Black Streaks on Your Roof: What They Are, and Why Pressure Washing Makes It Worse

The dark streaks running down an asphalt shingle roof are not dirt, soot or shadow. They are a cyanobacteria called Gloeocapsa magma, and it is living on the roof, feeding on the limestone filler used in asphalt shingles. It spreads by airborne spores, which is why it tends to arrive on a whole street within a few years of showing up on one house.

Why the north slope first

It needs moisture, and the slope that holds moisture longest is the one that sees least sun. On Long Beach Island that is usually the north and the bay-facing slopes, and if a taller neighbouring house throws shade — common through Brant Beach and Beach Haven Crest — the pattern follows the shade rather than the compass.

Why pressure washing is the wrong answer

Two reasons, and the second is the expensive one.

First, it does not work for long. Pressure removes what you can see and leaves the organism's root structure and spores in place, so it comes back — often faster, because the surface has been roughened.

Second, it damages the roof. Asphalt shingles are protected by a granule layer, and high pressure strips it. The Asphalt Roofing Manufacturers Association's guidance is explicit that shingles should not be pressure washed, and most shingle manufacturers exclude high-pressure cleaning from their warranty terms. A roof cleaned that way can look excellent and have years taken off it in an afternoon.

What is actually recommended

Low pressure and a cleaning solution — soft washing. The solution kills the organism rather than scraping it, so the result lasts, and nothing is applied at a pressure that can move a granule. It is slower and it is the method the manufacturers allow.

Questions worth asking before anyone gets on the roof

• What pressure will you be working at?
• How are you protecting the plantings and the run-off path?
• Are the gutters being flushed afterwards, since everything you take off the roof ends up in them?
• Are you walking the roof, and if so, is it a roof that should be walked?

A crew that answers those without hesitating has done it before. A crew that answers the first one with a number in the thousands of PSI is about to cost you more than the cleaning.