I have old plank subfloor installed at a 45 degree angle to the floor joists. enter image description hereHow should I install cbu(cement backer board) on top of this for a tile installation?

The entire area is approximately four feet by four feet. the joist are 2 inches by 9 inches by 11 feet

  • I have put down backer directly on t&g, if the t&g is not totally flat I put some thinset down prior to screwing it down and have never had any problems even with large living spaces +16x 24 on the last one I did for my last house on the 2nd floor living room.
    – Ed Beal
    Aug 14, 2017 at 0:56
  • this is not tongue-and-groove, it’s butt-jointed (the straight-cut planks are laid next to each other)
    – joe
    Aug 14, 2017 at 4:05

3 Answers 3


My experience is no, too much bounce. Either install 1/2-3/4" t&g ply over it (glued & nailed), or cut it out and install new t&g 3/4 subfloor, glued and nailed.

Also consider your floor joist width and span. In this question, that's undetermined.

In either case, then you can install 1/2" cement board.

  • I have updated my questions to include the floors dimensions. However the joists are a little complex i am going to simplify it and say that there is a 15 inch average span
    – joe
    Aug 13, 2017 at 20:07
  • I mean the length span. I think you are talking about the spacing between the joists. Can you re-edit the OP? You omitted one of the measurements.
    – NPM
    Aug 14, 2017 at 0:07
  • the length is 11 feet
    – joe
    Aug 14, 2017 at 12:59
  • 2
    Oh, little tiny thing. No worries, put down the cement board and start layin. Maybe to be sure, set the board in a bed of thinset with a 1/4" notched trowel.
    – NPM
    Aug 14, 2017 at 13:02
  • I see that the real question is a different then the one i asked. I will re-ask the question. I am looking for a source to your last comment.
    – joe
    Aug 14, 2017 at 20:37

Virtually all brands specify minimum 5/8" plywood or OSB as a base.

I have to say, if it was my house and there were significant challenges related to plywood installation (plank removal difficult, heigh problem with plywood over plank, etc.) I might consider mortaring and screwing it down without the plywood if and only if the plank subfloor was very sturdy (no flex/bounce/deflection when jumped up and down on) and very flat.


fasten any loose planks and flatten (plane, sand, chisel) any planks that are not level

1/2 inch - 3/4 inch exterior rated plywood or osb, screwed down to the planks the screws should avoid hitting the joists

thinset applied with 1/4 inch x 1/4 inch trowel applied on top of the plywood/osb

1/2 inch cement backer board on top of the thinset and screwed in just after the thinset is applied. it's O.K. to walk on the cement board before the thinset sets

  • 1
    I am just adding to NPM's answer
    – joe
    Aug 20, 2017 at 2:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.