How often should a commercial building actually be washed?

There is no single interval, and anyone quoting you one has not looked

The honest answer depends on the elevation, not the building. A south or west wall with sun and airflow can hold two or three seasons before it needs anything. A north or east wall in shade, especially under tree cover, can visibly need it every year. This is why a building often looks like two different buildings — and why a per-building annual contract is usually either overpaying on three elevations or underserving one.

What actually drives it

Orientation and shade first: organic growth needs moisture, and the wall that dries slowest grows the most. Tree cover, which adds both shade and organic debris. Irrigation throw, if sprinklers are wetting a wall or a walkway, which is both a growth driver and a rust source. Drainage — a gutter that overflows down an elevation will streak it every year no matter how often it is cleaned. Traffic, for flatwork, which is a completely different clock from the walls. And salt exposure, which in Michigan is the one that does structural rather than cosmetic damage.

The interval that matters most is the spring one

If a property gets one exterior visit a year in Michigan, it should be after the salt season ends. Chloride left in concrete over a summer keeps working through the following winter's freeze-thaw cycles, and the resulting spalling is not reversible by cleaning — the surface is gone. A spring flatwork wash is the cheapest structural maintenance available on a commercial property. An autumn wash is worth having and is mostly about appearance and leaf tannin.

When washing every year means you are paying twice

If the same elevation streaks dark every single year while the others do not, the cause is usually water: a blocked gutter run, a downspout that discharges against the wall, a failed sealant joint, or irrigation hitting the building. Cleaning it annually treats the symptom forever. Fixing the cause is one repair and then a much longer cleaning interval. This is worth asking a contractor about directly, and the answer tells you something about the contractor.

A reasonable default to start from

Entrances, walkways and any flatwork people walk on: twice a year, spring and autumn, with the spring pass the non-negotiable one. Dumpster pads: quarterly, monthly through summer for food service. Building elevations: every two to three seasons, except the problem elevation, which gets diagnosed rather than scheduled. Gutters: annually, in autumn after the leaves are down. Then adjust from what you actually see in year two.