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 '17 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 '17 at 4:05

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 '17 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 '17 at 0:07
  • the length is 11 feet – joe Aug 14 '17 at 12:59
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    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 '17 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 '17 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.

  • Its not very flat. It is very sturdy. – joe Aug 13 '17 at 20:01

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

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    I am just adding to NPM's answer – joe Aug 20 '17 at 2:16

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