I'm not to sure pressure washing is your best choice. If the timbers ae extremely soiled, a low pressure wash may help. High pressure can do serious damage to the patina, particularly in softwoods.

I wood recommend dry brushing with a short stiff bristled scrub brush. A little compresses air can help speed the process, but wear adusk mask. Airborne dust can contain racoon fecis which can cause some pretty serious health problems.