Flagstone Patio: How to Remove Calcium Stains

barbsullivan asks,

Hi, I would like to know the proper application of installing flagstone on an upper deck. We purchased a home in 2007, had a large lower (on the ground) and upper deck, all flagstone. The problem from day one is that the upper deck leaches calcium continually, no matter how much sealing we do. It drips down the steel poles, drips along the house, turns the flagstone white, etc… We were told that the owner had the upper deck installed in the winter and the concrete would not harden so a lot of calcuim was added. So, we are going to rip everything out and start all over, I would like to apply flagstone again but really need to know if there is a way to do this on an upper deck?

Our Answer

It's difficult to tell from your question if this is a problem caused more by the flagstone or the concrete. And you are calling this a “deck” but I will assume it is perhaps a tiered patio.

Both the concrete and the flagstone are extremely porous materials. Before tearing this out and starting again, I'd suggest that you try to seal the surfaces thoroughly with a product like Dupont's Exterior Stone Sealer. This will slow the absorption of the stone and concrete and hopefully reduce the calcium which leaks out as a result. Certainly worth a try if the alternative is to rip out an otherwise structurally solid patio.