Originally Posted by Jungle
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.
I have some lumber that can be used, and I also have a larger level to help in finding the right level. I happen to have a laser level too, but a basic type (I normally use it to cut drywall (so no need to scribe/mark the wall itself with a pencil).
I also plan one using liquid nail and small wood nails to hold the shims to the subfloor, then I am going to lay down the backer board with thinset mortar.
There are parts of the floor in this house that seem to have several layers of old floor on top of each other; basically they did not bother to uninstall the older floor and simply install newer ones on top of the older ones (since the 50s). Is that common? So many layers...
I also found lots of staples throughout the floor with some pieces of what may be carpet fiber and foam attached to them. In the following pic, the baseboard was removed and I found that thing (seems carpet fiber) under the backboard and on the corner of the wall):