Mold vs. Mildew vs. Algae: Identifying What's on Your House

Most South Florida homeowners lump every dark stain on their roof, siding, or driveway into one category: "that green stuff" or "that black stuff." But understanding mold vs mildew vs algae actually matters, because each one grows differently and responds better to different cleaning approaches.

Algae: The Most Common Culprit on Roofs

The black streaking most people call "roof mold" is usually algae — specifically a bacteria called Gloeocapsa magma, despite the name it's classified as cyanobacteria rather than a true mold. It feeds on the limestone filler in asphalt shingles and thrives in humid, shaded conditions, which is exactly what South Florida provides most of the year. Algae typically appears as streaking that runs vertically down a roof slope, starting near vents or shaded areas.

Mildew: The Surface Grower

Mildew is a type of fungus that grows on the surface of a material rather than digging into it. It typically appears as a flat, powdery gray or white patch and is common on painted siding, stucco, and outdoor furniture in shaded, damp areas. Because it stays on the surface, mildew is usually the easiest of the three to remove.

Mold: The Deeper Problem

Mold is a fungus that penetrates into a material's surface, not just sitting on top of it. On homes, it often shows up as dark green, black, or brown patches, and it can appear on stucco, wood, and even indoors around window frames and bathrooms if moisture is present. Because mold grows into the material, surface cleaning alone often isn't enough — it needs a treatment that kills the growth below the surface, not just what's visible.

Comparing the Three

Why the Difference Affects Your Cleaning Approach

Because algae and mold grow differently than surface mildew, a solution strong enough to kill mold at the root can be overkill (or the wrong chemistry) for a light mildew patch, and vice versa. This is part of why a proper roof cleaning uses different treatment than a house washing job on painted siding — the growth pattern and material both matter.

When to Call a Professional

• Streaking covers more than a small, isolated patch
• The growth keeps returning within months of cleaning it yourself
• You're unsure whether the surface (painted trim, stucco, roof tile) can handle DIY pressure washing
• The affected area is at height or otherwise hard to access safely

Homes throughout West Palm Beach and Fort Lauderdale deal with all three of these growth types regularly given the region's humidity, so getting the diagnosis right the first time saves both money and time.

Not sure what's growing on your roof or siding? Contact us for a free estimate and we'll identify it on-site. Call (561) 566-8138.