Green algae on concrete floor - removal tips?

February 10, 2008 4:55 AM

Outside on the concrete floor of the back yard, it's all green and slippery. What to do to get rid of the algae?

At least, I think it's algae. It's dark green, and slippy. And I don't like it.

Outside is normally shaded away from the sun, if that helps you to know. I'm kind of expecting to buy some liquid cleaner, like bleach I suppose, and brushing vigorously with an old brush, but I don't know if that's entirely the right thing to be doing.

posted by jamescridland to home & garden (13 comments total)

That's what I would be doing. It certainly sounds like algae. I'd use a pressure washer (because I have one), but bleach and brushing would work.

Algae are sensitive to copper, so you may be able to prevent regrowth by arranging some old electrical cable so that rain runs over it.
posted by No Mutant Enemy at 5:02 AM on February 10, 2008

I rented a pressure washer for the same problem. The results were amazingly good.
posted by about_time at 5:26 AM on February 10, 2008

You can clean it off quite easily with bleach or a pressure washer. And a pressure washer is great fun. But if it's damp, you'll have difficulty stopping it from coming back.

Shaded and damp, or bright and damp, it's the water that the algae needs. If the surface never "dries out", algae will start recolonising pretty quickly. The only thing you can do to prevent regrowth is to make sure that the surface dries out regularly. If it's shaded, you're going to have difficulty doing that.
posted by Solomon at 5:32 AM on February 10, 2008

Make a solution of 1 part bleach, 1 part dish washing liquid and 2 parts water. Get a floor broom and scrub it on the area. The soap makes the bleach stick to the area. Give everything a good scrub, let it sit for ten minutes or so and then wash away. Should be good as new!
posted by pearlybob at 6:02 AM on February 10, 2008 [1 favorite has favorites]

I used powdered laundry soap, a hose set on fine spray and a stiff pushbroom. Sprinkle the soap over the area, spray down with a light mist from the hose (enough to make the soap wet but not wash it away). Scrub with the push broom. Rinse with the hose.

I had this problem on a wood walk under a willow tree. I only needed to wash it every couple months but it is pretty dry around here generally.
posted by Mitheral at 6:57 AM on February 10, 2008

The suggestions for power-washing will work in the short-term.
Unless you want to regularly power-wash the concrete, you need to correct the environmental conditions that allow the problem to exist in the first place. Sounds like you have a drainage problem.
posted by Thorzdad at 7:14 AM on February 10, 2008

We have the same problem on our patio adjacent to the pool. Shaded areas develop algae and sunny areas do not - it's not a drainage issue for us as much as a "not enough time to dry" issue.

like those above, we also power wash our green areas - it works great in not very much time. I prefer this method to using bleach because I'm concerned about both getting the bleach solution into the pool and protecting the shrubs and grass in the area.

You can rent a power washer if you don't own one.
posted by Flakypastry at 8:29 AM on February 10, 2008

Power washer... and, the warning nobody else has given.. a powerwasher can be a bit dangerous... I always recommend that, before you start using it, get an old piece of wood and practice a bit... and show yourself how easy it is to cut through the wood , then imagine your fingers, toes or, eyes...

sorry to be gruesome, but it's easy to think "it's only water, not a chainsaw!"...
posted by HuronBob at 8:44 AM on February 10, 2008

I've used a powerwasher for this before. Not all powerwashers are dangerous -- the one I borrowed could be used to spray bare toes with nothing worse than stinging -- but some are, so HuronBob's warning is worth heeding.

But see if there is a way to fix the drainage to keep water off of that area, and avoid much of the algae growth in the first place.
posted by Forktine at 10:18 AM on February 10, 2008

Like Flakypastry, I'd be concerned about washing bleach into your yard. Not good for anything trying to grow there, and it's not environmentally sound to have bleach in the soil. Powerwashers alone are very effective; if you want a chemical cleanser, please try to find something less toxic.
posted by bassjump at 10:43 AM on February 10, 2008

We had black algae every winter in our carport from bad drainage patterns and bleach never worked. It was power washed a few years ago and it's only now starting to return.
posted by rhizome at 11:40 AM on February 10, 2008

I power washed our north facing deck this past summer. It's fun as all get out, but do heed the safety warnings. Wear safety glasses, heavy jeans, and boots.
posted by plinth at 5:51 PM on February 10, 2008

My neighbors have good success with using the power washer, but I find that a brisk going-over with a stiff brush works equally well. Copper really does work to retard re-growth, and surprisingly you don't need much. As long as the pooled water can pick up particles from a piece of copper, it's getting enough to disrupt growth.
posted by nakedcodemonkey at 10:26 PM on February 10, 2008