Mold and Mildew: What's really happening on your siding?

When homeowners notice green patches growing on the north side of their home, it raises concern. Rightfully so, as the green algae and dark grey mildew give off the impression of a dirty and neglected home. This is more than just a cosmetic issue, as there are some long-term issues if it is just left there. It can lead to siding degradation and long-term staining. 

A brick wall in Milwaukee, WI, covered in some dirt, moss, & algae

Mold, Algae, and Mildew: What's the difference?

• Mildew is typically flat, gray, and powdery.

• Mold is darker, closer to a blackish patch that is often raised and grows in colonies. 

• Algae is a greener patch work that spreads in damp, darker areas. 

These all grow in damp, constantly wet, and darker areas of the house. These generally are found on the north side of houses as the sun hits those areas less often in the Northern Hemisphere.

Why is this an issue? 

These growths retain moisture and cause surface-level issues on your home. When these micro-colonies form on your siding, they retain moisture and cause it to stay damp longer, taking the finish off your siding and eating away at your vinyl. This is very common in the Midwest. For example, northern Wisconsin cities of Eau Claire, La Crosse, or the Minneapolis metro areas.

On vinyl siding, mold feeds on dirt and organic debris trapped on the surface. While vinyl itself isn’t an organic food source, the grime that accumulates on it is. As growth spreads, it can stain the siding permanently.

Impact on Curb Appeal and Property Value

Even if structural damage hasn’t begun, visible staining communicates neglect. In real estate markets, first impressions are decisive. Mold-streaked siding reduces perceived property value and can raise concerns during inspections. A clean exterior signals proactive ownership.

Mold-streaked siding in Madison, Wisconsin

Why Pressure Washing Matters

Professional pressure washing, depending on siding type, removes biological growth safely and effectively. The key is using the correct pressure settings and cleaning agents to eliminate spores without damaging the surface.

Improper DIY washing can:

• Force water behind the siding

• Strip protective coatings

A professional approach ensures contaminants are removed at the root rather and prevents new spots growing rather than temporarily rinsed away. Your siding is a protective barrier between your home’s structure and the elements. Mold, mildew, and algae compromise that barrier over time.

Routine professional exterior cleaning can:

• Extend siding lifespan

• Protect underlying materials

• Maintain property value

• Reduces long-term repair costs

Mold on siding is not just a cosmetic issue. It’s an early warning sign. Addressing it promptly keeps small maintenance concerns from becoming major structural expenses. Professional pressure washing is the best way to avoid all these issues. Especially if you reside in those upper Midwest areas.