Flattening a subfloor

[Broked house]

The house is about 50 years old. One of the structural columns in the basement had lowered 3/4” and I was able to raise that up and shim out the sag with steel plates.

The floor is only 1/2” plywood and I have purchased some 3/4” sturdi-floor tongue and groove to put over that before installing the wood floor. Some of the joists must have been set with the crown in the wrong direction as the floor sags along the entire length of a few joists next to each other.

When I put a 9’ aluminum straightedge over it I have a dip of about 1/4” in most spots in the middle. In the past I solved small sections by layering roofing felt, but I am looking for an easier or better idea. Getting to the joists below would require removal of ceiling and working on top of a pool table (not ideal). I prefer not to sister the joists.

These are the three options I am considering and wondering which I should choose? To be clear options 1 & 2 would be sandwiched between the subfloor and underlayment. Option three would be from the basement.
1. Layer felt paper over a 15 foot span between 5 joists.
2. Find some kind of leveling compound or thinset that does not mind being screwed through or nailed through. Also would not cause squeaking.
3. Remove all the nails from the subfloor and shim the joists from the basement with 1/8” and 1/4” plywood.
4. Something easier other that is not sistering.

What have you done that works or would work? Thanks

 

[Deleted member 174758]

I'd skin it with plywood (not necessarily 18mm, 12mm glued and screwed on 150mm centres should do the trick and will give you a 24~25mm thick sub floor) then use an SLC on top it to level it all up. Make sure that the screw heads are all sunk under (requires a fast impact driver or a drywall driver). Whatever you want as a finished floor goes on top of that