Secure all the plywood or OSB sheets to the joists with 2-inch, coarse-thread screws, even if they are already nailed. Screws are more reliable than nails and won't pop out.
Vacuum the floor and look for the highest spot. Place one end of a 6-foot straightedge on that spot and, using it as a pivot, rotate the straightedge in a circle. Mark all depressions with a pencil. If a depression is more than 3/8 inches deep, mark it as such.
Move the straightedge to another high point and repeat the procedure. Continue doing this until you've covered the entire floor.
Fill depressions deeper than 3/8 inches with roofing shingles, stacking as many as necessary to bring the floor level. Drive one or two screws into them to hold them down. You don't need to screw the shingles down if you're installing a hardwood floor; the flooring cleats will hold them in place.
Use floor-leveling compound to fill shallow depressions. It comes as a powder that you mix with water; once mixed, it sets quickly. Prepare only as much as you can use in about 10 minutes, and apply it with a drywall knife. If you overfill a depression, you can sand down the compound after it sets.
Sand down high spots in the floor with a belt sander and a 60-grit belt. Use the sander to take down excess leveling compound as well as to level mismatched joints between sheets of plywood or OSB.
Vacuum the floor thoroughly before installing the underlayment for the floor covering.
Things You Will Need
- 2-inch screws
- 6-foot straightedge
- Roofing shingles
- Floor leveling compound
- Drywall knife
- Belt sander
- 60-grit belt
- If you're installing hardwood and you have floor sanders handy, use an edger to sand down the subfloor. It works more quickly than a belt sander.