We are planning a LVP project (our first attempt) that will be going atop an old knotty wide plank (6") floor that is most likely original to the circa 1880 building the apartment is located in. Obvious settling has occurred, with on large area that is depressed a good 1/2" or more.

From our first looks at installation guide videos, getting as solid a foundation as possible seems key to making this a project a success. The existing flooring has many gaps in between floorboards, with some that have been filled with various things over the years that are still holding, lots of other areas are loose or open. Our plan was to fill every opening, and use some subflooring for the recessed dip, so we can use a leveler to get a level foundation to begin with.

Here is what the floor looks like for reference.

Should we use a crack filler for the gaps and cracks (maybe with rope in the bigger ones as a base) to prep for putting down the self-leveling mix next? Seal it before self-leveling? New to this, so open to hearing all advice and suggestions on the project. We will probably have more questions as we proceed, but wanted to start with wrapping our heads around step one.

 

Your cracks have to be filled or sealed to prevent the self leveler from simply pouring out through the crack. It's not critical how you do it. If the cracks are not open at the bottom then they don't need to be filled or sealed. Caulk is easy to use on smaller gaps while a foam backer (rope) can be pushed into the gap. I've even seen cracks get taped over.

 

I have not done any LVP installation but from what I have read in this forum the main concerns are having a level and smooth surface. The former to prevent problems at the joints and the latter to prevent any irregularities from appearing at the surface. In your case I would expect to need an underpayment (plywood, Masonite, etc.) If there is a dip in one area you might have to shim or use leveling compound.

Wait for SamFloor to weigh in.

 

Now that your picture has loaded I don't really see anything that needs to be filled, especially since you said you were going over with a layer of subflooring. You might need to do leveling so closing the gaps will only be needed for the self leveler, but I can't see the gaps affecting the finished flooring.