By now, you have picked up on "dont do it"... So looking at the brightside- Carefully tearing out your nice wood floor (from above) you can reuse it or keep it for another project(s). Tearing it all out gives you options you wouldnt have had before. You can adjust your sink or toilet locations, move the bath if you like... Change the size of that closet... Get you some more room in the bath if needed.Or just change the floorplan. PLUS you can then finish it off with a desireable tile floor. Proper installation is very easy DIY project just get a video at a home center and take a look. You will see it isnt that bad..Good luck & hang in there.. Two things I have learned about home repairs is 1)Do not remove protective guards off saws (dont ask)& 2)A cat turd dipped on chocolate- is still a cat turd..

I'd install the thicker layer first. Screwing the second layer to the joists would be nice but it's not necessary. Fastening it to the first layer should suffice.

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.