Roofing Felt or Red Gard?
So glad you inquired, Where does this kind of advice come from? It’s certainly not from accredited installers. Cement board installed properly on shower walls, "do not need "to be waterproof". Walls in showers only need to deflect/shed water, they are not "swimming pools". As far as the shower pans go, there are many systems and the most common is to install a vinyl liner "folded tight" in the inside and outside corners and installed so the they run up the walls at least 8", as well as up and over the curb, and preferably without any cuts, unless the vinyl liner comes with its own "glue" and its own trim pieces for inside and out side corners.
The latter trim pieces take some expertise, and many installers ,DIY and professional, are now using preformed floor pitch strips and as well as "curb baskets", and then installing "dry pack" cement between and over them. Fasteners of any kind that might penetrate and compromise the integrity of vinyl liners must never be used below the 8" height on walls and never used on the curb. If you use products like "Red Guard" over the cement board, you are wasting money, and might even compromise the integrity of the installation's bond. Cement board is designed to hold tile without any help. "Red Guard", in order to function effectively as a "water proofer" must be applied in many coats to eventually get to a 1/8" final thickness.
"Red Guard" used to waterproof a shower pan floor is not recommended by novices, or professionals, who don't know what they are doing. Backer boards on walls must also be lifted several inches above the estimated height of the "dry pack" in order to prevent water wicking up the walls from the floor pack. These open areas must be filled in with dry pack cement so it effectively prevents the (siphon) wicking of water up into the boards and possibly getting the wood studs wet (which could eventually create mold). All 1/8" purposely left open joints between all cement boards, and especially in the inside and/or outside corners, must be filled in with "modified" thin-set mortar and then covered over with alkaline resistant backer board tapw and then recoated with additional mortar. Al mortar is allowed to dry and then tiled in the normal method. There must be a minimum of 95% transference of mortar to the backs of the tiles after they are set, and this should be tested by removing a previously set tile with a screw driver to see if the mortar spreading technique was effective, according to "NTCA" rules and guidelines.
As far as "roofing felt", it, or an 8 mil plastic liner is permitted on walls before the installation of backer boards, especially if the shower wall/s are "outside walls" in 3 or 4 season climates. Any type of protection that is used behind the boards must overlap the waterproof liner around the pan. When this additional protection is used it functions as a moisture barrier to prevent saturation of wood studs and the/any fiberglass insulation between the studs from getting wet from water condensation dripping down on the backsides of the backer boards during long “hot showers”.
All weep holes in shower drains must always be protected from invitable clogging with "dry pack" cement, and this mandatory protection requires a full 8 oz cup of pea gravel, or hand made "tile chips". This should be at least 2" thick and packed tightly around the sub drain to protect the weep holes from clogging before the pan is filled with the "dry pack" cement mixture. The shower pan must be pre-pitched a mininimum of 1/4" per running foot for proper drainage. A square drain frame around the drain is also easier to tile to. Armen Tavy