I have a subfloor in a third floor complex that is in various degrees of being unlevel. I don’t think the cement type floor leveler is suitable for this, since the old sub floor is fifty-year-old planking with wide gaps. What is the trick to building a shimmed floor on top? How do I assure that it’s level in all directions?
You are correct in assuming that a poured cement leveler is the worst choice in this case, they simply crack apart as you nail through them. And over time they will degrade. Shims are only meant for small 1/4″ localized dips in the floor, not much bigger than a foot across. But you can shim along a whole wall this way. I use 1/4″ plywood glued and stapled to the plywood. Then I use my floor edger and sand down the edges flush with the old subfloor with 16 grit sandpaper. Let the glue dry overnight and then lay the floor. Cedar shims are no good, as they crack when nailed.
Now that said, when you have a major humpy floor, you should bite the bullet and remove the subfloor planks down to the joists. Then sister the joists will LVL joists at exactly the level you want. Lay a new 3/4″ plywood subfloor, and Bob’s your uncle. You will have a level sound surface to lay your new floor on.
In some cases we will sink the subfloor between the joists on plywood ledges and on the other end of the room the sistered joist can raise it up to the desired level. This splits the raising over the entire area and minimizes any extreme ramp up. Yes, expensive, but a permanent and proven method. I cannot give you the whole method in detail by email, but if your carpentry skills are not honed, you really should hire a pro for this anyway.