Roof Washing

That's algae — Gloeocapsa magma — not dirt and not shadow. It's an airborne organism that lands on the roof, feeds on the limestone filler in asphalt shingles, and spreads downward in dark streaks. It shows up worst on north-facing slopes and shaded sections because it needs moisture to survive. Rain doesn't remove it. Scrubbing doesn't remove it. It has to be killed chemically or it grows back from what's left behind.

Yes, badly, and it's the reason we don't do it. High pressure blasts the protective granules off asphalt shingles — those granules are the shingle's UV protection and waterproofing, and once they're in your gutters they're not coming back. It also forces water up under the shingle courses. A pressure-washed roof can look great and have years knocked off its life. Ours is a soft wash: low pressure, the right solution, gentle rinse.

We apply a cleaning solution that kills the algae, moss, and lichen at the root, let it dwell, and rinse at a pressure no higher than a garden hose. The organism dies on contact; the black staining releases over the following days and weeks as rain and weather carry it off. That's why a roof often keeps improving for a few weeks after we leave. Killing the growth is also what makes the result last — pressure only removes what's on top.

Typically several years before streaking returns, depending on shade, tree cover, humidity, and which direction your slopes face. A roof under heavy trees on the north side will re-stain faster than an open south-facing roof. Trimming branches back so the roof dries out after rain is the single most effective thing you can do to slow it down.

Both. Moss is the functional problem — it holds water directly against the shingle, works its way under the courses, and lifts them, which is a leak in the making. Algae is mostly cosmetic but it does darken the roof, which raises attic temperatures. And some HOAs and insurers cite roof staining. If you're seeing green moss and not just black streaks, treat it as maintenance rather than appearance.