Sub floor prep for laminate

We are finishing up a kitchen remodel and have everything done but the laminate floor. When I look at the floor it is nowhere near the flatness spec that the manufacturer recommends. It doesn't satisfy me either. There are a couple spots that are a good 1/4" lower than other areas of the floor.

The sub floor lumber all appears to be in good condition. I can see the entire joist system from the basement. No cracks. When my wife walks across it I don't see any real noticeable sway or sag in any particular joist. the plywood is also solid. No apparent deflection between joists.

It really looks like there are a couple joists that are just at a different level than the others. Maybe slightly bowed or crowned.

What's the best way to level this floor so that I can get it real close to flat?

The liquid levels I've seen are pretty "liquid" like and I'm afraid it will run through the gaps between the plywood.

thanks for the help.

You may want to put a caulk in the joints before using a liquid leveler. Some can be feathered out to zero, some need to flood and cover the entire area to a minimum depth, so read the specs carefully. Some require a special primer to be installed first to ensure a good bond. A self-leveler over a wooden subfloor may require installation of metal reinforcing lath, so maximum thickness can be an issue.

You need to use a long straightedge and check how deep the depressions are. Then, you need to decide if it is in general level, or slopes from one side to the other. Then you can decide if you just want flat, or want to achieve both flat and level.

If you come back with some numbers, it's easier to suggest a fix.

alternative to SLC

I had a similar situation, trying to flatten an existing floor. I used strips of roofing felt to even out low spots. It doesn't compress, and was easier for me to work with than SLC

That is an acceptable method that was actually discussed in the installation instructions of some engineered wood flooring I installed. A 1/4" could be done, but is quite a few layers, though. It would looks sort of like a bunch of slices as seen on a topographic map...getting it right could be time consuming. SLC is one possibilty, but over wood, it would require more thickness than you probably want to afford (it's not cheap!). There are some other things that can be applied in thinner layers only where needed. SLC is not very forgiving plus it sets very fast so you can't fix your mistakes easily either. Works really well, but I don't think this is the rigth application.

Not familiar with that product. If the floor is within the flatness specs, it should work over their decoupling layer which is usually some sort of foam or plastic. Many of the laminates don't like moisture coming up from below, so it acts both as a decoupling layer, insulation, and many also have a little bit of give to them to make walking or standing on it more comfortable.