We just removed the old floor from our bathroom. My husband is going to put down 3/4" plywood and then 1/4" Hardibacker on top of that. Is it okay to use pre-mixed thinset mortar to set the Hardibacker on top of the plywood?

Also, do you use a square-notched 1/4" trowel to apply the thinset?

Thanks a bunch!

MarthaB

PS... I don't know if it matters, but we're going to put tile on top of the Hardibacker.

the premixed thinset mortar should work fine, but also use hardibacker screws as well. i'd say one screw about every 8-12 inches in both directions. you do want to use the square-notched 1/4" trowel as well, just be sure to put the thinset on evenly.

STOP!!! DO NOT USE THE PREMIX STUFF. It is junk to begin with, it takes forever to dry and it will compress. Use a thinset that comes in a bag, you mix it with water. This will cover your warranty with Hardie, premix will not. You can use the cheap unmodified thinset or a modified thinset. The primary reason for the thinset being under the Hardiebacker is not to bond so much as it is to fill in any hollow spots that may compress under weight. That is one of the main culprits that will cause your tiles to come loose or crack. You can use backerboard screws or 1& 1/4" galvanized roofing nails to anchor the board to the plywood. Also tape the seams with mesh tape and flash coat with the modified thinset you will use to set your tiles. The plywood seams should be gapped at 1/8" and should be filled with a cheap latex caulk before putting down the Hardiebacker. This will allow the plywood to expand without buckling as long as it is not tight to the walls, should be a 1/4" gap there left open or caulked, but no thinset in that gap either.