QUESTIONHow to Construct a Shower over a CMU substrate? I am installing new tile for my bathroom tub/shower. I have stripped the entire room. The back wall of the shower is an exterior CMU wall. What is the best way to install new tile over the CMU wall? Should I attached green board to the wall & tile over that or put the mortar bed directly onto the CMU wall?
ANSWERANSWER - Definitely do not install green board in a shower or wet area. Although it is technically moisture resistant, it is not recommended for showers per the building code and industry standards, because when exposed to excessive moisture it deteriorates and becomes a source of food for mold.
The best way to install tile over the CMU wall (concrete masonry unit) is to either attach metal lath to it and apply a mortar bed over it, or you can bond the mortar bed directly to it if it is porous enough and the mortar bed isn't too thick. If the CMU is flat enough, you can grind and fill the surface flat, apply a liquid applied waterproof membrane over it, and then adhere the tile to it with a modified thin-set mortar.
Of course the most important condition is the shower pan. It needs to have a presloped mortar bed applied to the floor, then a properly installed waterproof membrane over it and up the walls and over the top of the dam. Then a mortar bed on top of it sloped to drain. Make sure the drain weep holes are protected and left open for draining. I recommend applying the liquid applied waterproof membrane over the mortar bed to keep the moisture out to avoid musky odors and to have the added protection. Make sure all transition joints are filled with a proper sealant such as a urethane or mildew resistant silicone sealant. good luck.