How to Level a Floor

How to Determine an Uneven Subfloor

The subfloor serves as a crucial structural component that is integral to the overall strength and durability of the home’s flooring platform. It is imperative that this subfloor layer be stable, solid, flat, and level. Any irregularities, such as dips, ripples, or waves, will be transmitted to the surface floor covering, which could potentially cause failure in the flooring installation. For instance, if ceramic tile is installed over an unsound subfloor, it may result in the cracking of grout or the breaking of tiles.

What Tools You Need

Diagnosing Uneven Floor

  • Rotating laser level (optional)
  • Long straight board
  • Carpenter’s Level
  • Tape Measure

Installing Underlayment

  • Tape measure
  • Circular saw
  • Cordless drill
  • Sander
  • Straightedge trowel

Using Self-Leveling Compound

  • Bucket
  • Marker or pencil
  • Long straightedge
  • Paintbrush (optional)
  • Straightedge trowel
  • Sander
  • Shop vacuum

Reinforcing Joists

  • Hammer
  • Pry bar
  • Screwgun

What you need to do before installing new flooring?

Before you install new flooring, it’s usually a good idea to check the subfloor and fix any issues. But sometimes, if you’re putting carpet over hardwood or laminate over vinyl, you can just fix the subfloor underneath without taking up the old flooring. Making the vinyl flat with self-leveling compound is an easy fix before laying down laminate planks.

Please consider that the installation of new flooring over the old will result in an overall increase in floor height. If this is a concern, we recommend removing the old flooring, underlayment, and potentially even the subfloor to install new layers from the bottom up.

Issues related to subfloors can be generally divided into two categories: widespread sloping that impacts the entire floor plane, and minor inconsistencies that disrupt its flatness without affecting its overall levelness. While there are a variety of ways for individuals to address minor inconsistencies on their own, larger sloping and sagging issues are typically beyond the capabilities of most do-it-yourselfers and often require the use of jacks to level uneven floors.


1. Inspect Room Slope Thoroughly

One of the most effective means of assessing a floor’s overall slope is by utilizing a rotary laser level. Once positioned in the room’s center, the laser level emits a completely even line along all walls. To ascertain whether any significant sloping is present, measure the distance between the level laser line and floor at various points around the perimeter of the room.

One can also obtain an approximate sense of the overall slope of the floors by placing a lengthy straightedge across the middle of the floor, with one end situated at the foundation side of the chamber and the other towards the center of the residence. Position a carpenter’s level across the peak of the straightedge and observe the bubble in the central vial to ascertain if the chamber is approximately level or displays a noticeable incline.

2. Search for Minor Abnormalities

Place a lengthy straightedge horizontally on the floor, preferably a long and straight two-by-four, parallel to one side wall and positioned about one foot away from the wall. Observe for openings between the underside of the straightedge and the floor. As you shift the straightedge across the floor, the amount and dimension of gaps will determine the corrective technique for the subfloor.

Please move the straightedge across the floor in 1-foot increments and measure the distance between the bottom of the straightedge and the floor at various points. If you observe a sag in the floor at any point, it may indicate that a floor joist has sagged or is broken. In such cases, it is recommended that you reinforce the joist to correct the floor sagging. However, during the measurement process, you may notice irregular gaps beneath the straightedge and the floor as you move it over the floor.

3. Select an Approach for Repairing

If the irregular dips or sags between the floor and the bottom of the straightedge span no more than 4 inches and are no deeper than 1/2 inch, it is advisable to cover the subfloor with plywood. This will likely provide a flat surface suitable for almost any new flooring.

To rectify shallow waves or dips wider than 4 inches, we recommend using a liquid self-leveling compound to level out those areas. After the compound has dried, the floor will be considerably level and smooth, which will enable it to receive any new floor covering.

If one observes that sagging is present along a specific line in a particular section of the floor, it is probable that a warped or broken floor joist is not providing the needed support anymore. To remedy this issue, it is recommended to strengthen the damaged joist by adding a sister joist next to it.

4. Layout Strategy

Please ensure that when placing the plywood sheets, they run perpendicular to the floor joists. Kindly remember to stagger the joints in the underlayment in comparison to the subfloor by approximately half a sheet. Furthermore, it is essential to center the short sides of the plywood sheets over the joists to provide adequate support. Also, please do not forget to leave some 1/4-inch gaps to allow for some movement.

5. Trimming and Attaching Sheets

Please trim the first sheet, if necessary, to stagger the joints and ensure that the ends of the sheet fall over the joists. Place the initial sheet into position along one of the walls with a 1/4-inch gap along the edge. Secure the sheet to the floor joists using 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, ensuring that the screw heads are slightly recessed below the surface of the plywood.

When attaching multiple sheets, please trim them as necessary and place the next sheet in the first row, leaving a 1/4-inch gap between each end. Fasten the second sheet as you did with the first, and install the remaining sheets in the same fashion. Be sure to trim the last sheet to fit, as needed, and leave 1/4-inch gaps between each sheet. Thank you for your attention to detail.

6. Complete the Installation

Make sure to use full-width sheets for the second row and cut the first sheet a little shorter so it doesn’t line up exactly with the first row. And don’t forget to leave some little gaps between each sheet, like about a quarter inch or so.

Proceed with the installation of additional sheets of plywood across the room, ensuring that they are securely attached to the floor joists in the same manner. It is essential to maintain a 1/4-inch gap between each sheet. For the final row of plywood, please utilize a rip-cut method to ensure a proper fit within the remaining gap. Please do not forget to leave a 1/4-inch gap around the perimeter of the wall.

7. Fill the Missing Spaces

Prepare a mixture of self-leveling compound and evenly apply it over the seams and screw heads. Once the compound dries, use sandpaper to smoothen the seams and nail-heads. It is recommended to vacuum the entire floor before continuing with the new floor installation.

Leave a Reply

Your email address will not be published. Required fields are marked *