Well, you can either: a) put a piece of wood or two on the outside of the sheetrock, or b) you can cut out a piece of sheetrock, install the studs behind it, and then 'reinstall' the sheetrock. c) put the pedestal sink on legs
Personally, I found that it's much easier to put back the same piece of sheetrock than cut a new one, but of course you need to be careful when cutting it out. -- Wacky Races 2012!
