[wheelman4]

Remodeling a bathroom in an old house. Was going to put 1/4" hardiebacker on the floor (in pics) in preparation for tile, but now I'm not sure if I need the 1/2" or pair up the 1/4" with underlayment?

Existing floor is solid and flat, I added a lot more screws to be sure.

 

[Windows on Wash]

What is the flooring current thickness and Joist spacing?

 

[wheelman4]

plywood sections are 5/8" (or 19/32)

floorboard sections are 5/8" w/ 16" joist spacing

 

[F250]

Personally, if the transition path into the bathroom and your existing cabinetry/fixtures allow, I would put new 1/2" ply over the entire existing subfloor, making sure to overlap the existing ply and lumber portion to effectively tie it all together. Then, I would use at least 3/8" hardiboard over that. In that way, you would have a more unified substrate for your new tile (or vinyl, etc.).

 

[wheelman4]

Thank you for your comment. It looks like I can accommodate about 1/2" of substrate thickness before the floor tile is applied.

So that could be 1/2" of hardiebacker or 1/4" of ply + 1/4" of hardiebacker.

Either way, was planning to set the hardiebacker in thin-set.

If someone was to ever re-do this bathroom again, my guess is that they would thank me for not putting thin-set directly on the floorboards -- so the 1/4" ply + the 1/4" hardiebacker is what seems like the best choice at the moment.

Happy to have a forum like this to gain some opinions.

 

[chandler48]

Keep in mind cbu offers no structural strength so adding to thickness of cbu (1/4 to 1/2) won't do as much as another 1/2" of plywood would do with 1/4 cbu.

 

[wheelman4]

That makes sense. So in other words, if I can fit 1/2" of ply and 1/4" of CBU, then that would be ideal.

I can't seem to find 3/8" plywood very easily these days -- but maybe that's and option?

 

[jeffnc]

Thank you for your comment. It looks like I can accommodate about 1/2" of substrate thickness before the floor tile is applied.

The best way is to use 1/2" plywood, covered by Ditra (Ditra adds only 1/8" and acts as an uncoupling membrane as well, by far the better choice). There are other uncoupling membranes out there as well, so see what is available in your area, and follow the manufacturer's instructions. Others I'm aware of are made by RedGard, Mapei, and Laticrete, off the top of my head. These things are also far easier to carry, cut and install than backerboard. There are some confusing details with regard to waterproofing and type of thinset to use. I install Ditra all the time, and Mapei Mapeguard once, so ask if you have any questions.

 

[Old Thomas]

I have done what you are up against. I screwed down the existing boards, ground off high spots, then used 1/2” Hardi backer. A couple of times I used floor leveler before the backer.

 

[jeffnc]

As chandler already said, cementitious backerboards are not structural elements. They are effectively wasted height, in this case (although they do offer a better adhesive substrate than the existing subfloor.)


Also, look at the original photo again, and imagine the disaster a floor leveler would be!