Photo 1: Using a piece of all thread to drive down exposed sheathing nails. Not only does this prevent nails from raising the flooring but it also prevents damaging the sander pad when sanding seams. Photo courtesy of Daniel Boone.


Photo 2: Here we sand the edge seams to prevent telegraphing bumps in the finished floor. This is especially important with OSB. Photo courtesy of Daniel Boone.

The builder or general contractor calls and reports the building is ready for the flooring installation. From this point on the responsibility of the flooring performance is in our court. From the relation with the builder /contractor we likely know the general situation at the site. With some few builders we can load the truck with acclimated flooring and proceed to the jobsite, do minimal preparation and begin installation the same day of delivery. With others, it’s a crap shoot.



Photo 3: Completed seam sanding. If the sander hits a proud nail, the rubber backing pad can be damaged. Photo courtesy of Daniel Boone.

Establishing a “protocol” is important for any future issue of an installation. A preliminary site review with a checklist as a guide is almost always necessary to determine the extent of the necessary subflooring preparation before installing the flooring. The site review should include determining if the subflooring is sound, dry, and flat; as well as what will be required to remove all the construction debris.



Photo 4: This is an OSB installation and there are humps in the floor about every 21 boards. Floor samples showed the edges of the OSB were thicker by more than 1/16”.

There are typically two different subflooring systems: the raised wood framed system over a crawl space or basement; and the reinforced concrete slab on/above grade or suspended.   





Photo 5: Clean drywall compound and other debris from the plywood subfloor that can interfere with the floor flatness. Photo courtesy of Daniel Boone.

The raised wood framed system

With 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.



Photo 6: All debris must be cleaned from slabs. Typically, this is followed by disc sanding with a buffer.

Check the moisture content of the subflooring and record the readings on the site check list and on the floor along with date of the readings. This establishes your protocol for registering the site moisture conditions. Low moisture is generally not a concern as long as subfloor panels have the typical required 1/8” spacing around their perimeters for any likely expansion. If moisture content is too high, any preparation will include action to reduce the moisture to an acceptable level.  Be sure to check moisture of supporting joists along nailing lines with the long insulated needles. If the moisture of joists is very high this excessive moisture can affect the flooring in the future. During the review, check the crawl space for excessive moisture and a proper ground cover on the earth. An excessive moisture condition in the crawl space is unacceptable and must be remedied.

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. 



Photo 7: The calcium chloride test is used to test for vapor emissions from the slab. Manufacturers normally require 3lbs. or less for flooring installation to proceed.

The on/above grade concrete slab system

With 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.



Photo 8: This concrete slab vapor retarding system uses asphalt mastic with 6-mil poly film, then the plywood subflooring is placed over the system.

As with the framed system, we check the moisture of the concrete. Many contractors are using the calcium chloride kits for this. A concrete moisture meter can also work. If the readings/test are too high, drying the concrete, usually with continuous ventilation and/or heat, will be part of the preparation.  For minor high and low areas we can grind the high ones down and fill the low areas.  A buffer using very coarse (16 to 20 grit) paper or carbide inserts can work. For major deviations in the flatness of the concrete, applying a screed or leveling compound is the most efficient preparation. For any fillers or levelers follow manufacturer’s recommendations and re-check moisture before beginning flooring installation.  



Photo 9: The plywood is 16” planks placed perpendicular to the direction of the flooring. In this case the plywood was scored on the back across the width every 12” or so and additional asphalt mastic was troweled on the poly to glue the plywood. No fasteners penetrate the VB.

When the slab is ready, further preparation includes placing a proper vapor retarder for any solid wood product and sometimes with engineered wood products. For geographical areas with a high potential for moisture intrusion such as coastal areas and major river plains, the extra protective retarder is necessary as the 6-mil poly film will likely not be protective enough. This can be a trowel applied adhesive compatible material, the polyethylene film, or other manufacturer recommended material (Photo 8). Preparation for these materials will likely include buffing the slab to remove debris, other concrete additives such as sealers, and other minor bumps and knots on the slab.



Photo 10: With a sleeper system, irregularities in the slab can be addressed with proper thickness shims placed under the sleepers. These shims should be full width to support the sleeper. Wedges don’t work well as they eventually work out and create noises and depressions in the floor.

For nail down flooring a proper subfloor system must be installed over the vapor retarder. Plywood is the material of choice.  Full sheets can be used, but telegraphed edge movement can result in related seasonal gaps or humps. Installing on a diagonal or cutting the sheets into 16” wide planks and properly spacing the plywood sheets/planks will avoid these seam related issues (Photo 9). The plywood can be glued, shot or screwed down. Gluing does not puncture the vapor retarder. Be sure with pins or screws that they are properly set below the plywood face. Grind off or remove any proud fasteners.



Photo 11: When using pins or concrete screws to attach the plywood to the slab, proud fasteners will interfere with the finished floor. In this case the installed flooring had to be removed because of humps and noises associated with the proud pins.

Finally, the system is ready for flooring installation. Any solid wood product must be assessed for acclimation. This requires using the moisture meter on the wood. Check 40 or more pieces and note the overall range and average moisture content. If the average is less than 1/2% different from the expected in use average EMC and the overall range is within +/-  1  ½ percent,  the flooring is ready for installation. If moisture is too high, the flooring should be placed in an environment to remove/dry some of the moisture. If moisture is too low, proper field spacing during installation or additional exposure to a more moist environment for later expected expansion will be necessary. 

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.