How to Level an Uneven Plywood Subfloor

Leveling subfloor

A subfloor is the surface underneath a finished floor. Some subfloors may be made of cement, but they are usually made from sheet material like plywood. You can level a subfloor by using jacks to actually lift and re-position the structure; however, that is usually a job for contractors. An easier solution is to level out a subfloor using asphalt shingles to smooth out the hills and valleys. A subfloor that is flat and level will prevent cracks, splits, creaks and other annoying noises.

Things You'll Need

• 4-foot-long level
• 12-foot-long piece of string
• 2 small nails
• Utility knife
• Tape measure
• Circular saw
• Screw gun
• 1/16-inch asphalt roofing shingles
• 3/8-inch plywood
• 2-inch or 2.5-inch collated subfloor screws
• 30 pounds of construction felt
• Black marker pen
• Hammer

Instructions

Identify High and Low Areas of Floor

1. Tie a long piece of string around a nail and hammer the nail into the floor at the edge of the floor. Tie the other end around another nail. Stretch the string across the floor and hammer that nail in just deep enough to hold the string taut across the floor. Examine the string over the floor to identify high and low areas. Mark the high areas by writing the word "high" in black marker right on the floor and mark the low areas by writing the word "low" with the marker right on the floor. In the lowest spots, you can write "very low" or "lowest area."

2. Decide how you want to fill in the lower areas to level out the floor. The floor needs to be fairly flat, so there should be about a 3/16-inch rise or fall over an area that covers 8 to 10 feet. You may want to start at a very low area.

3. Fill in the lower areas using shingles and construction felt. Place the shingles underneath the plywood. If you are installing a hardwood floor on top of it, you may need to use construction felt between the hardwood floor and plywood subfloor if it is still not quite level. Use your level as you work around the floor to ensure that you are evening out the subfloor. Lower areas may require more shingles or overlapping of shingles to achieve the right height.

4. "Dry fit" the plywood in place first by walking on it while an assistant uses the level to ensure the floor is even. Fasten 3/8-inch plywood to the surface using 2-inch or 2.5-inch subfloor screws. Position the screws approximately every 12 to 14 inches and about once every square foot along the perimeter. Connect the screws to the joists as frequently as you can.

5. Constantly check the evenness of the floor with your level and ensure that any seams between pieces of plywood are not creating ridges. Stagger screwing into the joints on the long edge of the subfloor. Repeat Steps 3 to 5 throughout the rest of the floor.

Tips & Warnings

• The shingles and/or construction felt should eliminate air gaps between the original subfloor and the additional 3/8-inch plywood subfloor. It is more important that the subfloor is flat than completely level. A very slight incline of 3/16 inches or less over an area of 8 feet should not be detectable. If you install hardwood floors across the joists, it should further reinforce the overall floor surface.

References

• One Project Closer: How To Level A Plywood Sub-floor Using Asphalt Shingles
• Reliable Modeler: Want To Learn How To Level A Sub-floor?