I recently noticed a low spot or sag where my kitchen, hallway and dining room meet. I pulled up the carpet to find what appears to be 1/2 inch toung and groove OSB (the actual floor) that was not installed correctly and has separated at the joints causing the sag and squeaks. Unfotrunately it runns under all interior walls so replacing it seems out of the question. I am looking to install vinyl plank in place of carpet and and am looking for suggestions what to use as underlaymen. I was thinking of 1/2 to 3/4 inch plywood directly over the OSB, to stiffen up the floor and then using a felt or foam vapor barrier. This is an interlocking, floating floor and we plan to stay here a while. I unserstand that this will raise the floor and all doors will need to be cut to clear but I plan to replace all the trim and door jambs in the process. I have laid carpet and 1/2 inch hardwood but never vinyl so any help or advise would be greatly appreciated. Thanks in advance!
That I am aware of T&G is not available in 1/2". 5/8" is thinest, and if carpet is the choice of finish, it should be 3/4". But it doesn't mean it will be.
You will need to set the screws or any nails that are above the floor, and check if the subfloor is flat enough for the manufacturers requirements for their floor. Usually it is something like no more than 1/8" deflection in a 4' span with a straightedge. If that is all in order, I would add just a layer of 1/4" underlayment, not plywood, although underlayment I typically use is plywood, difference is it is rated as an underlayment. Underlayment does not have the voids in the laminations of the plywood so if a table leg or heel presses over a void, it will not poke a hole in your finish floor.
If the floor you have now is squeaky, now is the time to resecure it with 2" screws. Typical nailing/fastening for underlayment is 4" on all edges, and 6" in both directions everywhere else. I like using a pneumatic narrow gauge stapler for this, it makes fastening go real fast.
The proper prep wasnt done, all flooring requires the substrate to meet deflection tolerances. 99% of the installs i do require floors to be ground flat. I see this in osb subfloors the most and its always at the seems. Fortunately there also the easiest to fix, belt sander, electric planers, scrapers all work to remedy that. I run my hardwood sanders over them because they hook up to my vacuums. But all those methods work. Deflection is one of the biggest reasons for laminate failures, 80% of the flooring inspections i do fail for that reason, followed by lack of acclamation. Id call the installers back and have it fixed. Call them back and have it fixed before it causes more damage.