Cover the slices with cement board and tile

Using a 1/4 x 1/4 trowel and modified thin set, I covered the sauna floor with 1/4" cement board. I suppose if this were a house and I wanted to make sure there were no leaks I could have glued the foam boards together at the but edges with some form of butal caulk. Since the foam board is waterproof the only places that might leak are the joints between the boards. I didn't do that here and I'm not expecting any leaks.

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