remodel bathroom

I am remodeling a bathroom which is a 5x8 space and I am going to be installing a marble floor, I have a existing floor which is 3/4 " plywood over the studs which are 2x6 24" on center. I thought I would put 1/4" cement board over the existing floor is that ok, or should it be 1/2 " cement board. Also do I install this material under the new tub as well ,or do I abut it to the new tub which would raise the floor above the bottom of the tub??

Thanks Rode

install another layer of plywood (1 1/4" min) then set the tub. then butt 1/2" cbu to tub to lock it in.

You really need more info before you can decide on if your floor is suitable for stone. Your floor must have less deflection than L/720. 2x6 is a very shallow joist plus the 24" spacing. You need to know the length of the unsupported joists - how far do they span between supporting walls or beams? Knowing that, you can determine the stiffness of your floor. Stone needs twice (minimum) the stiffness vs ceramic. The reason is that stone has more natural imperfections. Check out www.johnbridge.com and on their advice forum, there is a menu bar. Use the 'deflecto' - it will calculate your floor stiffness.

CBU is not structural, so on a floor, 1/4" is fine unless you need the thickness to match other areas (not common - it is usually too thick, not too thin). Also, when you use it, make sure to follow the instructions - thinset is REQUIRED under it to meet manufacturers' warranty. Galvanized roofing nails are faster and easier to install vs the screws, but you can use either. IF using the screws, suggest you use those with the square drive vs Phillips - they are easier to get to seat without messing up the driver bit.

You may not be able to use any tile on that floor if you can't make it stiff enough. You need stiffness not only between the joists (tough on 24" but possible with enough thickness) as along them (also tough with a 2x6). If you can't get underneath the floor to add more joists (sister those that are there), you might be out of luck if you want the tile to survive. ANyway, check out that site - they'll set you straight.