The black streaks running down painted siding or vinyl are not mildew, though most people assume they are. They're almost always a bacteria called Gloeocapsa magma, which produces a dark pigmented sheath to protect itself from UV exposure. It spreads through airborne spores, which is why it tends to appear on multiple houses in the same neighbourhood around the same time. Once it establishes itself on a moist surface, it grows slowly and consistently downward, which is why you get those long vertical streaks rather than scattered patches.
It grows where moisture collects. The north and west-facing walls of a house see less direct sunlight in the Fraser Valley, which means they dry out more slowly after rain. Shade from trees accelerates it further. Gutters that overflow also run down the siding and create the exact conditions the bacteria needs to thrive. If you have streaks on one side of your house and not the other, the answer is almost always sunlight and drainage.
The problem with a garden hose, and with standard pressure washing, is that it removes what's visible but doesn't kill the organism. Gloeocapsa magma has a protective outer layer specifically designed to survive UV and physical damage. You blast it off, it looks clean for a few weeks, and then you see the streaks returning faster than before. That's because the underlying spores weren't treated. You've removed the surface growth and given it a fresh start.
Most homeowners in the Fraser Valley notice the streaks in spring when the sun hits the house at the right angle. By that point, the growth has usually been there through the whole winter.
Get a free quote→Soft washing uses a low-pressure application of a cleaning solution that contains a biocide, typically sodium hypochlorite at a specific concentration, along with surfactants that help it penetrate the bacteria's outer sheath. The bacteria dies. The staining fades, usually within a few weeks as the dead organisms oxidize. When done correctly, soft washing keeps the siding clean for significantly longer than pressure washing alone, because the organism has been eliminated rather than relocated.
In the Fraser Valley, late spring is the best time to treat it. The wet months of winter let the growth establish, but the moderate spring temperatures and lower UV index allow the cleaning solution to work properly. Treated in May or June, most homes stay clean through the following spring. If your house hasn't been washed in more than two years and you're seeing streaks, they won't stop on their own.
