Dirt, mildew and algae do more than look tired: they make walkways slippery and shorten the life of paint, wood and concrete. How often to wash depends on the surface, the shade around it and how much traffic it gets.
A schedule by surface
Driveways and walkways: once a year, and after any oil spill
Patios and pool decks: once a year, twice where trees shade them
Siding: every one to two years, more often on the north side
Wood decks and fences: every one to two years, before sealing or staining
Commercial entrances and sidewalks: monthly to quarterly, depending on foot traffic
Signs it is time
Green or black streaks on siding or concrete
Walkways that turn slippery after rain
Gray, weathered wood on decks and fences
Oil and tire marks that no longer fade
Best time of year
In the DMV area, spring cleans off the pollen and the winter grime, and early fall prepares surfaces before the leaves come down. Wash at least a few days before painting or sealing, so the surface can dry completely.
Pressure or soft washing
Concrete handles high pressure well. Siding, painted surfaces and roofs need a soft wash: low pressure with a cleaning solution that does the work. Too much pressure can force water behind siding, strip paint and score wood, which is why the right method matters more than the strongest machine.