Thanks to those that responded to underlayment question. Confirmation is always good. Now, next situation. This is a pier/beam, 12” floor joist 16” center. I have a floor lift of about 3/16” in the floor sheeting in just one area, visible and can feel the rise when walking on it. It is like a hump. Under the house, they doubled the joist on 2 primary joist for added support for entry on one side. Why I don’t know. (That is another support problem for another day). Both secondary joist end about 18” past the center lateral support beam ”. These are not tie to the primary joist. The short ends have bowed up, lifting the floor. There is a noticeable gap between the primary joist and the sub floor. That confirms the problem. I was thinking I can bolt or nail the joist together to eliminate any future lift. Then cut the short ends back past the bow or lift. I still will have about 6”s extending past the center support beam. With a lot of perseverance and patients I can use a recip saw to cut the ends partially through and chisel what is left.
Any do’s or don’t are welcomed or other suggestions. Thanks.