Eclipse Exterior Cleaning Blog

15 Oct Pressure Washing vs. Soft Washing: Which Does Your Home Need?

A Southeast PA Homeowner's Guide Both clean, but they work very differently. Using the wrong one can strip paint, crack siding, or leave mold behind to grow right back. Here's how to tell which one each part of your property needs. Ask most people what "power washing"...

13 Oct What Are the Black Streaks on My Roof?

A Southeast PA Homeowner's Guide

They're not dirt, they're not soot, and they won't wash away in the next rainstorm. Here's what's really growing on your shingles and how to get rid of it safely.

Drive through almost any neighborhood in Bucks, Montgomery, or Chester County and you'll spot them: dark, tiger-striped streaks running down the roofs, usually heaviest on the north side and under big shade trees. Plenty of homeowners assume it's dirt, tar bleeding from old shingles, or a sign that the roof needs replacing. In most cases, it's none of those. Those streaks are alive, and once you understand what they are, it's easy to see why they keep spreading and why the way you clean them matters so much.

What causes black streaks on a roof?

The culprit is usually a type of blue-green algae (technically a cyanobacteria) called Gloeocapsa magma. Its spores travel on the wind and settle on shingles, where the algae finds two things it loves: moisture and a steady food source. That food source is the shingle itself. Most asphalt shingles contain limestone filler, and the algae feeds on it. As the colony grows, it develops a dark outer coating to protect itself from the sun. That coating is what you're actually seeing. The streaks start small, then follow the path water takes as it runs down the roof, which is why they look like drips. Left alone, algae often makes room for more stubborn growth like lichen (the flat, crusty green or gray patches) and moss (the fuzzy green clumps). Those are a bigger problem, because they attach themselves to the shingle surface instead of just sitting on top of it.