I am installing cement board in my bathroom now that my tub is installed, and I have found that if I place the cement board over the tub flange, it bows out a bit and will cause my tiles to be crooked. I have read that the options are either to shim out all of the studs so that the board covers the flanges and is still even, or simply place the board on top of the flange and fill in the bottom with mud or thinset.

I definitely want to cover up the flanges since my screws on the flanges chipped away some of the paint there, and I'd like to cover it up completely. So which is the preferred method? Is it possible to sit the cement board on top of the flange and cover the flange with thinset?

 

Shim out all of the studs.

 

I like to use lattice strips to "face off" the studs over the tub. That will bring the wall in line with the flange on the tub. You will be meeting sheetrock on the sides and will have to make a slight transition, but it can be done with mud and you'll hardly notice it.

 

Another option (although, perhaps not ideal) is to use a router and either a straight or rabbet bit to put a thin groove in the bottom edge of your CB so that it would overlap the tub flange slightly. If you decide to try this, I'd recommend doing it outdoors, as it will generate a lot of nasty dust and such.

Shimming out the studs would probably be the preferred method, but this would ultimately produce the same desired outcome.