Does this sound right?

I was going to put hardibacker down over subfloor. Subfloor is 23/32. I beleive it's OSB. Here are the marking on it:
PS 2-92 PS-133
Exposure 1
23/32 24" O. C.
Mill 214
Construction sheathing 1F24
Tongue and grooved 47-1/2" CSA 0325

Joists are 16" apart, each is 13' long

I was planning on putting down 1/4" hardibacker with nonmodified thinset, then 16" tile with modified thinset on top of the hardibacker. Does this sound right? Should I proceed?

The 16" tile is ceramic, 1/4" thick

What size are the joists and what type of wood (pine, douglas fir etc?) This makes a huge difference on the amount of deflection your floor will have especially the size of the joists.

The size of the joists are 9" x 1 1/2". I think they are pine, but I don't know.

You should be okay defelection-wise as long as the joists are in good condition. It comes out to L/427 Since the maxamium deflection allowed is L/360 you should be fine as long as joists are not cracked or knotty.

Joists look ok. Thanks for your help Nate!

It makes a difference what species of wood they are, and also what grade. Assuming the house was built to meet minimum standards, I agree you should be ok for ceramic or porcelain tiles. How does the floor feel?

The subfloor sounds good to me, I would simply re-fasten it before installing the Hardie.

Jaz

Thanks Jaz. The subfloor seems sturdy. The house isnt too old, only close to 7 years, so it should be built to meet minimum standards, I would asume. I did notice one small spot in the subfloor where it looks like they cut a piece out and reattached it. It gives alittle when I step on it where the cut line is. It's less than a 1 foot section. I put extra screws in it and was going to brace it from underneath, but discovered 2 air duct tubes run between the joists in that section. The rest of the floor is very sturdy. Any suggestions for that one little section? Do you think it will be ok once the hardiebacker is on top?

That's what happens when they cut the t&g and just butt the sheets. Being that small an area hopefully it won't matter. Some people would lower the sheet metal and fix it, I probably wouldn't. Just pay special attention there with a few more screws. It would also help if you used modified thinset for both under the CBU and to set the tiles. That's what I would do regardless, why buy two different thinsets and have partial bags of each leftover?

Jaz

Thanks jaz. So it's ok to use the modified thinset under the hardibacker? I read somewhere in this forum that with the OSB your supposed to use the nonmodified thinset, then modified for the tile. Just want to double check before I start buying stuff.

The recommendation for the use of unmod under CBU's is made by the TCNA as a general rule. However, the manufacturer's directions always trumps. With Hardie you can use either. I recommended the modified because it will bond the CBU to the subfloor, helping your situation. As for that advice, I believe there are only 2 manufacturers that allow the use of unmodified under their CBU. One is Hardie, the other is FinPan makers of Util-A-Crete. All others recommend modified.

Jaz

It doens't really matter that much, the fuction of hardibecker is to give the tile a smooth surface to bond. Also it has less voids than plywood does making it a better substrate for tile and you are not aiming to bond the backerboard to the subfloor only to take imprefections out of the plywood so they won't telegraph through the tile and crack it. Other than that the thinset really has no purpose for backerboard whatsoever.

What you were probably reading was some of the pro's on this forum were saying that's it's a good idea to use unmodified thinset mixed with water, because believe it or not you actually want the hardibacker to deattach from the subfloor overtime as it actually serves to isolate it from vertical movement in the floor-structure.

Cement board and tile backer manufacturers vary with their recommendation about this. Follow the manufacturers recommendations.