The installation of underlayment is extremely crucial to the success (or failure) of any resilient installation. The proper fastening technique is the most important part of the installation. Additionally, all underlayment should be acclimated on the jobsite 48 hours prior to being installed. This allows the underlayment to acclimate to the conditions of the structure.

Make sure that each sheet is spaced correctly to allow proper air movement between each sheet. The best way is to place each sheet of underlayment in the center of the structure to eliminate any outside moisture influence. However, if this cannot be done, the underlayment should be stored in a dry warehouse, standing on edge. This allows the air to flow around each sheet.

Here are some additional tips to ensure a great underlayment installation.

Photo 1: Most installers will use a moisture meter to check the moisture levels between the subfloor and the underlayment. These results should be documented for future reference. If any major differences of 4 percent are noted, the installation should be held-up until an acceptable level is achieved.

Photo 2: Use the proper fastener 3d ring shank for panel thickness up to 1/2”, and 4d for panel thickness over 1/2”. Note the fastener should not be longer than the combined thickness of the underlayment and subfloor. Any blow-through of the fastener will result in poor fastener withdrawal resistance.

The use of staples is an acceptable method of fastening underlayment.  The staple recommended is a 1/4” crown staple, fastened with a mallet-driven stapler.

Photo 3: If stapling, the use of a mallet-driven stapler is recommended. While many installers prefer to use a pneumatic-type stapler, the air-driven stapler has a tendency to overdrive fasteners and/or leave oil spots on the underlayment that will result in staining of the resilient flooring.

Photo 4: The installer must leave a minimum of 1/4” around the perimeter of the room to start the installation. This allows for any expansion or contraction of the substrate, without any effect on the field of the underlayment.

Photo 5: Keep back a minimum of 6” in both directions from subfloor seams. At least two nail patterns will ensure that the subfloor will not affect the underlayment installation.

Photo 6: Sand all joints of the subfloor. Any ledges will create a hollow, which is likely to be hard to fasten and also to show through.

Photo 7: Place a chalk line in order to get started in a straight line. Be sure to leave a 1/4” at the perimeter walls.

Photo 8: Install the panels in a staggered joint pattern, never allowing all four corners to come together at any one spot. Always thoroughly fasten each sheet as they are installed.  This reduces the tendency to fasten-in any fullness.

Photo 9: When fastening, be sure to work from the seam sides to the open sides of each panel, to prevent any unnecessary fullness being forced into the panel. Following this step improperly may result in bubbles in the finished floor.

The panels are to be butted lightly with no fullness or gaps, and the fasteners should be within 3/8” of the edge of the panel. The interior nailing should be done on a diagonal fan pattern, to eliminate the straight-line pattern.

Photo 10: Belt sand every joint to ensure each joint is level with one another. Failure to do so will result in a show-through issue. With this type installation, it is not necessary to do any type of filling. If followed correctly, show-through should not be an issue. Remember that you must use factory edges throughout the installation.

Underlayment installation is just as important as the installation of the resilient itself, and failure to follow any of specifications can result in a failure of the entire installation. There is no cure for any show-though that takes place days after the installation is completed, short of ripping everything up and starting over. So make sure not to take shortcuts or ignore crucial steps. Do it right, and do it well.