Foundation
Check the degree of sagging by examining your floor. A floor that slopes from one side all the way to the other side indicates that a sagging foundation may have caused the sagging. To level your floor, in this case, you will need the help of a contractor.
Check the degree of sagging by examining your floor. A floor that slopes from one side all the way to the other side indicates that a sagging foundation may have caused the sagging. To level your floor, in this case, you will need the help of a contractor.
Sub-Flooring
A floor that is uneven in spots but does not have an even slope across the entire floor, is likely to be caused by bulging or loose plywood that has been used for your sub-flooring. You may need to rip up and replace existing plywood, or simply tack the existing plywood down more securely.
Joists
The most common cause of uneven floors is sagging or crowning floor joists. To remedy this problems you can either use a self-leveling product, or use shims between the joists and your plywood sub-flooring.
Source: www.DoItYourself.com