Mold, mildew and algae on the outside of a house
Green, black or gray, they all want the same things: shade, moisture and something to hold on to. Knoxville supplies all three.
What you're probably looking at
Green film
Algae. Common on north walls, vinyl, fences and concrete. Slick when wet.
Black spots
Usually mildew, a surface mold. Common under eaves and on painted trim.
Black streaks on the roof
Roof algae feeding on shingle filler, spreading down with rain.
Gray-green crusty patches
Lichen, tough and rooted, often on old roofs and stone.
Green cushions
Moss, in shady roof valleys, patio joints and on north-facing walls.
Rust-orange marks
Not growth at all: iron from well water, fertilizer or metal, needing a different cleaner.
Why here
East Tennessee gets more than 47 inches of rain a year, summer highs in the 90s with heavy humidity, and long shaded stretches under hardwoods. Houses on wooded lots, near the lakes, or with a hill blocking the sun on one side are the first to turn green. The University of Tennessee area's older rentals, with mature trees and little upkeep, are a classic example.
How it's removed
Growth on siding and roofs is killed with a soft wash solution and then rinsed off. Scraping or blasting removes what you can see but leaves the roots, and it's back quickly. On concrete, a cleaner followed by a surface cleaner does the same job. Pre-wetting plants and rinsing them afterward protects the landscaping.
Is it a health risk?
Outdoor algae and mildew on siding aren't the same issue as mold inside a home. The practical risks outside are slippery walks and steps, and faster wear on roofing, paint and wood. If you have mold indoors, that's a job for a remediation specialist, not a pressure washer.
Common follow-ups
Will bleach from the store work?
Diluted bleach kills surface growth, but strength, dwell time and plant protection matter. Too strong, and it damages plants and some finishes.
Why does it come back so fast on one wall?
That wall stays damp. Trimming back trees or shrubs that touch it helps more than anything else.
Can you treat it so it stays away longer?
A soft wash kills growth at the root, which buys time. Shade and moisture will eventually bring it back.