Field note 01
Black streaks on your roof: what they are, and the fix.
Those dark runs down your roof aren’t dirt, and they aren’t “just age.” They’re roof algae — a cyanobacterium, closer to bacteria than to a plant — that feeds on the mineral filler in shingles and spreads where the roof stays damp longest: the north face, the shaded runs, under overhanging trees.
That’s why it comes back after pressure washing. Blasting shears off the visible layer and leaves the roots in the surface. The colony regrows from what’s left — usually within a season or two — and the pressure that removed it also stripped granules, opened tile pores, and shortened the roof’s life.
Kill what pressure washing leaves behind — that’s the entire idea, and it’s on our homepage for a reason.
The fix is chemistry, then patience.
A soft wash puts a soap-based solution on the surface at low pressure — roof-safe, section by section — and lets it work. The solution kills the growth at the root, and what is dead cannot regrow — which is why a soft-washed roof stays clean far longer than a blasted one.
Your landscaping is part of the plan, not an afterthought: beds get pre-soaked and covered before we start, and everything is neutralized and rinsed after. The full method, tile and shingle side by side, is covered under roof cleaning; the same growth on walls is handled by a house wash.
Diagnose it before you call anyone.
Pick the thing you’ve been squinting at. Straight answers — including the parts that can’t be fully fixed.
What it is
Roof algae — growth, not dirt. It feeds on the mineral filler in shingles and spreads along the damp, shaded runs of a roof.
What we do about it
Soft wash chemistry kills it at the root, then it rinses away. Blasting only shears the surface off — the roots stay, and it grows back.
What it is
Algae and mildew, usually on the north side or anywhere the sun doesn't dry the wall. It sits on the surface but roots into texture and paint pores.
What we do about it
A soap-based soft wash at low pressure kills it and rinses clean without stripping paint. The dwell and the rinse are what change with the wall.
What it is
Efflorescence — minerals carried out of the masonry by moisture, left behind when the water dries. It's coming from inside the material, not landing on it.
What we do about it
It can be cleaned back, and what we can see of it goes in your written report.
The honest part
Because it's mineral, not organic, it can return until the moisture source is fixed — we'll tell you that up front.
What it is
Usually irrigation water or fertilizer granules — iron, deposited into the surface and oxidized in place.
What we do about it
Stain-specific chemistry, applied only where it's needed, lightens it substantially.
The honest part
Deep-set rust lives in the material itself. We'll lighten it as far as the surface allows — and tell you before we start how far that's likely to be.
Want it in writing for your actual roof?