cement backer board
There is no choice in the matter. You can not install any backer over plank subfloor, whether they have gaps or not. Too unstable. If your 10" planks were in perfect condition 1/2" ply underlayment exposure 1 might do. Often the planks are cupped or bowed in which case a thicker ply is necessary. You still need to re-screw them with at least 2 screws per joists of course.
Once the new ply is fastened only to the planks, properly spaced etc, you can install your favorite concrete backer board using the specified thinset mortar. Then tape and tiles etc.
Tile 4 You LLC Troy, MI
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