Green Algae and Black Streaks on Siding: What Causes It in Portland and How to Remove It Without Wrecking the Paint

What the green stuff is

The green haze on the north or east wall is algae, most often Gloeocapsa magma, a cyanobacterium usually called algae and the same organism that makes black streaks on roofs, or Trentepohlia, which can look orange-red on some walls. It feeds on moisture and the minerals in dust, and it grows wherever the siding stays damp and shaded: north walls, under eaves and decks, behind shrubs, and along the bottom two or three courses where splash-back keeps things wet. Portland's long wet season and shade from tall trees are ideal for it, which is why the same house can have a clean south wall and a green north one.

Algae is not eating the siding, but it holds moisture against it, and on painted wood that moisture is what eventually lifts the paint. On vinyl and fiber cement it is mostly cosmetic until it thickens into a film that traps dirt and stays permanently damp.

What the black streaks are

Black streaks come from three places. The first is the same algae in its mature, dark phase, which is why the streaks follow the moisture paths: under windowsills, below the roof edge, down from a leaking gutter corner. The second is mold and mildew, which grow in the same conditions and often alongside the algae. The third is runoff: dirt, roof granules and the tannins from cedar and fir needles washing down the wall from a gutter that overflows. Streaks that line up with a gutter corner or a downspout elbow are telling you where water is coming over. Cleaning the siding without fixing that is a two-year fix.

How to remove it without damaging the siding

The method is a soft wash: a cleaning solution applied at low pressure, allowed to work for several minutes, then rinsed at roughly garden-hose pressure. The solution does the work, not the water. On algae and mold that means a diluted sodium hypochlorite solution with a surfactant, or an oxygen-based cleaner on surfaces that cannot take bleach, with the plants below wetted before and rinsed after. The wall comes clean the same day and stays clean because the organism is dead, not just knocked off.

By siding: vinyl and fiber cement take a standard soft wash well. Painted wood needs the paint checked first, because chalking or failing paint will come off with any wash, and it is better to know that before the hose is on. Cedar and other natural wood get a gentler solution and a lower rinse so the grain is not raised. Stucco is rinsed very gently and never blasted, because it is porous and the coating can be scarred.

The DIY version for a single-story wall is a pump sprayer with a diluted house-wash product, a soft brush on the worst spots, and a garden hose rinse from the top down. Wet the plants first. Keep the sprayer off the windows, and do not use a pressure washer on siding at any setting; the gain is not worth what it can drive behind the laps.

What pressure washing does wrong

High pressure aimed up under lap siding drives water behind the wall, where it sits against the sheathing. It lifts and feathers paint, raises the grain on cedar, scars stucco and can crack vinyl in cold weather. It also does not kill the algae; it knocks the surface off and leaves the roots, so the green comes back faster. On siding, pressure is the wrong tool. It is the right tool on concrete, which is why we carry both.

How long a clean wall stays clean

A shaded north wall in Portland usually stays clean three to five years after a soft wash, longer where branches have been trimmed back and the gutters above it stop overflowing. The bottom courses behind shrubs and the splash zone by the driveway green up first. Most homes do well on a house wash every three years, with the concrete done at the same visit, and the worst wall done more often if it bothers you.

Questions homeowners ask