What thickness of wonderboard?

I have 3/4 plywood over 24" floor joists spaced 16 o/c. Will 1/4 Hardibacker attached to plywood with thinset and screws be sufficient for long-term installation? I will be installing a 1/4 thick 12x12 tile.

 

Hardibacker is not spec'd for anything higher than 19.2" oc joists. There are a few methods for 24" oc joists structures, but none involve Hardi. If it is t-g plywood and not square edged plywood, then Custom Building products will guarantee the instalation of 1/2" Wonderboard set in their Flexbond thinset and fastened to the subfloor per their fastener schedule.Personally, I would add a layer of 3/8" bc or better plywood and then use Ditra as my underlayment. The 3/8" ply would be the absolute scraping the bottom minimum. Thicker ply, even 1/2" would be better still. Ditra only adds 1/8" to the floor height. With 24" oc, you don't have a lot of options. Any way you could add a few joists?

 

I'm confused.

the statement "24" floor joists spaced 16 o/c. " does not sound right.

Your joists are 2 feet wide, spaced 16 o/c?
that is some mighty big board.

are they 2x12's? spaced 16 o/c?

 

Jp, missedthat part. You are right, very confusing. I saw 24" and read it as 24" oc and thought he meant not 16 oc. Never saw a 2x24, but boy would that make for a solid floor

 

The joists are prefabricated floor joists 24" deep. The top and bottom of the joist is about 2" wide. They are fabricated from wood and are spaced every 16" on center. The house was built in 1992 . . . so construction is fairly current.

 

You will be fine with the 1/4" over the subfloor. You'd be fine with just about any acceptable underlayment. Ditra, wonderboard, hardi. I like hardi because of ease of use, cost, and availability.
My next house will have 24" joists, then you can hide all plumbing, hvac and elec. in the joists for a nice flat basement cieling.
Have fun
Bryan