That green film on the shady side of your house is algae, not dirt. Why it grows, why hosing it off fails, and how to keep it gone longer.
That green or grey film on the shady side of your house is algae, and on the Emerald Coast it can come back within 6 to 9 months of a wash. The good news: it does not mean your house is failing. The bad news: scrubbing it or blasting it off with a pressure washer does not actually solve it.
What is the green stuff, exactly?
It is a living organism, usually algae with some mildew mixed in. It needs three things: shade, moisture and something to hold on to. Here, with humidity above 70% for most of the year, a north-facing wall gives it all three. That is why it looks like dirt but behaves like a lawn — cut it back and it grows again.
Why is it only on one side of the house?
Because that side never fully dries. Look for the usual suspects:
- The north wall, which gets little direct sun
- Walls behind shrubs or under trees, where air cannot move
- Spots where the sprinklers hit the siding every morning
- Under gutters and eaves, where water runs down after every storm
Is it actually hurting my house?
Slowly, yes. The film holds moisture against the paint all day, which breaks the paint down faster. On stucco it roots into the texture. On walkways it gets slippery. And on a rental or a house you plan to sell, it is the first thing people notice in the photos.
Can I just blast it off with a pressure washer?
You can, but it will be back in a few months — and you may do real damage on the way. High pressure only cuts the growth back; the roots stay in the surface. On siding it forces water behind the panels, on stucco it widens hairline cracks, and on painted wood it lifts the paint.
The method that works is a soft wash: a cleaning solution applied at under 500 psi, left to work for 10 to 15 minutes, then rinsed from the top down. It kills the growth instead of trimming it. More on how that works in our soft washing guide.
How do I keep it from coming back so fast?
- Trim plants so there is about a foot of air between them and the walls
- Point sprinklers away from the siding
- Keep gutters clear so water does not pour down the wall
- Wash on a schedule: every 9 to 12 months for most homes, every 6 to 9 if you are close to the Gulf
If it comes back within 3 months of a proper wash, the problem is water or shade — not the wash.
Not sure how bad yours is? Send us a photo. Cucol LLC is based in Santa Rosa Beach — find your area or ask for a free estimate and we will tell you honestly whether it needs a wash now or can wait.
