Why algae always seems to hit one side of the house first

Algae, mildew, and mold all need moisture to establish themselves, and direct sun is what breaks that cycle by drying a surface out between rain, dew, and morning humidity. In the northern hemisphere, the north-facing side of a house gets the least direct sunlight year-round, so it simply stays damp longer after every wetting than the walls getting baked by afternoon sun.

That gap shows up as a pattern homeowners can usually spot on sight: the front or south wall looks clean while the shaded side — often the one also crowded by a fence, hedge, or tree canopy that blocks airflow and shade it further — carries a green or black film that keeps coming back no matter how often it's treated. It's not that the house was built or painted differently on that side; it's that the drying time between wet spells is just shorter everywhere else.

That's also why treating the whole exterior on the same schedule can miss the mark — the shaded side is working against a head start that sun and airflow give the rest of the house for free.