Soft washing, in almost every case. Siding in this climate is dealing with algae and moss growth rather than baked-on dirt, and growth comes off chemically rather than mechanically. Direct pressure on lap siding drives water behind the boards and can strip paint on older trim. Low pressure with a cleaning solution removes the growth at the root, which is also why it stays clean longer.

Spring and early autumn, not midsummer. March through May clears the growth that built up over a wet winter, and September through October gets surfaces clean before the rain returns and gives moss something to sit in. Summer feels like the obvious window because it is dry, but by then the growth has had months to establish. Most properties here do best on two visits a year rather than one.

It can, and that is the whole reason the method gets decided before we start. Older concrete, particularly driveways poured decades ago with surface aggregate already exposed, will etch under a narrow-tip wand held too close. Painted siding and older wood trim will lift. We use a surface cleaner on flatwork so the pressure spreads evenly, and low pressure on anything painted or wooden.

It depends almost entirely on shade and drainage. A south-facing driveway in full sun can stay clean the better part of a year. The same concrete under fir canopy, or a north-facing wall that never dries out, will start greening again within a few months. Where a soft wash was used with a cleaning solution the growth takes noticeably longer to return, because the roots came off rather than just the surface.

Commercial work brings requirements that residential jobs usually do not, and they turn on where the wash water ends up. A lot draining to a storm inlet is treated differently from one draining to landscaping, and cleaning that lifts oil or paint residue changes what can be allowed to run off. We sort the containment and disposal question out with the property manager before the job is scheduled rather than on the morning of.