One method of leveling a floor before applying flooring is the use of a self-leveling compound. This compound is similar to concrete, and you can apply it over wood or concrete floors. If the difference is greater than 2 inches, you can add aggregate or apply it in layers.
The manufacturer supplies the compound as a dry powder. Mix it with water using a heavy duty drill motor and paddle blade to form a thick compound. Apply the self-leveling compound using a trowel and let it dry. As it dries, gravity pulls the mix into lower areas and thins it in high spots to level the floor.
Keep traffic off the compound for two hours. If you are applying ceramic tile, the surface should dry a minimum of four hours before beginning the installation. For flooring that is sensitive to moisture content, the self-leveling compound should dry a minimum of 16 hours before installation.
For floor variations greater than 2 inches but less than 5 inches, place a layer of pea gravel with no standing water over the areas, bringing the surface to 1/4 inch below grade. Pour the self-leveling compound over the gravel and mix it with a rake. When applying the compound in this manner, the minimum drying time is 24 hours.