problem with stone

by Gail Albin
(Alameda, CA)

We had a flagstone patio installed in our yard about 10 years ago and we have developed a problem that has gotten worse over the past 5 years. It was layed over a concrete patio. The edges were set in concrete and the center was layed in sand with planting of baby tears in between the stone. It is a very damp area with sun only about a 2 to 3 hours a day. The stone started with black spots on it and now it is turning black and all the stones are affected some more than others but it looks dirty. I had it pressure washed and nothing came off. What can we do and what causes this. I am afraid to replace the stone or it will do it again. Any ideas?

Thanks Gail Albin

Comments for problem with stone

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Try this too
by: David

Go to this page. It might help you out.

Black spots
by: Anonymous

WE have the EXACT same problem with Arizona flagstone on our patio. What is the cause? Is it mildew? Will a mildew remover "kill it". Used a pressure washer at over 1000psi with some improvement, but not a lot.

The right nozzle
by: David

Dear Gail,

We've had good luck with pressure washing the black spots away, however, you need the right nozzle for doing the job. We used what's called a "turbo nozzle." It costs $100 for the nozzle. I'm guessing the nozzle that you got was not strong enough--enough PSI.

The pressure washer also needs to be strong enough to force the water through the nozzle.

If all else fails, you many need to consider sand blasting.

Great luck!

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