Following proper procedures are crucial to the successful wood panel underlayment installation. Panels should be smooth enough so the texture/graining will not show through the finished flooring. (Photo courtesy Halex Corp.)

It is definitely surprising that with all the great wood underlayments available, there are still a lot of underlayment concerns plaguing our industry. These range from underlayment joint show-through, to fastener problems, noise from movement, discoloration and moisture. The following are some of the things we need to look at to prevent these problems.

Underlayment Selection.Be sure to choose the proper type underlayment for resilient floors. They must meet the following requirements: 1. The subfloor or substrate should be structurally sound. Any movement to the floor will weaken the fasteners and cause noises. 2. The underlayment must be designed for use with resilient flooring. There are a lot of products available not designed to be used as an underlayment.

The right underlayment should have a minimum thickness of 1/4” or heavier if the site conditions warrant; panels should be smooth enough so that texture/graining will not show-through finished flooring. This includes sanding marks and any plugs used.

The top layer should be a wood species that resists dents and punctures from concentrated loads, plus the veneer layer under the face ply should be the same as the face ply without voids.

The panels should be free of any substance that may stain vinyl such as edge patching compounds, marking inks, paints, solvents, adhesives, asphalt, dye, synthetic crack and knot fillers, etc. The panels should not be treated with any type of insect or fire retardant.

And the panels should be installed in strict accordance with the board manufacturer’s recommendations.

Substrate Preparation.Preparation of the substrate is key. Follow these steps for success. Existing materials should be well-bonded and firm. Do not install over any soft flooring materials like a cushioned-backed material. If you install over a soft material, the fasteners will work up and down, creating squeaks and ultimately weakening the fasteners’ hold. This can lead to possible fastener show-through.

The subfloor should be smooth, clean and dry. Residues like adhesives, wax, oil, varnish, paint, etc., need to be covered. Adhesive, oil or wax residue needs to be covered with an appropriate material. If the residue is tacky, place a layer of non-saturated scribing felt, builder’s paper or polyethylene sheeting over the residue to prevent a cracking or popping sound when walking on the floor.

Check the moisture content of both the subfloor and the underlayment panels. The subfloor should be no more than 12% and there should be no more than 3% difference between the panels and the substrate.

Fasteners.Once the panel and substrate requirements are met, take a look at the type of fasteners: screws, staples or nails. I am not aware of any underlayment board manufacturer that recommends the use of screws. The reason: screws are easily stripped and in some cases the heads snap off, causing the holding power to be severely compromised.  

Some panel manufacturers recommend using a narrow crown staple (generally 1/4”).  I find the mallet driven stapler works best; however many installers prefer the pneumatic type. The reason I like using the mallet type is the greater control of the staple depth. The pneumatic staple requires constant monitoring of the air pressure and a large enough air supply. Without constant monitoring, it is easy to overdrive the staple. The staple needs to be flush with the surface of the panel and not through the top ply.

Nails.Nails have the best holding power of all fasteners. Many installers will dispute this fact, but I have seen the test results that support that statement. When selecting a nail, you need to be aware of the shank and the length of the nail. The shank should be either a ringed shank or a spiral shank. The type most commonly used, because of its availability, is the ring shank.  

The correct length of the nail is determined by the thickness of the panel to be installed. Panels that are less than 1/2” will used a 3d nail (1 1/4”) and thicker panels from 1/2” – 1/4” will require a 4d nail (1 1/2”). In looking at underlayment problems, I find many installers are using 6d and even 8d nails. The problem with a too long nail is the fastener blow through the subfloor. A too long nail takes the grain of the subfloor with it, severely compromising the withdrawal resistance of the nail.

Fastening patterns are extremely important especially along the edges of the panels.  The fastener spacing is set by each board manufacturer based upon grade and thickness.  The important factor often overlooked is the distance from the joints edge. The fastener should be within 3/8” from the edge. The edge spacing allows for greater board stability and resistance to board movement or peaking.  

The field fastening is also based on the thickness of the panel and is set by the board manufacturer. When fastening the field of each panel, always work to the open side of the panel. Do not to nail any fullness into the panel. An important thing to remember:  Fasten only one panel at a time.

Gluing.Many installers, retailers and flooring contractors think it is a good idea to glue the panels to the substrate. Not true. I had a discussion with the Engineered Wood Association about this and the only time they recommend gluing is if you want to enhance the structural integrity of the floor. In that case, you should use a 5/8” or 3/4” panel. There is a lot of popping and crunching sounds that are a result of gluing.

Installation.When installing the panels, be sure to follow a few simple rules: When laying out the room, leave an expansion zone around the perimeter of the room. Offset subfloor joints and underlayment joints a minimum of 6” to stop any transfer of movement.

Use only factory panel edges of the underlayment for joints. All cut edges should be placed along walls. Next, butt the underlayment joints together lightly. Make sure to fasten only one panel at a time.

Underlayment joints treatments.There is some controversy as to whether an underlayment joint should be patched or not. My feeling is not to patch every joint. The underlayment joint should be butted-up lightly with no fullness in the panels; joint unevenness should be sanded to smooth; and only underlayment board joints gapped more than 1/32” should be filled.

Do not skim coat the entire installation – if an underlayment is that bad try another board or brand. I have tried this type of installation in both arid and damp environments and feel it is one that works well for both. The key is a good underlayment, proper fasteners, proper fastening patterns, dry substrates and proper joint control.