Why the North Side of Your House Turns Green

The north side of your house turns green because it never gets direct sun and never fully dries out. That shaded, damp surface is where algae grows, and on a Twin Cities house it shows up on siding, soffits, trim, and the bottom corners of the windows on the north and east walls. It is not dirt, and it is not mold inside your walls. It is a living film sitting on the outside of the surface, and it washes off.

Why the north side and not the south

Sun is the whole difference. South and west walls take direct light for hours a day, so whatever moisture lands on them overnight is gone by midmorning. The north wall gets indirect light only. After a dew, a rain, or snowmelt running off the roof, it can stay damp all day, and in October it can stay damp for a week.

Algae needs three things: moisture, mild light, and something to hold onto. The north wall hands it all three. The south wall gives it light and a surface, then bakes the water off before anything can take hold. That is why the same house can look clean from the street and green from the back yard.

Shade does the same thing to any wall. A wall with a spruce or a tight row of arborvitae planted against it stays wet the way a north wall does, whichever direction it faces.

What the green actually is

Most of it is algae, a green film a few cells thick. The dark streaks running down a north-facing roof are a related organism that feeds on the limestone filler in asphalt shingles. Lichen is the crusty gray-green patch with a raised edge that looks almost painted on. Moss is the one you can pull off in a clump.

None of them eat into siding or grow into the wall. They sit on the surface and hold water against it, which is the actual reason to deal with them. Paint fails faster when it never dries, and wood trim under a mat of moss stays wet long enough to rot.

Where it shows up first on a Twin Cities house

Usually in this order: the bottom two feet of north-facing vinyl, where splashback off the ground keeps it wettest. Then the soffit and the band board under the gutter. Then the lower corners of north and east windows, where water runs off the glass and sits in the frame. Then concrete steps and the north edge of a driveway. The north slope of the roof is last to be noticed and usually the worst.

Windows are where people see it from inside. A green haze in the bottom corner of the glass is almost always algae on the outside of the pane, not a failed seal. Check the inside of the glass with a finger first. If the inside is smooth and the green is still there, it is on the outside, and it comes off with a normal window cleaning.

How to wash it off

Soap and dwell time do the work here, not pressure.

• Wet the wall first, working from the bottom up, so dry siding does not drink in the cleaner unevenly.
• Use a cleaner with sodium hypochlorite in it, or household bleach cut with water and a little dish soap. That is what kills algae. Plain water just pushes it around and it is back in a month.
• Let it sit about ten minutes and keep it wet. Dried cleaner leaves streaks.
• Rinse from the top down with a garden hose or low pressure.
• Soak the shrubs under the wall before you start and again when you finish. Bleach solution will burn leaves.

That is a soft wash, and it is how siding is supposed to be cleaned. High pressure on vinyl drives water up behind the laps and into the sheathing, and on old painted wood it strips paint and carves lines into the grain. If you are hiring out pressure washing, ask what pressure the person plans to run on siding. On siding the answer should be low, with the soap doing the work. Concrete and brick are where real pressure earns its keep.

Keeping it from coming back

You cannot change which way the house faces, so the only lever is drying time. Cut shrubs back about a foot off the wall and prune the low branches so air moves through. Make sure the gutter above the wall is not overflowing down the face of it. Keep soil and mulch from piling against the siding.

Do that and a north wall here goes two or three years between washes. Leave a gutter dumping down the same wall and it will be green again by spring. The same logic drives how often windows need cleaning in Minnesota: it is less about dirt in the air than about how long a surface stays wet.

If you want the green off the north side of your house: pressure washing starts at $150 and window cleaning starts at $99. Get a quote or book a time.