Yes i had this problem, subfloor was serious unlevel.
First i would take out anything rotten or looks bad poor structure. Then use lumber 2x10 x 12' you can shim it up by cutting pieces of lumber or using small bits of ply wood. You should get a laser level. Make sure the lumber you buy is very straight of course. You could measure from the ceiling, assuming that's level.
First i would take out anything rotten or looks bad poor structure. Then use lumber 2x10 x 12' you can shim it up by cutting pieces of lumber or using small bits of ply wood. You should get a laser level. Make sure the lumber you buy is very straight of course. You could measure from the ceiling, assuming that's level.