Black streaks, algae and roof cleaning
It is not dirt, it is not soot, it is not granule loss and it is usually not a reason to replace anything.
Drive any older street in Bryan or College Station and look at the north facing slopes. Perhaps half of them carry dark streaks running down from the ridge. Homeowners read this as dirt, as mould, as pollution or as a sign the roof is finished.
It is none of those. It is a cyanobacterium, commonly Gloeocapsa magma, and it has been spreading through humid parts of the United States for decades. It grows on roofs because modern asphalt shingles contain limestone filler, which it feeds on, and it colonises the slopes that stay damp longest. North faces get it first because they dry slowest.
Does it actually matter
Mostly cosmetically. A roof with heavy staining can easily have years of life left, and the staining itself is not a structural problem.
Two caveats worth having. A dark layer absorbs more heat, and a hotter roof in a climate like this ages a little faster, though that effect is modest next to ventilation. And if you are selling, it reads as neglect to a buyer and will appear in the inspection report whatever its real significance. That is an argument for cleaning, not for replacing.
Moss is a different matter. Moss is raised, holds moisture against the surface and lifts shingle edges, and it does shorten roof life. If what you have is green and three dimensional rather than black and flat, treat it as more urgent.
Cleaning it without wrecking the roof
The granules on a shingle are its sunscreen. Strip them and you have exposed the asphalt, which is the actual failure mechanism of an old roof.
So cleaning should be low pressure, with an appropriate cleaning solution left to do the work and rinsed gently. The industry guidance for asphalt is a soft wash rather than a pressure wash, and the difference is not a detail.
Protect what is below. Roof cleaning solutions are hard on planting, and the runoff goes exactly where your beds are.
Stopping it coming back
Zinc or copper strips at the ridge. Rain picks up trace metal as it runs and inhibits growth down the slope. Cheap, effective and visually unobtrusive.
Algae resistant shingles at the next reroof, which have copper bearing granules distributed through them and carry a specific warranty against staining. The upgrade is modest and in this climate it is one of the better ones.
Reduce shade where you reasonably can. An overhanging limb keeps a slope damp all morning. Trimming it helps the algae problem and the debris problem at once.
Questions people ask
- Will pressure washing fix it?
It will remove it and it may take years off the roof doing so. High pressure strips the granules that protect the asphalt. Cleaning should be low pressure with a cleaning solution doing the work, and anybody who arrives with a pressure washer and a wide grin should be sent away.
- Does it come back?
Yes, unless something changes. Cleaning alone buys a few years. Zinc or copper at the ridge, or algae resistant shingles at the next reroof, is what actually stops it.
- Is it the same as moss?
No. Algae is a flat stain. Moss is a raised green growth that holds water against the roof and lifts shingle edges, and moss genuinely does shorten roof life. Moss is less common here than algae but it turns up on heavily shaded roofs.