i am installing engineered hardwood floor over plywood subfloor. and was making great progress until I hit a room with significant waves in the subfloor in one room.

I called in contractors, and they have done a number of things (some of them poorly) to fix it.
1. they added some jacks under some of the joists, which raised the largest low spot.
2. a smaller area had a low spot about 8'x2'x 3/16" deep. First they tried to raise this area by cutting out the plywood, shimming up the joists, and laying new plywood, but they got the level wrong and didn't fix the issue. The last solution was to use self-leveling concrete in another smaller area. They said i would simply have to use glue in that end of the flooring (I am using staples everywhere else). Unfortunately, the "self-leveling" part of the concrete did not actually work the first time, so they are re-did it and didn't getting it right the second time either. My guess is that not enough water in the mix created a long 1/2" high crest in the middle of what had been the low area.

First, is using lightweight self-leveling concrete like this considered acceptable?

Second, 3 days later, with little foot traffic in this area, I am seeing at least one hairline crack in the concrete, and I am concerned that the concrete may end up cracking as the subfloor flexes, and developing a crunching sound under the felt paper even with the flooring glued to the concrete. Are my concerns valid?

I want to make sure this is done right, before I add any more of my work to the mix, and if the contractor is doing anything that is risky, I'd love to know before I go any further.

If the directions for application are followed the patch should be OK. Better read them. They are printed on the bag. Sometimes the self-levelling part doesn't go perfectly. I use a flooring edger with 16 grit sandpaper after the patch has hardened some, usually first thing the next day, to sand the high spots after checking with a straight-edge.
I use the edger with 16 grit paper to sand the high spots down, setting nails as necessary where they start to show after sanding some of the sub-floor down. This should not affect the strength of the sub-floor/ finish floor results. If the self-levelling latex/concrete layer is not too thick, I staple right through it. The staples should penetrate ¾ inch into the sub-floor.

thanks for the response!

i am guessing the compound could be sanded to reasonable flatness, but the contractors did not prime the subfloor floor or use metal lath mesh as a substrate as indicated in the instructions. those lapses make me worry about installing my flooring over the top of it.

The primer is essential for correct use of SLC, boy what dummies. The lath is more for deep pours. If the low spot is just 3/16 simply build it up with 30lb roofing felt from furthest point inward. Problem solved.

That's the sad thing...they have had sooo many chances to get it right.

If they had been able to shim up the subfloor from the joists the first time, it would have been fine. Their failure to do that led them to the "easy" solution of SLC. Their 2x failure on the result and method of SLC means (i think) they need to tear out the whole area of subfloor and recover it with plywood so it is flat.

I've used SLC over plywood but only for glue down installs, so I can't comment on how stapling through the patch will work out (my guess is that its not ideal)

I can add that the SLC should be runny, a bit thinner than pancake batter. You should not have to trowel or smooth it at all. Pour and let it settle, then check with a straightedge right away, and add more as needed. A lot of guys will wait til its dry to check and because it will shrink as it dries, a second application is usually needed. If you check while you do it, you can save yourself the trip back out to fix it.

I don't understand the use of SLC under a fastened floor considering it has a compressive strength similar to or higher than that of traditional concrete. Which could easily cause deflection of the fastener.
I also would be afraid of it busting up under the wood flooring.

I had not considered using SLC as a solution, but the contractors said I could just glue down the flooring on that area, and staple on the rest of it as i had been doing.

the contractor is bringing another contractor (different company) over tomorrow to evaluate the work. I can't see another solution to this mess other than tearing out the subfloor in this section and replacing it at level with the rest of the floor, since they applied the SLC without priming the subfloor first.

No...... you cannot glue one area then naildown the next!!!!

Different installation methods allow the wood to move at different rates, it will be a disaster.

good to know!

the contractors came back with a guy from another company to get his opinion. he said he would have sistered in another joist to raise up the floor level (since the joists span too long), and would not have used SLC as they did.

the original contractors said they were not interested in doing it according to the other guy's recommendation, and just threw in the towel. Luckily I have not paid them for this last work, so now I am in the joist jacking business myself.

i'll be pulling up the subfloor they laid, supporting the joists with additional jack points, and seeing if I can raise them up to get the floor flat, and if not, shimming on top of the joists to get the new subfloor flat.

I'd go with the guy who recommends sistering the joist.

that is plan B.
when i asked him to do it, he hemmed and hawed about how busy he was with bigger jobs (probably partly because the original contractor was standing there).

at this point i am tired of waiting around and being disappointed in the work of others, and i'd rather be disappointed in my own work

Sistering is easy. Go for it.

thanks! from my research, it seems pretty straightforward:
1. tear up existing poorly installed plywood subfloor sections
2. glue and clinch nail correct thickness plywood into chord faces of 2 i joists according to mfg specs
3. sister 2x12 joists onto faces of i joist at level required to set correct height for subfloor
4. cut new plywood subfloor to fit
5. check level/height of new subfloor section
6. glue, screw subfloor in place
7. profit!

thanks to all the assistance and guidance here, i'd like to report on my success.

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I cut out the subfloor they had added and realized instead of scabbing 2xSomething onto each of the joists at a higher level, the contractors had just scabbed a single 2x6 onto the one i-joist closest to the wall, and didn't fasten it well to the i-joist. The result was that when they screwed down the subfloor, the 2x6 was able to pivot away from the i-joist. This meant they did nothing to change the height of that section of the subfloor.

I researched the best method to brace the existing i-joists and ended up adding reinforcement to both sides of each of the i-joists (according to the mfg specs/method) running 1" plywood inside the chord of the i-joists from their start at outside foundation wall through to a point under the far end of the closet.

I then added blocking across the two joists where they ran under the closet (face hangers on the joists, and short sections of i-joist between), then installed some shims between the top of the blocking and the bottom of the plywood under the closet edge to straighten it out a bit. I thought about using jacks, but with the joists reinforced, they should be able to handle any additional load.

I got a few different thicknesses of plywood, and added them to the tops of the i-joists as needed to get a flat surface, glued and nailed them in place, then installed new subfloor.

Filled in the cracks, planed/sanded down a few small high spots, added felt paper over the top, and have installed 1/2 of the hardwood floor in the room over the last few days.

Total cost of supplies = $300
Total time = 12 hours
Total savings over contractors = $1500

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huge thanks to all who helped point me in the right direction!