How to Identify Mold vs. Mildew vs. Algae on Your Florida Home's Exterior
The black, green, and brown staining on Florida homes looks similar but has different causes. Here's how to identify what's growing on your exterior surfaces.
Florida homeowners frequently call exterior cleaning companies asking to "remove mold" from their home's exterior — but what they're seeing may be algae, mildew, lichen, or Gloeocapsa magma bacteria rather than true mold. While all of these biological contaminants are addressed by professional soft washing, understanding what's actually growing on your surfaces helps you recognize the problem earlier, understand the cleaning approach, and set expectations for how long results last.
Green Algae: The Most Common Florida Exterior Growth
Green algae is the most prevalent biological contaminant on Florida exterior surfaces. It appears as a green film or spots on walls, driveways, walkways, roof shingles, and any surface that retains moisture. Algae requires moisture, sunlight, and a surface to colonize — concrete, stucco, and roof materials provide all three in Florida's climate. Green algae on a driveway creates a genuine slip hazard when wet. Soft washing with sodium hypochlorite kills algae at the root level; pressure washing without chemistry removes visible growth but leaves root systems that quickly regenerate.
Black Mold vs. Gloeocapsa Magma
The black staining most visible on Florida roof shingles is typically Gloeocapsa magma — a cyanobacteria (technically not mold) that appears as black or dark gray streaks running down roof slopes from the peak. This organism produces a dark protective pigment that appears black and is air-transmitted, meaning it spreads from roof to roof across neighborhoods. True black mold (Stachybotrys) is rare on exterior surfaces in Florida — it requires cellulose material and indoor moisture conditions. What most homeowners call "roof mold" is almost always Gloeocapsa magma bacteria.
Mildew: The Powdery White or Gray Growth
Mildew appears as white, gray, or light yellow powdery growth, often in shaded, damp areas like exterior wall corners, under soffits, and in sheltered porch ceilings. Unlike algae which is green and smooth, mildew has a distinctive powdery or fuzzy texture. Mildew responds well to soft washing chemistry and typically requires less chemical concentration than algae for effective treatment.
Lichen: The Stubborn Crusty Growth
Lichen is a combination of algae and fungi — it appears as circular, crusty patches in green, gray, or orange-brown coloration on roof tiles, natural stone, and older concrete. Lichen is the most difficult biological growth to remove because it adheres deeply to the surface substrate. Heavy lichen colonization may require two treatment cycles and extended dwell time to fully kill before cleaning removes the physical growth.
Professional identification and treatment of all exterior biological growth. Contact Caldwell Clean — call (937) 776-5094.