First of all, you still haven't specified whether your floor is "not flat" or "not level." Whatever else you have (or have not) done to get where you are, it would help a lot if you could say whether your problem is level or flat. Also, just to be sure here, if I am understanding you correctly, you installed the 2x4s ACROSS the 2x6s that make up the underside of the house? (Notice the very very NON-technical terms...so be aware that I'm not a builder or knowledgeable in this stuff like some of these people that post here...just sympathetic to rising note of panic in your posts....) The only reason I'm trying to clarify that is because I am wondering whether you installed the 2x4s "upright" (so the "4" dimension is vertical) or "laying down". I'm not trying to pile on to what you have *already* done, but I don't know what the proper distance for SPANNING that 2x4s have, and I don't know whether they can span two full feet if they are flat. This makes a difference because you want to install tile. If your subfloor is bouncy or bends ("deflects" I think is it), then all the tile you install will just crack and plague you and your heirs until the end of time...until you rip the whole floor and subfloor out again and re-do it. Or put in linoleum. Whichever comes first. You say in the top post that the cracks are like 1/4", but in your last post you write that they are up to 1". That's a big difference? Which is it? Does it vary from 1/4"-1"? Have you actually measure or are just eyeballing or going from memory? IF YOUR CRACKS ARE VERY SMALL: IF it were me (and it isn't), I'd probably just put down a very tight layer of Wedi board (thinset and screws/washers), seal the seams so it can hold the next layer, then put down a thin layer of self-leveling compound (though you still haven't said whether your floor is wavy or slanted), and then tile. IF YOUR CRACKS ARE 3/4'-1": Maybe use a layer of plywood (which would be cheaper but harder to cut), maybe tape and thinset the seams (ask someone on this board about this because I'm just brainstorming here), then use the SLC, then the tile. Only plywood will actually help form a floor (which I think you need to do with 1" gaps), not backerboard of any kind. Instead of Wedi, you could use probably any backer board (ask someone here what to do for the seams). But I only know about Wedi because that is what we went with, and I like it because it is light weight (good if you are a female working alone). I don't think your solution is in one step. It feels like you are going to have to create a tight subfloor, THEN level or flatten. I think you have a non-standard thing going on there, and that might not be addressed so easily with just one step before tile. One more thing, just out of curiousity, WHY do you think your floor must be level? Oh yeah, and I would start making your peace with a threshold. Given how non-standard your set-up is, AND the fact that this is DIY (or at least the subfloor), I'm not sure it is realistic to think you can get away with no threshold.

Okay. If you're just trying to level out the floor versus strengthen it, all you would need is something like 2x6's, but you'd want them to span wall to wall, so that the subfloor would be fully supported. It will be fully supported so long as ONE of the two-- the original joist, or the sistered joist, hit the bottom of the subfloor when laid down flat (the operative word there being FLAT). As for order of materials, If you're just doing a normal ceramic floor, you want a layer of 3/4" exterior grade plywood, then either 1/4" cement board or 1/8" Ditra, and then your tile. NO tar paper, unless you're going to do a mudset. Figure on 3/8" for the thickness of tile and thinset, and if you need any height past what these layers would give you, then my recommendation would be to do it in plywood for anything over 1/4", and for under, switch up with your underlayment by using 1/2" cement board, or something along those lines.

Keep existing structure, complete gut the space, new footer(s) inside existing structure, load bearing structure above those, floor joists above that, subfloor and refinishing. Have engineered plans/permits before starting. Basically an insulated and interior crawlspace below the new floor when done. Have access to down there, you could use it for storage also. Not impossible. Plenty of work but not impossible. Sounds like it would need a big reno anyhow from description. Just more work than a standard DIY gut and redo due to structure work involved. This is something a GC and crew normally does. This does give you a chance to make it how you want it. Hopefully selling price reflects the need for a large reno. what unexpected or nasty surprises did you encounter? I have found a lot of things in my current house where I ask myself, what were they thinking? So you will probably find a fair amount of stupid behind the walls. When I redid the drain system, there was an old 1-1/2" drain line from the kitchen that had so much gunk built up there was maybe 1/2" left for actual draining. Also the electrical was mix of grounded and ungrounded wire, and tons of junction boxes in the basement, I redid all that mess. Found a lot of the wire nuts where the wires were loose or arcing was happening and it welded the wires and nuts together, at least they used junction boxes or the house probably would have burned down... In laundry area, an ungrounded range receptacle was used instead of a grounded dryer receptacle even though the wire they used did have a ground. At the panel the fuse was rated at 30A so it was safe, just annoying I had to get a different cord to plug it in when I moved in. I eventually changed out the outlet for future owners... Oh yea that old fuse box the main fuse connection started arcing! Talk about fire hazard! So that was replaced with a breaker panel. Also in the laundry room the washer standpipe was a 2 inch that went into that same 1-1/2" from the kitchen with a vent tee instead of a sanitary tee... (so much fail). It now has its own vent and drain to the soil stack. Higher radon level than wanted, had to add active mitigation. Kinked gas line multiple places... Bath vent in attic went to above a soffit grate so 'some' moisture went outside but some got in attic. Not enough attic ventilation. Lots of other stuff, but cant remember it all lol.

I have been in my current house for about 3 years. In that time I have gotten the weeds under control and have some pretty thick turf to work with. The problem is that there are at least 3 types of grasses and textures. They all green at different times and Temps. And my yard is extremely bumpy. It feels like you are walking on golf balls. This year I have decided to do a leveling project and hopefully to reseed on the leveling mix. My question is, if I were to add 1-3" of topsoil/leveling mix to the yard(depth will depend on if it's a low or high spot) would the new seed grow on top of, and overtake the grass underneath it? Or would I need to kill and scrape up the grass underneath before the leveling and reseeding? I plan on using bluegrass on the reseed.