Shims to be placed under sleepers are full bearing; wedge-shaped shims driven from the sides tend to work out.


These shims flatten the problem dip and will stay in place.

The performance of the finished wood floor is directly affected by the performance of the subfloor. We’ve all heard that the house built on the rickety foundation will fall and the house built on a firm foundation will stand the tests of time. Therefore, subfloor selection and preparation is most important in the ultimate performance of the flooring. How many times do we have to say-Flat, Clean and Dry? These are the basic requirements of subflooring in any wood flooring installation situation whether over concrete or a wood joist system. When the actual installation of the flooring begins the contractor has approved the subflooring conditions. Any issues that are reported after installation and relate to the subfloor will likely be contractor responsibility. As always, the caveat “Follow manufacturer’s directions/instructions,” applies.

What to do to achieve the proper subflooring and proper subflooring conditions? The basics are to:
  • Make sure the substrate is flat to the specified tolerance.
  • Determine that the substrate is adequate for the application involved.
  • Clean the subfloor of any debris or substances that will interfere with the application.
  • Determine that the subfloor is dry enough for the material to be applied.


Note the 2-by-4 stiffener against the subfloor. It is screwed and glued to the subfloor but not to the engineered I-joist. The 2-by-4 was placed to support a broken plywood joint. This can also be used to block where short runs of flooring run parallel to joists such as those with borders.

The typical method of installation-mechanical fastening (nails, cleats, or staples) or glue down, will determine the requirement for how flat the subfloor should be. Nails, etc., force a board into place and hold it there. Adhesives rely on continued contact to “stick” the boards; too much variation in flatness and the boards don’t contact or stick.

The NOFMA standard for adhesive application of solid wood flooring is that the subfloor must be flat to a maximum variation of 3/16” in 10 feet. This is for either slab or joist construction. This is also the generally accepted standard for engineered flooring, but always first check with the manufacturer for these products. The wood flooring contractor is responsible for determining that the substrate is sufficiently flat. Rolling a 10-foot length of pipe over the floor can identify those areas that are not flat. Pulling a string line across the flooring or using a laser level can also be used to measure variation. If the variation is greater than the specification, deflection, hollow sounds, creaking, and or movement can result.

The framing was cut and allowed to deflect to remediate a 3/16" hump over this wall.

The options for flattening the substrate for adhesive applications are: 1) grind/sand the high areas, 2) fill the low areas, or 3) pour a leveling topping over the entire substrate. The materials used must all be compatible with the substrate and the adhesive.

Who will do the required work? And, most important: Who is responsible for the cost involved? A preliminary site review will ID the issues and cost can be assessed and reported to the general contractor/builder. If the flooring contractor begins to do this work for free, the expectation is this is a free service and expected at no charge on the next job. In the real world when small areas or marginal conditions are encountered, the flooring contractor will likely take care of these procedures.

For slabs, grinding high areas can sometimes be accomplished by using a wood flooring buffer with a sanding disc and open coat, very coarse 12 to 24 grit sand paper. There are also stone and carbide grinding accessories that can be used. Where low areas are encountered they can be filled with either cement or latex-based products. Choose the most effective product according to the required time line. Cement products are generally the most cost efficient, but they use water and cannot be covered until sufficiently dry, which may take days. The latex types can generally be covered quicker; with some, this can be applied the same day.

Check the subflooring for moisture particularly at doorways. Be sure to make a habit to record the readings. This sets a precedent should a problem occur.

For the badly out of tolerance slab, applying a self-leveling topping is the likely choice. In my experience this operation is beyond the duty of the flooring contractor. These products are best placed by a concrete contractor using a pumping truck. Sometimes the builder/general will just say - “Take care of it the best way you can or I will get someone else to do the work.” Be sure to recommend a good competitor, since their reputation and money will be on the line when the call back occurs. Again, be sure all the products that are used will be compatible with the adhesive.

With wood frame construction and adhesive application, the grinding, filling, and overlay takes on a much different prospective. Too much grinding/sanding of a wooden subfloor can affect the structural performance. When using a leveling compound the flex of the system must be considered, too much movement and the material can crack and loosen, not holding the flooring sufficiently over the long term.

The best way to treat high areas is to lower them by dropping the appropriate framing members. The builder/general should be responsible, since most flooring contractors are not framing contractors. Again another way to treat the high area is to pour a self-leveling compound over the entire subfloor. If a self-leveling compound is used, the system must be checked for the extra dead load weight. Also consider the amount of water introduced into the surrounding components.

For nail down flooring on a slab, the NOFMA standard of variation is 1/4” in 10 feet. The same procedures can apply for flattening the slab as do with adhesive application except that the greater variation is allowed. Where sleepers are used, they can be contour-cut on a band saw to conform to the allowed variation. Be sure that enough thickness remains for nailing, at least 1 inch. Also, the sleepers can be shimmed over low areas. Use flat cut shims not tapered ones, as they tend to work out over time. Also, use group #1 dense wood species for all sleepers and shims.

For conventional wood joist framing and nail down applications there is no established standard except the requirement established by the local building codes. The NAHB says that unevenness of up to 1/4” ridge or depression within any 32” measurement is acceptable. Installing flooring over such a condition will result in not getting paid or at least a call back. So, in the real world this has to be fixed before installing flooring.

Check the subfloor. These ram set pins are too high and were a cause for the noises and humps in the flooring that was removed and replaced. When the flooring was taken up, the contractor could not deny this was his problem.

Where a 3/4” plywood subfloor is used; different thicknesses of plywood can be used to build up low areas. These can be sanded along the edges to create a smooth transition for the primary subfloor. Remember strip flooring can span 10”, so the tapers do not have to be completely feathered. However, with engineered flooring more continuous support is required. Sleepers can also be used to bridge unacceptable conditions like sloped or depressed areas. They can be scribed to the variation and contour cut on a band saw. Use wood that is equivalent to Group 1 dense soft wood species. For the more minor variation up to 1/8” in a small area, placing layers of asphalt roofing shingles in the depression can support the flooring. Wooden shingles can also be used, particularly in sloped areas of transition between different areas or different subflooring systems. Don’t use a leveling compound as it will break up when the flooring nails go through it.

Did we start by saying-CLEAN? Debris interferes with flatness and adhesion of any glue. It also covers up problems within the substrate that can affect the flooring.

Use common sense. With adhesive applications flatness is the key to performance as long as installation procedures are correct. With nail down application flatness is critical but extra variation is allowed. The subflooring therefore becomes the critical variable. It must be dense enough and thick enough to adequately hold the fastener. Another critical variable is deflection or movement, too much and fasteners loosen and movement increases. Stiffening the system can require major remediation such as putting additional joists and support beams. Overlaying with an additional panel thickness can help. Use a panel thickness that is similar to the primary subflooring. Thinner overlays don’t really stiffen the system. Remember when adding height you can create problems with transitions and stairways.

As flooring contractors we encounter many different situations both with new construction and existing construction. We are the expert on wood flooring application and should install only when the system conditions can assure good performance of the flooring product. We will be the responsible party, if questionable situations are accepted and installation proceeds, the liability likely rests with us.