1) You can brace or block the joists per standard construction, but what might be easier is to strap the bottom edges of the joists. Typical strapping is done with a 1x3, but a lot of times I'll simply use a 2x4. Run a 2x4 across the bottoms of the joists, perpendicular to the joists. Add a dab of PL adhesive and use a couple of screws at each fastening point. You have specific needs here, so spacing the 2x4s as you need will work very well. What it does is if a single joist wants to deflect downward, the strapping transfers that load to the adjacent joists as well. So it's good as a floor stiffener. For a washer/dryer set up, it also dampens the entire floor platform in terms of the possibility of the spin cycle causing the floor to reverberate. The strapping serves the same purpose as cross-bracing or solid blocking between the joists. It's code-approved, though code only requires it for 2x12 joists, with the strapping being made of 1x3, and the strapping spaced every 8'. The strapping is more effective long-term than bracing or blocking, due to fitment, installation, and shrinkage issues. 2) Do the whole floor. There are only restrictions on maximum spacing (every 8'). So if you make it every 2', or 3', or 4', no worries. Strapping is also easier because you're not wrestling with any of the utilities that take up space between the joists. 3) Your joists span 8-1/2'. For simplicity, why not do run three pieces of strapping? One at 1/4 span, one at half-span, the other at 3/4 span. Just a little over 2' spacing. 4) 3/4" ply over the diagonal boards is plenty. It does not have to be exterior grade. 5) Your floor sandwich can be made up of: -Tile -Ditra -Floor heat wires encased in SLC (no wires under the washer/dryer area through, see comments later) -Plywood underlayment covered with SLC primer -Diagonal plank subfloor -joists I usually advocate a small platform under the washer/dryer stack. It's more for damping than load spreading. I've done sandwiches of plywood/cement_board/plywood, or plywood/MLV/plywood. MLV is mass-loaded vinyl. Screw through the top layer of ply, through the middle material, and into the bottom layer of plywood to secure it together. Or just use construction adhesive. I drill a hole through the middle of the platform sandwich to accommodate the flood pan's drain line. I usually put a square of rug pad on the floor. The inexpensive, maybe 1/8" thick, rubbery mesh or open weave type of rug pad. It acts as a bit of a damper, but also holds the platform in place. I then set the platform on the rug pad. Then put the flood pan over the platform with the drain plumbed through the hole in the platform. Then set the washer in place and the dryer on top of that. I recommend you tile the entire floor, but again, no heating wires under the location of the washer. You'll be putting the sandwich platform on top of the tile in that area, and the insulative value of that platform can shorten the life span of the heating element. Pricklypear, the Ditra-XL is simply thicker than regular Ditra. It really wasn't designed for a specific tile size. It's sole purpose is to allow the thickness of "Ditra plus tile" to match up with the thickness of a typical hardwood floor.

Rigid sheathing over the patch should do just fine. The height difference in the two floors isn't something you can help. If you feel that it is worth the effort and the price, you can ask him to stop and REMOVE the top layer of subfloor. Then ask him to REPLACE IT with NEW plywood. This will NOT change the roller-coaster waviness of the surface. That is inherent in the house. It will simply give you smooth "new" while keeping the two floor heights as close to even as possible. If the height variation upsets you, then you have two options: choose sheet material like sheet vinyl or sheet linoleum or rip out and rebuild the entire kitchen floor and *possibly the entire main level. My suspicion is the KITCHEN floor was raised to meet/match the other flooring. If that is the case, it is possible to remove the current plywood and then drop in slightly THINNER plywood. Again....this all hangs on the question as to whether or not the kitchen subfloor was raised to meet the other flooring. You are not allowed to go "too thin" with a subfloor. Most industry standards require (depending on where you live and how old the house is) TWO layers of subfloor that equal or exceed 3/4" total thickness. Sigh. It might be possible to drop the kitchen subfloor a snick. But the amount of money and effort it will take could be a deal breaker. It should have/would have been talked about BEFORE he started patching. And I mean WAAAAAAAY before he started patching. Like several WEEKS before he stepped on site. This would have been discussed because you would have CHOSEN your floor and discovered the difference and would have raised the possibility at the time. It's not too late...but then again you don't want to see his face after you tell him to rip out his work and start again...with new plywood, etc.

We had the same exact situation. We hired a flooring contractor but did the prep ourselves to save money and he advised us to go through the entire floor and put in two screws on each side of the board about every 16” or so. It’s back breaking work and you need ear plugs, but it prevents squeaking later. The nails allow too much give. You don’t need to worry about the gaps. I’m not sure about your vapor barrier. We were over a basement and didn’t need that but did have to insulate the ceiling in the basement to prevent moist air coming up. Here’s a photo that shows our subfloor.

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!