Those black streaks on your roof are alive

Almost every roof around here gets them eventually. Dark streaks running down the slope, heavier on one side than the other, worse each year. Most people assume the shingles are wearing out. They are not, at least not yet.

What it is

It is a blue green algae that feeds on the limestone filler used in asphalt shingles. It arrives airborne, it settles, and in a climate with this much humidity it never really dries out enough to die off on its own. The streaking runs downward because that is the direction the spores travel every time it rains.

Why the north slope is worse

Shade. The side of the roof that gets the least direct sun stays damp longest, and damp is the whole requirement. Add a couple of live oaks over the house, which most streets around Gretna have, and the shaded side never gets a chance to dry out properly between storms.

Why it matters beyond looks

It is eating the filler in the shingle and it holds moisture against the surface. Neither is fatal quickly, but neither is doing your roof any favors across ten or fifteen years of Louisiana summers.

What removes it

A no pressure soft wash with a cleaning solution that kills the organism rather than blasting the top layer off. Pressure would take the streaks off and take the granules with them. Done properly the roof comes back clean in a single visit and stays that way far longer, because what was growing there is dead rather than relocated.