What causes black streaks and green mildew on your house, and how to remove them

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Short answer: Green staining on your house, especially on the north or shaded side, is mildew and algae taking advantage of Florida's heat and humidity. Black streaking is usually the same kind of organic buildup layered with dirt, and both respond well to a soft wash rather than needing to be scrubbed off by hand.

Why the shaded side is always the worst

The north-facing or tree-shaded side of a house stays damp longer after rain and gets less direct sun to dry it out. That combination, moisture plus shade, is exactly what mildew and algae need to establish and spread across stucco, siding or block.

Walk around most houses in the county and the shaded side tells the story on its own: it's almost always the greenest wall, while the side that gets full afternoon sun stays noticeably cleaner for longer stretches between washes.

Why Florida makes it worse than most places

Heat, humidity and afternoon storms are a near-daily occurrence for much of the year here, which means the conditions that grow algae and mildew barely let up. A house that would stay clean for a couple of years elsewhere can show real staining well before then in Volusia County.

How it comes off

A soft wash, a cleaning solution applied at low pressure, breaks down the growth without the cracking or paint-stripping risk that high pressure carries on stucco, siding, block or brick. It works on the biology, not just the surface dirt, which is why the result holds up better than a quick scrub would.

Corners, under eaves and areas around downspouts often need a closer look too, since those spots trap moisture the same way a shaded wall does and tend to show the heaviest buildup.

What might not come all the way out

Mildew and algae respond well, but older, deep-set stains like old oil or heavy rust can lighten with a wash without fully disappearing. That's a fair expectation to set going in rather than a surprise afterward, especially on a house that hasn't been cleaned in a while.

Rust staining, often traced back to irrigation water with iron in it, behaves the same way: a soft wash can lighten it noticeably, but a stain that's been baked in by the sun for years may still leave a faint mark behind.

Keeping it from coming back as fast

Since shade and moisture are the real drivers here, staining tends to return fastest on the same shaded walls where it started. A recurring schedule, like Puma Pass every 6 months at 15% off, tends to catch that regrowth early rather than letting it build up for another full year before the next visit.

In Volusia County

The typical house wash across Volusia County's 235,236 homes prices at $279. That number applies whether the exterior is a light dusting of green or a heavier, longer-neglected buildup, since the price is based on square footage and story count rather than how dirty the house currently is.

Questions people ask

Will the stains come right back after cleaning?

Algae and mildew can return, especially on shaded walls, which is why many homeowners keep a recurring schedule rather than a one-time clean and hoping it lasts.

Is the black streaking the same thing that shows up on roofs?

Roof streaking comes from an algae that specifically feeds on shingle material. The black on your house is more often general organic buildup and dirt, though both grow in the same hot, humid, shaded conditions.

Can I scrub it off myself instead of washing the whole house?

Spot scrubbing can help in a small area, but it won't reach growth across the whole exterior the way a soft wash does, and aggressive scrubbing on stucco risks surface damage.

Does painting over the stains work instead of cleaning?

Painting over active algae or mildew doesn't remove the growth underneath, so it tends to show through again. Cleaning first is the more lasting fix before any paint goes on.

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