You need a level floor if you install flooring, including hardwood flooring; engineered flooring; laminate flooring; and ceramic, porcelain, and quarried stone tiles; VCT (vinyl composition tile); and other flooring types. The difficulty of leveling a floor depends on how far out of level the floor is. Peaks in the floor are more difficult to fix than dips. But any problem of this kind can be solved.
Place a level directly on the floor. The length of the level depends on the size of the floor: small rooms requires a 2-foot level; medium-size rooms, a 4-foot level; and large rooms, an 8-foot level.
Look at the leveling bubble to determine where the floor is out of level. Mark the areas that are out of level with a pencil: Mark a straight line directly on the floor in areas that are not level.
Lift one end of the level so the leveling bubble indicates the level position. Use a tape measure to determine how far out of level the floor is. Make a note on the floor next to the pencil line.
Repeat this procedure over the whole surface of the subfloor.