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Question:

Can I use cement board as a subfloor instead of plywood?

We have an older house (1957) that has 1 thick beams as a substrate floor. Currently, there is 1/2 plywood on top of the substrate floor. We are going to rip this up so that we can apply mortar to the substrate for leveling purposes. We would put the porcelain tile directly on top of the cement board.The guy at Home Depot (a former contractor) said we can just use 1/2 cement board directly on top of the substrate floor without a layer of plywood underneath. The substrate is old, but sturdy real-wood diagonal beams that lay on top of the joists. He said they are plenty strong and we don't really need to add a plywood subfloor if we use 1/2 thick cement board.What do you think? Do you think we could get away will laying down cement board on top of the substrate floor without plywood for added strength?Thanks!

Answer:

If the diagonal sub floor is solid ( along with the beams) and the beams are 16 oc. The floor boards must tight together as apposed to some that have gaps you can. But to level out the floor with mortar may or may not work. It will depend on how much or how deep you need the mortar. There are other methods you may have to pursue. Mortar or thin set hasn t got a high compression strength so it can only be done to certain depths. Any questions you can e mail me through my avatar and check my qualifications there. GL
May 11, 2017
Cement Board Subfloor
May 11, 2017
If you have any reservations about the strength of using just the compressed cement board, why not lay that compressed cement board on top of the ply that's there now and save the hassle of ripping it up and quell those doubts. As for using a mortar for leveling purposes, why wouldn't you just use a tiling compound to rectify the levels as your tiling. After all how bad could it be, as a carpenter I'd say the guy who built the sub-floor frame couldn't have been more than a few mils out, here of there. BTW the compressed cement board will easily take the load without the ply.
May 11, 2017
To meet the structural requirements for a sub-floor, you will probably need 3/4 T&G plywood or OSB. Chip-Board is a non-structural material meant for shelves and cheap cabinets, and is not water resistant at all. I am not aware of any concrete sub-floor products currently on the market (some are being developed for commercial & multifamily construction - as a means of fire resistance -but I don't think they are available at this time). The current offerings by James Hardy & Sons, Durock (USG) and Wonder-Board are not sub-floors, they are underlayment materials, and should be treated as such. (Install & use them only as the Manufacturer's Installation Instructions indicate.) I prefer the USG product, but that's only my opinion; using any one of these products as an underlayment is far superior to plywood - NEVER use Luan plywood as a ceramic tile underlayment, it is not suited for wet or damp locations and does not bond well with the thin-set cement. The best bet is a quality installation of the plumbing (especially the tub or shower base) and then good maintenance (like caulking, keeping tile and floor grout in good repair. etc.). I suggest any of the cement-board underlayments available, just be sure you ask for the Manufacturer's WRITTEN instructions wherever you purchase the product, then follow them exactly. **************************************..... DO NOT use treated plywood without consulting the manufacturer of the treatment chemical used on the product; most are pretty toxic and need to have contact with outside air until all out-gassing of the chemical has stopped.
May 11, 2017
If joists are spaced 24 inches then you need 1.5 inches of subfloor under the cement board. if joists are spaced 16 inches then you need 3/4 inches of sublfoor under the cement board Those are the generalities.
May 11, 2017

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