Black streaks on your roof: what they are and how to remove them safely

The dark streaks running down the north and shaded slopes of asphalt roofs across Louisiana are algae, Gloeocapsa magma. It is not rot, it is not mould inside your attic, and it is not a sign your roof is failing. It is a cosmetic organism that thrives on humidity, shade and the limestone filler in shingles.

Why it is worse here

Warm nights, high humidity and heavy tree cover are ideal conditions, which is why streaking shows up so strongly in areas like Greenwell Springs and shaded older neighbourhoods. It always appears worst where the roof dries slowest, which is why one slope looks filthy and another looks new.

The mistake that turns cosmetics into damage

Do not pressure-wash an asphalt roof. The granules on a shingle are its UV armour, and a pressure washer strips them. A clean-looking roof with bald spots has been aged years in an afternoon, and most shingle manufacturers treat it as abuse that voids the warranty. Roofs also become genuinely dangerous underfoot when wet.

What actually works

The accepted method is a low-pressure soft wash with a diluted sodium hypochlorite solution per ARMA guidance: wet the vegetation and landscaping first, apply, dwell, rinse gently. Streaks fade over the following weeks rather than vanishing instantly. Done properly it removes nothing but the algae.

For prevention, zinc or copper strips near the ridge release trace metal in rainwater and slow regrowth, and algae-resistant shingles with copper-infused granules ship with warranties specifically covering streaking. That is the right time to solve it, on a replacement, not by scrubbing every three years.

When streaking is not the real problem

Streaks are cosmetic. Granule loss in the gutters, curling or cupped edges, bald patches, exposed nail heads and dark blistering that does not follow the drainage pattern are not, those are wear indicators. If you are unsure which you are looking at, a photo-documented roof inspection will tell you, and we will happily tell you your roof is fine and just dirty.

Want an honest read on whether it is algae or age? Ask us to take a look.