Minimum would be 3/8". You already have the strength in the planks. You only need the plywood because it's required that you go ov er engineered wood panels with any kind of tile underlayment, whether it be cement board membrane or other. Now, two things-- first, cement board will not hide rolls in the floor (I'm going through the same thing with my house-- it's 101 years old). It'll only conform to the shape of the subfloor. Your best bet would be to use jacks (adjustable lollicolumns) to straighten out the joists (I'm assuming here you don't have a finished basement! :-) ) The nice thing about using the jacks to straighten out the joisting is it would also give you the added support you'd need for a stone floor at the same time. :-) If you don't want to go that route, there's always self leveling compound OVER the cement board or membrane.

Any area of subflooring that shows water damage and or is soft, will need to be replaced. Locate your joist first and make the cuts so the new section hits on the center of a floor joist. You will also need perimeter blocking for the part between each joist.

You have lots to get lined up before you get to install your bamboo. The subfloor moisture rating MUST be taken once it has been installed. The moisture rating for the wooden subfloor (the new layer) must be LESS than 12%. But more importantly it must be within 3% of the bamboo. That sounds logical until you realize that wood moisture meters have a hard time dealing with strand bamboo. They must have a special setting. Please make sure your install has access to this type of meter....or you could be voiding your CaliBamboo warranty before you have laid a single plank. The other thing CaliBamboo is SUPER SERIOUS with: Vapour Retardant over wooden substrates. Here's what the installation instructions say about that: https://www.calibamboo.com/content/installation/instructions/cali_bamboo_flooring_installation_guide.pdf "Wood Subfloors: For best results, use a vapor retarder with a perm rating between 0.7 and 50 when tested in accordance with ASTM E-96 Method A. Install as recommended by the manufacturer. Examples of acceptable vapor retarders for wood subfloors include: Asphalt laminated paper (UU-B-790a, Grade B, Type I, Style 1a.), Asphalt-saturated kraft paper, and Silicon Vapor Shield." This requirement is very different from many other floating flooring projects. Most floating floors just need to click the product together and move on. CaliBamboo (click together bamboo is a WHOLE DIFFERENT UNIVERSE) is not the same as installing laminate or vinyl. Remember: Bamboo can be as much as 30% ADHESIVE BINDER. The binders react to moisture DIFFERENTLY then the bamboo strands themselves. That's why you must have TIGHT humidity control. I know CaliBamboo says 30% - 60% RH to be the living condition requirements BUT (and this is a B I G BUT) your day-to-day conditions MUST NOT swing more than +/- 5% in either direction. A +/- 15% swing (as shown above) is FAR TOO MUCH. You cannot have 45% one day, then 60% the next (+15%) and then 30% the next (-15%). You will have massive problems inside of a year. In other words, if your floors were installed/acclimated to 35% humidity (which is low for bamboo) the swing cannot be more than 30% - 40% RH (relative humidity). Please ensure your new HVAC has the ability to add AND subtract humidity at the touch of a button. Whew! Bamboo is tricky!

Thank you, Joseph Corlett. The floor has already been raised, shimmed, and levelled. That was the first thing she had done. She was told at that time that the original floor doesn't need to be replaced; it's in reasonably decent structural shape, but that if she wants to put in vinyl tile she would be advised to lay a 2nd layer of plywood on top of the original in order to form a smoother and more stable surface because of the old rough top layer on the original (which has sat for several years with no other flooring laid on top of it) and to "anchor" the original subfloor more tightly. This is a cabin in the woods...not top flight suburban architecture, but it will be her home...not simply a weekend getaway, and thus, it needs to be a long lasting foundation and subfloor.