The short answer
For most homes in the Triangle, once a year keeps siding clean. Homes with heavy shade, lots of trees, or a north-facing wall that never gets sun often need it more often, because that's where mildew and algae grow fastest.
Signs your house needs a wash
Green streaks on the shaded side of the house usually mean algae. Black spots or streaks are usually mildew or mold. A yellow-green film in spring is pine pollen, which feeds that growth if it sits. If you can wipe a finger across the siding and it comes back dirty, it's time.
Why soft washing lasts longer
Blasting siding with high pressure knocks off what you can see but leaves the roots of the growth behind, so it comes back quickly. High pressure can also crack vinyl and force water behind the siding. Soft washing uses low pressure and a cleaning solution that kills the growth first, so the house stays clean longer.
When to schedule
Right after spring pollen season is a popular time. Any time you see green or black streaks is a good time. If you've received an HOA letter, book as soon as you can so the work is done before your deadline.
Written by Marcus, owner of Southern Surface Solutions in Fuquay-Varina, NC.