If you don't address the sagging joints, anything else you do will just be another bandaid.

The engineer has the right idea, if the joists are sagging, the sag may get worse.

I have an old house with uneven floors due to sagging joists and have talked through a few options, but eventually just put on flooring that made the uneven spots less obvious. Getting shims in all the right places and at the right heights is gonna be really difficult if not impossible. It could end up worse or more likely just squishy feeling which will cause problems with whatever flooring you use. If a structural engineer said nothing is dangerous that's good. Leveling compound is an option, but can also be tricky to do right and make sure you understand how much it weighs before you pour it and some versions are made to weigh less. I kinda came to the conclusion that the only way I would want to fix this is pulling everything out and sistering the joists which ultimately wasn't worth it for me, but it sounds like you're down for a lot of work so I would just go all the way. I also did one very small section only on my house (a bathroom) with leveling compound and it turned out good. That was a cracking tile floor though that needed to get redone and I wanted a nice level base for that and the tile can get laid right onto the leveling compound so it made sense. If you do go with leveling compound, seal everything up and add the maximum water recommended to thin it out and mix it with a strong drill with a handle on the side to be confident there's no clumps in it and it'll spread smoothly and even then you'll need a rake or squeegee to spread it out. I also put metal mesh stapled into the subfloor before pouring out to really reinforce it Incase there's anymore movement to prevent the tile from cracking.

Need to make structural fix first. Otherwise leveling it is pointless - it will be out of level again quickly. Add the beams necessary to support, wait, and see what happens after it re-settles (6-12 months, at least).

Otherwise, again, you'll just be re-leveling it later.

How thick is self-leveler? unless it is thin enough that it can be easily scraped, that's gonna be a pain in the ass. Most SLC is very fine aggregate, and doesn't come out in chunks. It will also be insanely dusty I would rent a demo hammer, zipwall off the area, and use a lot of vacuum collection + full face respirator (not just half-mask).

You need an engineer to prescribe a structuring plan for the undersized joists. Why would you think adding more weight without reinforcing first would work.

Sounds like an expensive project! I’ve attempted several floor leveling projects in our 110 year old clapboard house with its floors being angled this way and that due to joist sagging etc., and have had the most success combining a couple of approaches. First I would definitely remove the new floor (d’oh!). Then cut out the subfloor in the worst affected areas, i.e. the most saggy. Sister in new joists with lots of glue and screws but make the new joists as level as possible, that way you can avoid fiddling too much with shims, etc., although getting the new subfloor even with the old does take some finagling. There will almost certainly be areas where sistering is not possible due to other infrastructure in the way, so it’s up to you how involved you want to get with that (moving electric, plumbing, ducts, etc.). Sometimes you can sandwich plywood on either side of an existing joist to strengthen it and that takes up less space and works pretty well too. The main idea here is to add strength to the joists and also level them a little bit.

Once that’s done, and the new subfloor has been added, you can use leveling compound to level the floor out the rest of the way, and you won’t need to worry as much about the added weight from the leveling compound. I’ve only done this in areas where the floor was going to be tile or carpet although I believe this will also work with engineered wood flooring (but not regular hardwood).

Good luck! This is a big disruptive job that will take a while, just be careful and don’t fall through the floor.

Install temporary relief in the garage (if possible) and wait a few weeks to see how your house reacts. Then proceed accordingly.

You should hire a contractor to fix the sagging joists with a beam. Ripping up subfloor is a PITA and just sistering joists won't fix the problem if the joists are resting on something that's sagging. You don't need a hammer drill to remove the flooring or self leveler, you should.just borrow or rent a burke bar. Even glued down stuff will come up with some elbow grease. Put down your own flooring if you want to save $$.

We had a not totally dissimilar problem with a room in our house.

The contractor we hired sistered and nogged the existing joists, so they created a new floor level (slightly proud of the old joists) that was flat and level.

This video is similar to what we had done. https://youtu.be/5-Rvv06_A0k

Because every joist was doubled up, the floor was much stronger and then a new subfloor and glue down hardwood added. Because you have a long span, this may not be sufficient to address the sag.

yeah without pictures its hard to say, but you've pretty much have done everything wrong. and Because you tried leveling sagging areas, things will only get worse.

joists have maximum spans depending on species, grade, thickness and width. they also different allowable levels of sag, and periods of time where excessive loads can cause deflection/sag.

also, without blocking, joists/ beams will twist. because a joist/beam is in compression AND tension on the outer edges of the horizontal member, those members get their strength from the height [up and down] and stability from the width/ flange. a joist essentially wants to twist when put under load, to the point its flat, especially in the center. the flatter it gets obviously the weaker the member is. this is exceptionally troublesome in big beams. as you lose upright positioning you sag more as you lose "strength".

wooden joists have no flange, they're usually just 1.5" to 2" depending on age. to fight this twisting, you need lateral support. members acting on members. this is where blocking comes into play. you've seen two pieces Xed between joists, or solid, and sometimes two 1x or 2x boards at top and bottom. if you don't have hangers at joist ends, blocking will significantly help whether it's a wood joist or steel beam. usually blocking is only required on something like 16?ft spans or so? for 2x12s or larger? but what blocking does is help distribute the load, add stiffness by preventing deflection which decreases sag.

this next part depends on span. so what you can do is jack up the floor and sister joist members with same size or LARGER members if you can take the height difference. every 2 inches you add to board height essentially doubles the load they can hold, every board you sister is slightly less. wood quantity for wood quantity, moving to the next 2x size is significantly more efficient/ cheaper and stronger with less deflection. you can notch the ends of the joists to fit a bigger member. while not ideal, it will still greatly benefit. you can also add end blocking on top of that and run joist hangers to support the bottom addition.

you can also add a beam, steel, wood, engineered lumber, again depends on load/span. you'll still likely be sistering the existing joists as they'll likely be permanently deformed. the beam can usually be mounted flush, and joists hangered, but you can also run beam beneath. there's some decent math to do here and no one here has enough information and really shouldn't Because of safety and insurance.

your floor may be within spec and the sag is okay, without pics, or measurements etc its hard to say.

but yeah you made your job a bit harder as once you jack up your floor, your previous adjustments will be very proud. and you can shave the floor if it's a wood product, you likely only have a 1/4 or so before you get down to the tongue. I forget what floor you said you're trying to save. it might look like shit when this is all said and done and you'll be doing more work and money in the end.

if you try something like shimming the floor from underneath, trying to fasten underneath. lol. hope I get to see that thread too. the half assed leveler is likely going to be the problem and was improper assessment. depending how it was discussed, you hired a flooring guy to install floor. it's like going to a boob surgeon and complaining you walked out with big boobies. they work with a lot of out if Level garbage and a lot of people don't do the work to level because they don't care and are likely going to flip it. next time try to assess the issues properly and come up with a plan. you can start now. at least you know who to point your fingers at when you complain about pulling certain things out [yourself]

you may find you had beams or supports and someone removed them. this is the case with a lot of houses because of open concept flipping. you can stay open concept but you have to do it right

Its best to fix the problem causing the issues, not create a bandaid that adds extra weight to the problem