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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

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joe
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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.

Jimmy Fix-it
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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.

NPM
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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

joe
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