Green Algae on the North Side of Your House: What It Is and How to Get Rid of It
Walk around your house in the Midwest, and you'll see it. The front and south sides look fine. Then you turn the corner, and the siding has green algae on the north side of your house.
That's almost always algae. It isn't a sign your house is falling apart, and it's usually easier to get rid of than people expect. What matters is getting it off without wrecking your siding or your landscaping, and then making the north side a less friendly place for it to come back.
Is it algae, mildew, or just dirt?
They often show up together, and the fix is similar for all three. Here's a quick way to tell what you're looking at:
What you see | What it probably is |
Green film or tint, smooth or slightly slimy, worst on shady walls | Algae |
Gray, black, or dark green speckles, sometimes fuzzy | Mildew or mold |
Black streaks running down a roof | Roof algae (usually not the same as what's on your siding) |
Brown or gray film that wipes right off and is everywhere, not just the north side | Plain dirt and pollen |
White powder that comes off on your hand | Oxidation, common on old aluminum siding |
A quick test: wipe a spot with a damp rag. If it comes off green, it's algae. If it smears dark gray or black, mildew is more likely in the mix.
Why green algae grows on the north side of your house
Algae needs two things: moisture and time. The north side of a house gives it plenty of both.
It never really dries out. The north wall gets little or no direct sun, so morning dew and rain hang around for hours longer than on the other sides. Sun does more than dry things out, too. Building scientists at Green Building Advisor point out that direct sunlight, with its heat swings and UV, tends to kill and bleach out algae. The north wall just doesn't get that.
Trees and shrubs make it worse. Big shade trees, overgrown bushes against the foundation, or a neighbor's house close by all cut down on sun and airflow.
Water keeps landing on it. Clogged gutters that spill over, a downspout that splashes back on the wall, or a sprinkler head that hits the siding every morning will feed algae all summer.
Dirt gives it something to grab. Siding itself isn't food, but the pollen and dust that collect on it are. That's why a wall that hasn't been washed in a few years gets greener every season.
Ever seen algae in neat vertical stripes? That's not a coincidence. On some walls, the framing studs behind the siding let a little more heat leak through, so those strips stay slightly drier. The damp spaces between them grow algae, and the wall ends up looking pinstriped.
How to get rid of it: start gentle, step up only if you need to
The biggest mistake people make is grabbing the strongest chemical or the most pressure first. Light algae on vinyl siding usually comes off with very little. Work your way up this list and stop as soon as the wall is clean.
Rinse and look. Hose the wall down from the top with a regular spray nozzle. Sometimes that alone takes off more than you'd think, and it shows you how bad things really are.
Pressure wash. If you use a pressure washer, use a wide-angle tip, stand back, and keep the spray level or pointed slightly down. Spraying up under the laps pushes water behind the siding, and too much pressure can damage vinyl.
Spot-treat only what's left. If a few areas still won't come clean, that's where a diluted sodium hypochlorite-based cleaner makes sense, applied only to those spots, with the plants below soaked with water first and rinsed after.
Skip wire brushes, abrasive scrub pads, and blasting the whole house at full pressure. They scratch vinyl, strip paint, and leave a rougher surface that holds onto dirt.
Not vinyl? Painted wood needs a lighter touch. Brick, aluminum, steel, and stone can handle a bit more, but the mortar between bricks can't. When in doubt, test a small spot first.
Keeping it from coming back
You can't move the sun, but you can make the north wall a lot less comfortable for algae:
Trim back shrubs and low branches so air moves along the wall, and it dries faster after rain.
Clean the gutters and make sure downspouts carry water away from the house, not onto the siding.
Aim sprinklers away from the house. One sprinkler head hitting the wall every morning is enough to keep algae fed all summer.
Wash the north side every year or two. Light growth comes off easily. Five years of buildup takes a lot more work and usually more chemical.
That last point is the one people skip. A quick, gentle wash before it gets bad is much easier on your house than a big cleanup later.
Where we see north-side algae the most
We wash houses across four states, and green north walls show up everywhere. Some areas just get it worse.
Area | What we see |
Lake humidity and mature shade trees. North walls near the lakes can turn green in a season or two. | |
Cool, damp springs near Lake Michigan. Homes closer to the lake tend to grow it faster than those out west. | |
Humidity off the bay and Lake Winnebago, plus plenty of older neighborhoods with big trees. | |
Wooded lots and heavy shade. Some north walls barely see the sun at all. | |
Minneapolis, St. Paul, Bloomington, Maple Grove, Minnetonka | Long snow cover and wet springs keep the bottom few feet of siding damp. |
Cedar Rapids, Iowa City | Humid summers. Newer siding put up after the 2020 derecho starts going green too, if it's on the shady side. |
Houses are built close together, so the north wall is often shaded by the house next door. | |
Long, hot, humid summers. Algae has a longer growing season here than anywhere else we work. |
Want us to take care of it?
If the north side of your house has gone green and you'd rather not spend a Saturday on a ladder, we can help. We start with the gentlest method that will work, protect your plants, and only use stronger cleaners on the spots that actually need them.
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