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How To Level A Subfloor

With many home improvement projects, creating a proper base for the finishing material is the key to a having a professionally looking finished project, and nowhere is this more important when the finish material is hardwood, ceramic tile or natural stone such as marble and granite.

Many home handymen rush the completion of the subfloor and/or do not pay enough attention to providing a sound and flat base for the finish material. They believe that the finish material will hide and or correct any discrepancies and misalignments with the subfloor and this couldn’t be farther from the truth.

Note: Although most handymen and even professionals will discuss the attributes of having a “level” subfloor in order to install hardwood or tile, in fact, the level of the subfloor has little if any relevance to the appearance of the overall completion of the project. What is required is a “flat” subfloor. If because of settling, your floor is on a slight angle, but is flat you will end up with a professionally looking finished project without making any adjustments or modifications to the floor.

Some of the problems associated with a subfloor that has not been properly prepared are:

  • The finished hardwood material buckles.
  • The floors sound hollow when they are walked on.
  • The floors make a variety of noises without anyone on them.

Making sure that your subfloor is "flat" (level).

Wood subfloor:

Note: The problem of subfloors that are not flat, occur in new and old houses. Do not assume that because your home was just built that the subfloors are going to be flat!

Prior to starting the installation of your new flooring material you must first strip the floor of its current material. Once you have the bare subfloor it is time for an inspection and some possible corrective actions.

  1. Make sure all of the nails and/or screws that hold the subfloor boards or plywood to the wood joists are flush with or recessed with the top of the underlayment.
  2. Note: Do not add any additional fasteners to the subfloor at this time.

  3. Vacuum the subfloor and make sure that all debris, no matter how small has been removed from the surface of the subfloor.
  4. Using a truly straight (This in itself could be a monumental problem. Much of the lumber we purchase today is very poorly milled and finding a straight board can be a daunting task) 8 to 10 foot 2 x 6, 2 x 8 or 2 x 10 board lay the edge of the board on the floor and look for high and low spots. Move the board around so that you cover the entire room floor area.
  5. Note: If you cannot find a truly straight piece of lumber. Rip 3 pieces of plywood lengthwise (8 feet long) on a table saw so that the edges are true, approximately 6 in wide. Join the 3 boards together so that you have a perfectly true edge surface and screw them together. As an alternative and for not much more than the price of a straight piece of 2 x 8 you can purchase an aluminum straight edge that is 8 feet long, as shown in Figure 1.

Aluminum straight edge

Figure 1 - Aluminum straight edge

Use some chalk to mark the high and low areas. Any area that varies by 3/16 of an inch over 10 feet should be corrected. It is not unusual to identify both high and low spots.

Correcting the subfloor:

Only you can decide whether to raise the low parts of the subfloor or to lower the high spots and this is generally predicated on which you have more of.

Lowering high spots:

To lower high spots in the subfloor you will have to remove the subfloor and then using a plane or use a belt sander to remove some of the wood from the floor joists.

Note: You can also use a belt sander and/or a floor sander to remove high areas by sanding the subfloor itself. In many cases this is logistically easier than trying to remove the subfloor to get at the floor joists. Floor sanders can be rented from most rental facilities and home improvement centers.

Raising low spots:

Note: Every professional flooring installer will have his or her own methods for raising subfloors to make them flat. There is really no right or wrong way as long as the finished subfloor is properly fastened to the floor joists with an end result that the subfloor is flat.

Raising low spots usually means removing the subfloor underlayment, so that some sort of shim, (in many cases roofing shingles are used), can be used on top of the joist.

Other methods include adding sisters to the side or sides of the joist in order to make it level. From my experience adding sisters is usually logistically difficult. You may be fighting with insulation, water pipes, duct work and electrical wires.

So I opt for adding a shimming material between the top of the joist and the bottom of the subfloor underlayment.

Completing the subfloor:

Once you have the floor flat and the underlayment material is in place add some flooring screws to the underlayment where it is in contact with the floor joists and/or shims. I usually find that the original fastening of the subfloor underlayment to the floor joists is inadequate if you want to avoid having a floor that squeaks in years to come.

Concrete subfloor:

Identify the high and low spots of the concrete subfloor in the same manner as for a wood subfloor.

Once you have identified the high and low spots on your concrete subfloor you can determine what remedial actions to take.

Low spots on the concrete subfloor can be filled by using a concrete floor leveling compound.

High spots on a concrete subfloor can be removed using a concrete grinder, which can be rented or purchased.

Note: Most quality concrete grinders use water. Make sure that all of the water has left the concrete before attempting to install the finished hardwood or tile flooring.