If the shady side of your house has turned green, gray, or streaky black over the summer, you’re not imagining it and you didn’t do anything wrong. Charlotte’s heat and humidity are close to perfect growing conditions for algae and mildew. The good news is it comes off, as long as it’s cleaned the right way.
The short version: Green stains are usually algae. Black or gray patches are usually mildew. Both feed on moisture, so they show up first on shaded, damp walls. Clean them with a low-pressure soft wash, not a high-pressure blast, and control the moisture that lets them grow back.
What is that green and black stuff, really?
The green film is algae. It floats through the air as spores, lands on your siding, and settles in wherever there’s enough moisture to live on. The darker gray and black patches are usually mildew, a type of mold. Both love the same thing: warm, damp, shaded surfaces. That’s why you see them worst on the north side of the house, under tree cover, behind downspouts, and anywhere a sprinkler hits the wall.
Is it just ugly, or is it a problem?
Mostly it’s a curb-appeal issue, and a big one. But left alone for years, that growth holds moisture against your siding and trim, and constant moisture is what breaks materials down over time. Mold and mildew can also stir up allergies for people who are sensitive. As the EPA puts it, the only real way to control mold is to control moisture, and the same idea applies outside your walls as much as inside them.
Why you shouldn’t just blast it off
It’s tempting to point a high-pressure nozzle at it and let it rip. Please don’t. High pressure can drive water up under siding and behind trim, crack older vinyl, and strip paint. On a house exterior, the safer approach is soft washing: low pressure plus a cleaning solution that actually kills the algae and mildew at the root instead of just blowing the surface layer off. It lasts longer and it won’t hurt the house.
How to keep it from coming back
You can slow the regrowth with a few simple moves:
- Trim back shrubs and branches so sun and air reach the walls.
- Point sprinklers at the lawn, not the house.
- Keep gutters clean so water isn’t spilling down the siding.
- Plan on a gentle exterior cleaning every year or two, depending on how shaded your lot is.
Helpful resources
- EPA: A Brief Guide to Mold, Moisture and Your Home
- NC State Extension — research-backed home and yard guidance for North Carolina.
Want that green gone the safe way? 🌿
We soft wash Charlotte homes so the algae and mildew come off and the siding stays intact. Request a free quote or call 704-946-7659.
Frequently asked questions
Is the green stuff on my house mold or algae?
The green is almost always algae. Black or gray patches are usually mildew, which is a type of mold. Both are common on shaded, humid walls in the Charlotte area.
Will pressure washing damage my siding?
High pressure can crack older siding, strip paint, and force water behind the walls. That’s why exteriors should be soft washed with low pressure and a cleaning solution instead.
How do I stop it from growing back?
Control the moisture. Trim vegetation for better sun and airflow, keep sprinklers off the house, clean your gutters, and plan a gentle wash every year or two.