We're retiling a tub shower. I have stripped the tile and old sheet rock to the studs all the way around. So far so good, no wood rot. Whenever the house was built, it appears that they used two layers of sheet rock for the tub/shower walls. I plan to use a hardy board type substrate going back, so the question is, since what was there was the thickness of two layers of sheet rock do I go back with two layers of hardy board or one layer of sheet rock with one layer of hardy board on top? I could go with double hardy board layers, however, I'm not sure if the special screws for hardy board are long enough to go through two layers.