Flooring Tile - Floor underlayment

I am preparing my upstairs bathroom floor to lay new tiles. According to the instructions the underlayment should be at least 1 1/4" but I've seen somewhere that a minimum of 5/8" can be used. The floor is currently 5/8" plywood and I would like to know if I need to add another layer of plywood before I lay the backer board. Does the 1 1/4" refer to the thickness of the plywood and backer board or just to the plywood. Also if I need to add another layer of plywood, how should it be applied, nailed to the existing plywood anywhere or does it need to be nailed over the floor joists. Lastly should I use 1/4" or 1/2" backer boards.

Hi
1 1/4 inches is the total thickness needed for your sub floor, so adding 1/2 backer board is just about right...that is of course, if your floor joists are 2x10 16 inches on center...if it's less you need to add more thickness. The backer board should be nailed or screwed down, but should also be bedded in a layer of thin set. This helps to fill any void under the board, giving you a more solid base for your tile.:rolleyes: