Water seems to be leaking from an upstairs bathroom to the downstairs bathroom ceiling. It seems to happen when water winds up on the floor (spill over from the right side of the sink and the left side of the shower). Just running the water in the bathtub and the sink does not cause any dripping. The house is old and floor also pitches down towards the corner in between the sink and the tub. I would like to redo the floor and level the corner. Underneath the tile is plywood. After removing the old tile and any backer board, should I throw on some leveling compound in the corner? For further security, can I place some sort of plastic liner in between somewhere before the final tile goes on?

Hello...I'm putting up the 12"x12" mesh sheets of 1" Travertine tiles...on a backsplash. They are about 1/4" thick. Based on my measurements and the many switchplates...I'm anticipating having to cut some of them...what is the best way to cut these?