How can I prevent water pooling on a windowsill from getting behind wall tiles in a shower?
I would never use tile tor the window sill. The tile itself is waterproof, but the grout seams will always crack, the thinset is permeable, and the water will find a way to get behind the wall. Get a piece of stone custom fit, make sure it does not puddle water near the wall, and then you only have to worry about the edges which can be sealed with a silicone caulk