Hey everyone

I have a home that was build in 1980 with some floors that are badly in need of some leveling before install. Appears there are a few different problems and I have a few outstanding questions I'm not finding answers to..so hoping for some help...

The install is on second story with 3/4 plywood and 16" OC joists. The plywood was lifting up quite a bit. After screwing it the floor is rock solid(no movement).

The joists aren't terrifically flat across most rooms. I have one room where every other joist is low. Generally, this looks like about 1/4"-3/8" across a span of two joists. I've done measurements across longer spans and generally get about the same variance. Overall, no single spot looks impossible but there is a lot of leveling. I'm partial to nail-down installations as I have experience with it and tend to be a slow worker(leveling compound/glue dries while you nitpick). Is leveling compound going to be easier to level something like this or is it just a matter of choice?

Assuming nail-down, I have read that shingles, plywood and tar paper are the preferred fill to level this. One thing i didn't understand was how much unsupported gap there can be between your fill and your subfloor. I've tried to draw this out below. The black X represent a cross section of the subfloor with a depression. The F represents fill like 1/4" plywood. Unless the fill was feathered out with great detail there will be some gap between the XX and the FF on the sides where hardwood above would not be well supported. Any guidance on how best to go about this and how much margin?

XXXXXXXX FFFFFFFFF XXXXXXXXXXXXXX
XXXXXXXXXX FFFFFFFFF XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Separate question...I had two installers quote my house and suggest using some Ardex compound with Latex(didnt specify which product) and then using nails. I have read repeatedly here that leveling compound and nails don't mix. Is there some product that I missed? Is this common practice even though the manufacturers of the leveling compounds don't seem to recommend/support it?

Thank you

I'm not a big fan of leveling compounds, under a nail down installation. IMO your just asking for problems, with fill popping, breaking up, etc.

Not sure how to translate your "XXFFXX" deal....But I would go with the shingles, plywood and tar paper as my preferred fill to level. As far as "unsupported gap there can be between your fill and your subfloor".....You really want as minimal as possible, "unsupported gap". It's just a matter of taking pains using the materials mentioned to gradually "step" to flatness.