a sign the roof is failing. The organism is Gloeocapsa magma, it feeds on the limestone filler in asphalt shingles, and the staining it leaves is almost entirely cosmetic.
It comes off with a 50/50 mix of water and chlorine bleach. It does not come off with a pressure washer, and trying is the most expensive mistake homeowners make on this particular problem.
Why Does It Streak Downward?
Because it starts high and travels with the runoff. Algae takes hold near the ridge where the surface stays damp longest, and every rain carries spores down the slope. That’s the whole explanation for the drip pattern.
It’s also why it shows up first on north-facing slopes and under tree cover. Those surfaces hold morning dew and rainwater longer, and algae needs moisture more than it needs anything else.
And it’s why your neighbors got it around the same time you did. The spores are airborne. It isn’t a sign anyone neglected anything.
Does Roof Algae Actually Damage Shingles?
Mostly no. It’s staining. It doesn’t eat asphalt, it doesn’t cause leaks, and it doesn’t shorten the roof’s life in a way you’d be able to measure.
There are two real costs, though, and they’re worth being straight about:
- Resale. A streaked roof reads as an old roof to a buyer even when it has twelve good years left. That perception costs money at the negotiating table.
- Heat. Dark streaks absorb more solar energy than the shingle around them, so a heavily streaked roof runs a hotter attic and works your air conditioning a little harder. It’s a small effect, not a crisis.
Algae, Moss and Lichen Are Not the Same Problem
This is the distinction that decides whether you can take your time or need to act.
- Algae is flat. Dark stain, no texture, no height to it. Cosmetic.
- Moss is green and three-dimensional. It holds water against the shingle and its rhizoids lift the shingle edges, which lets wind get underneath and lets water travel sideways. Moss is a genuine problem and it needs to come off.
- Lichen is the worst of the three. Crusty gray-green patches bonded directly to the shingle surface. Removing it usually takes granules with it, so on an older roof, heavy lichen is often a signal the roof is closer to the end than the middle.
If what you have is flat black streaking, there’s no emergency. If it’s raised and green, or crusty and stuck down, have someone look at it.
How Do You Clean It Off?
The Asphalt Roofing Manufacturers Association’s guidance is a 50/50 solution of water and chlorine bleach, applied gently, left to sit for 15 to 20 minutes, then rinsed at low pressure. That’s the whole method. No scrubbing, no agitation, no pressure washer.
Before you start, soak the plants and shrubs below with water and cover them, then rinse them again afterward. Bleach runoff kills landscaping. Rinse metal gutters, flashing and downspouts as well.
What not to do, in order of how much damage it causes:
- Don’t pressure wash. It strips the granules that are the shingle’s entire UV protection and drives water up under the shingle courses. It’s the fastest way to turn a cosmetic problem into a replacement, and it will void most shingle warranties.
- Don’t scrub with a stiff brush or scrape. Same problem, slower.
- Don’t use a product the shingle manufacturer hasn’t approved. Some cleaners will void your warranty on their own. Check yours before you buy anything.
Should You Do It Yourself?
On a low-slope, single-story, walkable roof, with the right footwear and a spotter, some people do.
We’d rather you didn’t. Wet shingles with bleach solution on them are exceptionally slick, you’re handling a chemical that has to stay out of your eyes and off your plants, and roof falls are what they are.
There’s also a judgment call in it that’s worth having someone make first. If the roof is old enough that cleaning would take granules with it, cleaning is the wrong move and you’re better off putting the money toward replacement.
How Do You Keep It From Coming Back?
Three things, roughly in order of how well they work.
Zinc or copper strips at the ridge
Installed under the ridge cap, these release trace metal ions every time it rains, and that runoff suppresses algae down the slope below them. Copper works better and costs more. The catch is that a strip only protects what’s beneath it, so it has to run the full length of the ridge to do the whole roof.
Sunlight and airflow
Cutting back the limbs that shade the roof does more than most people expect. A surface that dries faster gives algae much less to work with, and trimming overhang is good for the roof for several other reasons anyway.
Algae-resistant shingles, at replacement
Nearly every shingle line we install is available with copper-containing granules and a stain warranty attached. GAF calls it StainGuard, CertainTeed calls it StreakFighter. If you’re replacing the roof regardless, there’s no good reason not to specify it.
The Question Under the Question
Most people asking about black streaks are really asking whether the roof is bad. Usually it isn’t.
But streaking tends to appear around the same age as several things that do matter, so if you’re looking at the roof anyway, it’s a sensible moment to have someone check the flashing, the pipe boots and how much granule loss has happened. That’s a free inspection, and it either settles the question or catches something worth catching.
Get a Straight Answer About Your Roof
L&L Contractors will tell you honestly whether what you’re seeing is cosmetic or structural. We work across Murfreesboro, Nashville, Franklin, Brentwood, Smyrna, Spring Hill and the rest of Middle Tennessee, and the inspection is free.
Call (615) 577-9103 or book a free roof inspection online.
