North walls never dry out
A north-facing wall gets no direct sun. It holds water after every rain, and moisture plus a humid
Wisconsin summer is exactly what algae and mildew want. It is almost always the worst wall on
the house.
Freeze-thaw works all winter
Water gets into the surface, freezes, expands and opens it up a little more each cycle.
Concrete and masonry take the worst of it, and a rougher surface holds dirt better next season.
Road salt gets carried up
Salt off the street ends up on driveways, walks and the lower courses of siding all winter,
then sits there until something washes it off.
No lake, and it still grows
People assume green on siding is a lake-town problem. It is not. Algae needs shade and
moisture, and a humid Wisconsin summer supplies the moisture whether there is water nearby
or not.
Commercial walls go the same way
A building is no different from a house here. The shaded elevations hold moisture, growth
takes hold, and it reads sooner on a large flat wall simply because there is more of it to
look at.
You stop seeing it until someone else washes
Growth comes on slowly and evenly, so the owner stops noticing. What people notice is the
comparison — a wall that has drifted a shade off next to a neighbor who was done last
year. That is usually what prompts the call.