I can only add 1/2 inch, what should I choose?

New Construction 3/4" OSB over 11 7/8" I-joist spaced (19.2) oc 12' max span. 250 sf kitchen dining area.

1/2" cbu std latex mod thinset -or-
1/2" bc ply with std thinset and latex liquid add mix.

Since you didn't mention the type of tiles, I'll assume you're using ceramic or porcelain, and not natural stone.

If 1/2" was positively the goal, I recommend adding 3/8" underlayment grade plywood, then Ditra. This combination will add strength with the plywood and an uncoupling membrane giving you a great worry-free floor.

Your first choice above will also give you a pretty good floor, again if 1/2" is magic. Half inch backer though is not better than 1/4", neither adds any structural strength. Your other choice of installing tile over a second layer of plywood, will get thumbs down with any pro I know.

Jaz

Thanks, the tile is 12" ceramic.

Anyone of them with Nobleseal, it will give you much more movement being a Crack Isolation membrane and is only adding 30 thousands of a inch. Also provides waterproofing!

I dont see how noble seal or any other isolation membrane would help if I go with 1/2" cbu? I understand how it would help with the 2nd layer ply-tile over, seasonal skrinkage-expansion breaking the bond.

If I go with 1/2" cbu bonding issues are minimal/gone but I assume the risk is slight floor flex causing possible grout cracks and pops?

I would add 1/2" plywood, then 1/4" CBU or a membrane of choice.

1/2" max

Some company needs to come out with a new -hybrid- thin (1/2') sheet product that combines, a very stiff wood ply panel, demensionaly stable, no moisture related shrinkage expansion and coated with a bonding surface that all thinsets stick to. Lets make it in light weight 4'x8' sheets as long as I am dreeming.