Cast iron tub has 1/2 gap all around it. Need to shim studs, so backer board will above the low lip of the cast iron tub.

I would just rip (to cup long ways) a 1/2 shim out of 2x4 and nail it to the existing stud. That way the shim is equal to the thickness of the stud.

U can use anything that is 1/2'' thick by width of 2x4. Plywood works well and is less work then ripping a 2x4.

Chuck

If you don't want to do all that ripping, or don't have access to a table saw, 1/2x 1 1/2 lattice or lathing boards should be available at your local lumber yard.

As Creahands says, plywood is a good choice. We often cover the entire bathroom with plywood - it means you can hang the towel rods/ mirros/ TP holder/ soap dish etc anywhere because you have backing around the entire wall.

That is true, plywood is a good choice if you can afford it to cover all walls. However, I don't think that's what Creahands meant."U can use anything that is 1/2'' thick by width of 2x4. Plywood works well and is less work then ripping a 2x4." was his quote.I took that as meaning to rip it 1 1/2 " wide. Maybe I missed something?

Ur right tommybwiz that is what I meant. Rip 1/2'' plywood to width of 2x4. Is the least expensive way to go and u don't have to worry about splitting wood.

Chuck

Cover an entire bathroom with ply is silly. Why waste your money like that? A smart builder/renovater will just put 2x10 or 2x12 backer in any possible locations for toilet paper rolls, towl bars , etc.