The raised wood framed systemWith the raised system, the subflooring is already installed, so any issues with the specification of the subflooring and framing will have to be addressed to the builder. Even though it is not a part of subfloor preparation, the thickness of the subflooring and the span or spacing of the joists can be very important in flooring performance. For too thin subflooring, or too wide joist spacing for the flooring that is to be installed, advise the builder, in writing, of the performance issues involved. For too thin subflooring add an additional layer of performance rated plywood, typically 1/2” or thicker. Orient the plywood in the same direction as the existing subflooring; however, offset the long edge of the overlay in the center of the existing subflooring and move the ends two joist spaces over so no seams fall along the original subfloor seams. For too wide joist spacing or spans that have too much movement, adding joists or mid-span cross support beams will stiffen the system. Adding an additional layer of subflooring generally won’t add stiffness to the deflection and movement associated with span or joist spacing issues.
Walk the floor and check for loose and un-nailed areas. Also, check for any damaged or broken sheathing. Any of these areas must be re-nailed and or replaced before flooring installation. Check for flooring flatness, particularly at changes in joist direction and supporting structural beams. Check for high joists, and low joists. For humps at structural members or high joists, sanding the subfloor sheathing may be a preparation option. However, if sanding results in too thin subflooring advise the builder again with a written note that the area needs to be lowered. Residential Construction Performance Guidelines allow up to 1/4” ridge or depression in 32”. A hump of this extent will likely result in a complaint by the consumer and sanding 1/4” off a 3/4” subfloor leaves a too-thin subfloor. All these items are normally the responsibility of the builder/contractor; however, for the minor condition we will likely address the condition as part of preparation.
Preparation will also include cleaning the floor of all the construction debris. Drywall compound, mud/dirt, and general litter all have to be removed. After debris removal, set all raised sheathing/framing nails, this may require the all thread and hammer punch shown in Photo 1. Next sand the sheathing seams flat, as shown in Photos 2 and 3. This is particularly important where OSB is used as sheathing since edge swell is more pronounced than with plywood. The result of not doing this is humps every 4 feet in the finished floor, as shown in Photo 4.
The on/above grade concrete slab systemWith the concrete slab system, the flooring contractor will typically select and install the subflooring, so the choices related to the subflooring are his responsibility. Any inadequacy in the slab should typically be addressed by the builder. However, the flooring contractor must inform the builder of issues with the concrete such as humps, low areas, cracks, and high moisture.
Subfloor preparation is critical to performance of the finished floor as illustrated by the problem floors of the photos. Proper thickness subflooring that is: Clean, Dry, Flat, and Sound, with the necessary vapor retarder will assure good performance. Any shortcut sets you up for costly repair or damage to your reputation. One of the NWFA’s educational schools is on Subfloor Prep, which also includes preparation for glue down installation; for information call (800) 422-4556.