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Green algae or mildew on siding? Here's Who to Call
Green or black streaks on vinyl, wood, or fiber-cement siding are usually algae (Gloeocapsa magma) or mildew colonies fed by moisture and shade. Left alone they trap moisture against the surface, accelerating rot in wood and degrading paint. A professional pressure washing ($200–$500 for an average home exterior) removes these growths in 2–3 hours using a soft-wash technique — low pressure (under 1,500 PSI) with a sodium hypochlorite or oxygen bleach solution that kills spores without damaging siding. Standard high-pressure washing can force water behind siding laps on vinyl and splinter softwood grain, so soft-wash is the industry standard for building exteriors.
What's going on
How to fix green algae or mildew on siding
Some "green algae or mildew on siding" cases look simple, but getting it wrong almost always costs more than calling a pro: hidden leaks, fire risk, code violations, and insurance headaches. A licensed cleaner will diagnose the exact cause, use the right professional tools, and stand behind the repair. Most local pros offer free quotes and can visit the same day or the next.
Who to call
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Symptoms to look for
- 01
Green, black, or dark streaks on siding that don't wash off with a garden hose
- 02
Siding feels slimy or slippery to the touch in shaded areas
- 03
Discoloration worsens on north-facing walls or under trees
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Musty outdoor smell near the house foundation
FAQ
Frequently asked questions
Who should I call for: Green algae or mildew on siding?
Call a licensed cleaner. cleaners have the specialized training, tools, and insurance to diagnose green algae or mildew on siding safely and stand behind the fix. Most offer free quotes and can visit the same day or the next.
Can I fix this myself?
Some minor cases can be handled DIY, but most require a licensed cleaner to avoid costly rework and safety risks. A free quote takes under a minute.
How fast can a pro come out?
Most cleaners offer same-day or next-day visits for non-emergency issues. True emergencies (burst pipes, gas smell, no heat in winter) are typically handled within 1–4 hours.
How much does it cost to fix?
Costs vary by severity and your location. Submit a free quote request to get real pricing from local pros in your area.
What are the signs of: Green algae or mildew on siding?
Green, black, or dark streaks on siding that don't wash off with a garden hose. Siding feels slimy or slippery to the touch in shaded areas. Discoloration worsens on north-facing walls or under trees. Musty outdoor smell near the house foundation.