How to Level a Floor

How to Diagnose an Uneven Subfloor

1. Check Overall Room Slope

   The best way to check a floor for general sloping is with a rotary laser level. When set in the center of the room, the laser level projects a perfectly level line onto all walls. To determine if there is major sloping, measure between the level laser line and the floor at various locations along the room’s perimeter.

   You can also get a rough sense of the overall pitch of the floors by laying a long straightedge across the center of the floor, with one end at the foundation side of the room and the other toward the center of the house. Place a carpenter’s level across the top of the straightedge and observe the bubble in the center vial to determine if the room is roughly level or is noticeably sloped.

   Tip

   Major settling problems are sometimes indicated by wall cracking that occurs directly above one corner of a window or door. If you see such cracks running up the wall from the corners, this may indicate that you have a more serious problem to deal with.

2. Look for Minor Irregularities

   Lay a long straightedge across the floor (a long, straight two-by-four works well), parallel to one side wall and about a foot from the wall. Look for gaps between the bottom of the straightedge and the floor. As you move the straightedge across the floor, the number of gaps and their size will indicate what strategy to use for correcting the subfloor.

   Move the straightedge across the floor in 1-foot increments, taking measurements between the bottom of the straightedge and the floor at various points. If you notice that the floor sags at one point across the room, for example, it may indicate that a floor joist has sagged or is broken. This means you may need to reinforce that joist in order to correct the floor sagging. More likely, though, you will notice various irregular gaps beneath the straightedge and floor as you move the straightedge over the floor.

3. Decide on a Repair Strategy

   • If the gaps between the bottom of the straightedge and the floor are irregular dips or sags, spanning no more than about 4 inches and no more than 1/2 inch in depth, then covering the subfloor with sheets of plywood will probably flatten out the floor sufficiently to accept almost any new flooring.
   • For shallow waves or dips that are wider than 4 inches, the best fix will be to level out those areas with a liquid self-leveling compound. Once the compound dries, the floor will be quite level and smooth, prepared to accept any new floor covering.
   • If you determine that sagging seems to be occurring along a line in one area of the floor, it’s likely you have a warped or broken floor joist that is no longer providing the necessary support. The repair here will be to reinforce the bad joist by adding a sister joist alongside the old joist.

   Tip

   If you are laying ceramic tile, then covering the subfloor with sheets of 1/2-inch cement board is usually enough to smooth out relatively minor dips.

   Keep in mind, though, that plywood or cement board laid directly over the existing subfloor will raise the height of the floor. Where this is a problem, the solution may be to tear out the entire floor down to the joists and install brand-new 5/8- or 3/4-inch tongue-and-groove subfloor sheathing over the joists. While this is a labor-intensive job, it’s not particularly difficult and will create a very solid and durable subfloor upon which to install your new flooring.

   How to Correct an Uneven Subfloor With Plywood Underlayment

   Adding plywood underlayment is usually sufficient to correct a subfloor that sags in areas that are no more than 4 inches wide and less than 1/2 inch in depth. It’s best to use the thickest plywood that is practical for your situation, but even a 1/2-inch layer will greatly improve a subfloor that has relatively minor irregularities.

4. Plan Layout

   Plan the layout of the plywood sheets so the long dimension runs perpendicular to the floor joists. Also, make sure the joints between sheets in the underlayment are offset (staggered) from the joints in the subfloor by about one-half sheet. The short sides of the plywood sheets must be centered over joists to support the ends.

   Plan the layout so there are 1/4-inch expansion gaps at the walls and between sheets.

5. Trim and Attach Sheets

   Trim the first sheet, if needed, to stagger the joints and ensure the ends of the sheet fall over joists.

   Place the first sheet into position along one of the walls with a 1/4-inch gap along the edge. Fasten the sheet to the floor joists with 2 1/2-inch screws driven through new underlayment and subfloor and into the joists below. Drive a screw every 6 to 8 inches along the joist, making sure the screw heads are slightly recessed below the surface of the plywood.

   If attaching more than one sheet, trim as needed and place the next sheet in the first row. Leave a 1/4-inch gap between the ends of the sheets. Fasten the second sheet as with the first. Install the remaining sheets in the first row, trimming the last sheet to fit, as needed. Make sure to leave 1/4-inch gaps between sheets.

6. Finish Installation

   Complete the second row with full-width sheets, but trim the first sheet so the end joints between sheets are offset from those in the first row by about 4 feet. Leave 1/4-inch gaps between all sheets.

   Continue installing sheets of plywood across the room, attaching them to the floor joists in the same way. Make sure to maintain 1/4-inch gaps between the sheets. On the last row of plywood, rip-cut the sheets to fit the remaining gap, leaving a 1/4-inch gap around the wall.

7. Fill Gaps

   Mix self-leveling compound and trowel the mixture over the seams and screw heads. Allow the self-leveling compound to dry, then sand the seams and nail-heads smooth. Vacuum the entire floor before proceeding with the new floor installation.

Use Self-Leveling Compound to Correct Subfloor

Wide sags and depressions in a floor sometimes can’t be adequately bridged even by sheets of plywood, so it’s best to fill these areas with self-leveling compound. When mixed with water, the compound settles into low areas and dries to a smooth, level surface. Self-leveling compound comes in large pre-mixed buckets or in dry form that can be mixed with water.

1. Identify and Prepare the Low Spots

   Mark the areas of the floor that need filling in to bring them level with the surrounding areas. A long straightedge works well to identify these low spots. If the manufacturer recommends it, apply a primer to these areas and allow it to fully dry.

2. Mix and Apply the Self-Leveling Compound

   Mix the floor leveling compound with water in a bucket, following the manufacturer’s directions. Pour a generous amount of leveler onto the subfloor and spread it over the floor with a towel, allowing it to settle into low areas. The product will seek its own level (much like water), but it must be spread out so it covers the affected area without pooling.

3. Lightly Sand

   Allow the leveler to dry completely, then sand off any high areas and vacuum away the dust. The surface does not need to be glass-smooth, but you should be able to draw a straightedge over the surface without noticing any wide gaps. Shining a work light across the surface as you check with a straightedge helps to identify any gaps you missed.

Reinforce Sagging Joist to Correct Subfloor

Where the cause of floor irregularities can be traced to a warped or broken floor joist below, it’s best to correct this structural problem to ensure the floor’s structural strength.

If you have access to the joists from underneath the floor, a sagging or cracked joist can be reinforced with a sister joist inserted alongside the bad joist and screwed to it. You may need to temporarily remove any cross blocking in order to move the sister joist into place. Ideally, the sister should be the same length as the bad joist, with one end resting on the foundation wall and the other on a support beam.

Because the subfloor may have sagged slightly, it may require some heavy hammering and prying to drive the new joist into place before you screw it to the old joist. When contractors do such work, they may brace or slightly lift the floor with adjustable steel columns before sistering the bad joist into place alongside the new framing member.

When to Call a Professional

If your evaluation of the floor shows that the room, as a whole, slopes to the side or middle, the repair project is usually a major one requiring the assistance of a contractor. Such large-scale sloping often means there is a structural problem with a foundation wall, a center support beam, or both. A contractor, in consultation with a building engineer, is usually the best option in such situations.

A professional contractor may also be the logical choice in some cases where a floor joist needs to be repaired or reinforced. Reinforcing a bad joist can be done by a do-it-yourselfer where there is easy access to the floor from below—either in an exposed crawlspace or unfinished basement—but it can be quite complicated in second-story situations where the floor space is enclosed by a finished ceiling below. In these cases, floor joist repair may require that you open up the ceiling below, and such a project is beyond the comfort level of many do-it-yourselfers.

If you decide to undertake the project by yourself, it may be helpful to consult with a structural engineer to make sure that you are staying within load limits for your intended repairs.