To check for small high or low spots that may not even be visible, place a long  level or straight pipe on the subfloor. Move it around to different areas and mark any high or low spots you find with a pencil or marker. Most floors will have at least some slight variations. 

If the subfloor is plywood or particle board, start by screwing down any loose boards or plywood sheets. Add more wood screws, if needed, to secure them. Replace any nails with screws, since nails that popped up will loosen again. Make sure to remove stray nails or staples.

When leveling floors made of wood, you’ll need to sand down any high spots. If you expect to create a lot of sawdust, wear a dust mask and use painter’s tape to tape plastic sheeting over doors and air vents so the dust won’t spread. 

Use a sander to sand the high spots.  Your local Home Depot store may have rental tools available. 

Keep checking the floor with a level as you sand it. 

When you’re finished, vacuum the floor thoroughly with a wet/dry vacuum or sweep it with a broom. Make a final check with the level to be sure all the high spots are gone.

If your floor is concrete, use a concrete grinder for large, open areas or a 4-inch or 8-inch angle grinder with a cup-wheel attachment made for use on concrete. Grinding and repairing a concrete floor can create concrete dust, so be safe and wear a respirator. A dust mask will not give you enough protection. 

Keep checking the floor with a level as you grind the concrete.

When you’re finished, vacuum the floor thoroughly with a wet-dry vacuum or sweep it with a broom. Make a final check with the level to be sure all the high spots are gone.

Use an underlayment, floor leveler or floor patch product to fix low spots in a wood or concrete subfloor. 

An underlayment is a thin layer of material sandwiched between two other materials. It's used to help insulate, absorb sound and reduce wear on your floor. It can also serve as a vapor barrier.

Floor levelers are cement-based products that come premixed or as dry substances to mix with water. Choose a product recommended for your application and mix it, if needed, following the manufacturer’s directions. 

Floor patches are products that help repair holes, cracks, small dips and other imperfections. Follow the directions on your product for how to apply it.

If the manufacturer recommends it, go over the subfloor with a damp sponge before you apply a floor leveler. This will help keep it from drying too fast, so you’ll have time to spread it around.

Pour the liquid-like floor leveler onto the subfloor and smooth it out with a trowel. Gravity will help it settle into the low areas. Remove any excess floor leveler and feather it around the edges so it blends with the rest of the floor. Let the floor leveler dry overnight or as indicated by the manufacturer.

After the floor leveler dries, sand down any high spots to make them flush with the rest of the floor.

Use a wet-vac to vacuum up dust and debris. Make a final check with a carpenter's level or straight pipe to be sure all the low spots are gone. 

Laminate is a budget-friendly, attractive alternative to hardwood flooring. Laminate planks snap together and “float” above a subfloor, which means tiny subfloor imperfections are okay although again, a level subfloor will help prevent damage.

To level a subfloor for installing laminate, follow the directions above to remove high and low spots. After vacuuming or sweeping the subfloor to clean it, add a plywood layer if the subfloor is in very bad shape, but do not overlap the sheets. A plywood layer is not necessary if you are putting down wood laminate.

Finish by installing the laminate according to the manufacturer’s instructions. 

Before you install tile flooring, follow the directions above for fixing high spots and low spots in a wood or concrete subfloor. Then vacuum the floor with a wet-dry vac or sweep it with a broom. Make a final check with the level to be sure all the high and low spots are gone.

To prepare a wood subfloor for installing tile, install a cement-based backerboard following the manufacturer’s instructions. 

To prepare a concrete subfloor for installing tile, you’ll need to do a different kind of preparation. Use this Home Depot guide, How to Install a Tile Floor, for more information. Tile leveling systems can help keep your tiles level, aligned, properly spaced and held in place while the tile mortar cures. Used correctly, tile leveling systems can make the process of installing tiles go faster and improve your result.

Before you install vinyl flooring, follow the directions above for fixing high and low spots in a wood or concrete subfloor. You do not need to add a layer of plywood unless your subfloor is in very bad shape. Finally, follow the vinyl manufacturer’s installation instructions. 

Before you install hardwood flooring, follow the directions above for fixing high and low spots in a wood or concrete subfloor. Make sure you apply a cementitious flooring leveler you can nail through later. 

Follow the hardwood flooring manufacturer’s installation instructions and use this Home Depot guide, How to Install Hardwood Flooring, to learn more.