I took out my existing shower/tub and am replacing the backerboard. Does anyone know if it is okay to use 1/2" backerboard right over the studs, then placing the tile right on the backerboard using thin set? Some people are telling me to use green board sheetrock over the studs, then use backerboard over the greenboard....do I need to use greenboard at all??
Do I also leave the backerboard 1/4" off the tub and fill that gap with silicon caulk, or do I bring the backerboard all the way down and touching the tub?
You don't need the greenboard. You can put the cbu directly on the studs. You may want to put up something like plastic or tar paper and then the backer board, depends on if this is a shower, too.
Concensus seems to be 1/8 -- 1/4" gap. Does the tub have a tile flange? If not, then you may want to do something like Schluter's Dilex-AS trim piece instead of caulking it. (Well, you still caulk, but it is hidden behind the Dilex trim - easier to make it look neat, if you like the look at all!). Suggest you look at www.johnbridge.com for tiling info, too. Alot of active threads - you can learn a bunch from them.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013