Why Does the North Side of My House Turn Green? (Gulf Coast Algae)

If one wall of your house is turning green while the rest looks fine, you're not imagining it — and it's almost always the north side. It's one of the most common things people call us about in Mobile and across the bay. Here's what's actually growing on your siding and why one side gets it worse.

The green is alive

That film isn't dirt you can hose off. It's algae — a living organism, most commonly a type called Gloeocapsa — and it feeds on moisture, shade, and the microscopic organic debris that settles on your walls. Our climate on the Gulf Coast is a near-perfect greenhouse for it: high humidity most of the year, warm temperatures, and plenty of rain. Give algae a damp, shaded surface and it moves in fast.

Why the north side loses first

The north side of any house gets the least direct sun. Sun dries surfaces out and its UV light suppresses growth — so the walls that bake in afternoon sun stay cleaner longer. The north wall, by contrast, stays damp for hours longer after rain or morning dew and rarely gets the direct light that would keep algae in check. Same story for any wall shaded by big live oaks, a fence, or a neighboring house. Damp plus shade equals green.

Why hosing it off doesn't work

Because the growth is alive, knocking the surface layer off with water — or worse, a high-pressure wand — doesn't kill it. The algae regrows from what's left in the pores of the siding, and within a few weeks in our humidity you're right back where you started. High pressure also risks cracking older vinyl and forcing water behind the panels, which creates a whole new problem inside the wall.

What actually removes it

The fix is soft washing — low pressure plus a cleaning solution that kills the algae and mildew at the root, then a gentle rinse. Because it treats the organism instead of just blasting the surface, a proper soft wash stays clean for a season or more, not a few weeks. It's the method siding manufacturers actually recommend, and it's safe for vinyl, fiber-cement, and painted wood.

Keeping it from coming back

You can't change the humidity, but you can slow the regrowth:

• Trim back shrubs and branches that keep the north wall in shade and hold moisture against it.
• Redirect sprinklers that hit the siding — you're literally watering the algae.
• Clean gutters so overflow isn't running down the wall.
• Plan on a wash roughly once a year here; the shady sides may need it slightly more often.

If the green is spreading, don't wait for it to take over the whole wall — algae etches into siding over time and gets harder to remove the longer it sits. Get a free quote and we'll soft wash it off the right way, whether you're in Mobile, Daphne, or anywhere across the bay.