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.
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.
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.
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.
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.