Black Streaks on Stucco or Siding: Mold, Mildew, or Algae?

Black or dark green streaks running down stucco or siding are almost always algae, not mold or mildew. Algae grows on the surface of the paint film wherever moisture sits the longest, typically on north-facing walls, under roof overhangs, and around sprinkler heads. It’s a cosmetic problem, but it’s also a sign your current paint’s mildewcide has worn out.

How to tell algae, mold, and mildew apart

Algae: Shows up as greenish-black streaks, usually running downward from a roofline, gutter, or window ledge, in the direction water flows across the surface. It’s most common on the shaded side of a house.

Mildew: Looks like a flat, dark gray or black film rather than streaks, and often smells musty up close. It spreads across a surface rather than running in lines, and tends to show up in areas with poor airflow, like behind shrubs planted close to the house.

Mold: True mold growth on an exterior wall usually means there’s a deeper moisture problem, like a leak or ongoing water intrusion behind the surface. It often has more texture or fuzziness than algae or mildew, and can appear in multiple colors, not just black.

If you’re not sure which one you’re looking at, a simple test helps: dab a small amount of diluted bleach on the stain. If it disappears within a few minutes, it’s algae or mildew. If it doesn’t lighten at all, it may be a dye stain or a sign the growth has gotten into the substrate.

Why this is common on Gulf Coast homes

Sarasota’s humidity keeps exterior surfaces damp longer than drier climates, which gives algae and mildew more time to take hold between rain events. Homes with mature landscaping, shaded courtyards, or walls facing away from direct sun tend to show streaking years before their south- or west-facing walls do.

Does this mean you need to repaint?

Not necessarily. If the streaking is limited to a small area and the paint underneath is still sound, a proper cleaning and treatment can handle it without a full repaint. But if the streaking keeps coming back within months of cleaning, that usually means the paint’s mildewcide additive has broken down, and the whole surface is due for a fresh coat with a coating built for high-humidity conditions.

What we do

We pressure wash and treat affected areas before painting, and when we’re repainting a home with a history of algae or mildew, we use coatings formulated to resist regrowth in humid, shaded conditions. Cleaning alone gives you a few months. The right prep and paint system is what keeps it from coming back.

If you’re seeing recurring black streaks on your home, book a free estimate and we’ll tell you honestly whether you need a cleaning or a repaint.