Be sure to check the moisture content prior to the installation of an underlayment. The moisture content should be no more than 13 percent for wood underlayments. Shown is Wagner Electronics MMC220A moisture meter, courtesy of Wagner Electronics.

Part of being a professional is knowing when to seek help or advice. Ever since I began writing this column, there have been certain questions from my readers that have come up again and again. Consider this my Top 10 list of frequently asked questions when installing wood underlayments.

Q: When installing wood underlayment is it necessary to allow for an expansion zone (similar to installing hardwood flooring)?
Yes, all wood underlayment installations should leave about a 1/4” expansion zone at the walls or fixtures. The reason for this is all wood moves with humidity, and if a wall or fixture is too tight the underlayment panel will tend to buckle or the joints will peak.

Q: Which type of underlayment fastener has the best holding power?
A ring or screw shank nail has almost double the holding power of a staple. (This is providing you use a ring or screw shank nail.) If I am using a stapler, I would use more fasteners than if I were using a nail. A smooth shank nail used for underlayment has a diminished holding power.

Q: Is there a moisture limit for subfloors when installing an underlayment over it?
Like any hardwood floor, there is a maximum moisture content of 13% for wood underlayments. Unfortunately, not many installers check the moisture content prior to underlayment installation. This needs to be completed as part of new construction and remodel installations.

Q: What is the proper length of the fastener to be used when installing underlayment?
According to the Engineered Wood Association (APA) the fastener’s length should be as follows. For 1/4” – 1/2” panel thickness, use 3d – (1-1/4”) ring or screw shank nails with a minimum 12 gauge (0.099”) shank diameter. For 19/32” – 3/4” panel thickness, use 4d – (1-1/2”) ring or screw shank nail with a minimum 12 gauge (0.099”) shank diameter. The fastener should not be longer than the combined thickness of the underlayment and subfloor.

Q: How close to the underlayment edge do I need place a fastener to prevent peaking of the underlayment panels?
For 1/4”and 3/8” panels, the fastener should be within 3/8” of the panel edge; for thicker panels, stay within 1/2”.

Q: Some underlayment manufacturers do not recommend the use of a patching compound. What are your thoughts on patching panel edges?
In years past, I used to always patch underlayment joints, but construction practices have changed and my opinion about patching has as well.

In the construction practices of the past, the lumber was kiln dried, but today the lumber is surface dried and has higher internal moisture content. When installing underlayment today, butt the panel edges lightly and sand them to smooth, with no patching. Always use factory edges so the panel joints are straight and clean. If you patch the joints you stand a good chance of pushing up the patch, creating an underlayment joint show-through issue which cannot be repaired.

Q: How far should you offset underlayment joints from subfloor joints?
The offset placement of underlayment joints should be a minimum of 6”. I plan on two rows of fasteners past the joint in the subfloor. This offset needs to occur in both directions.  

Q: Some installers believe that gluing down of underlayment panels is a good idea. What are your thoughts?
My question to that is what do they hope to accomplish? If the goal is to enhance structural integrity of the substrate and the underlayment is 1/2” or thicker, it is something to consider. If it is just to bond the underlayment to the floor, I find it presents more problems than solutions.  

Q: Are there ever any circumstances for using a vapor retarder under underlayment?
Yes, I have observed the use of a 4-mil poly under underlayment in new home construction where they did not want the underlayment to be affected by the moisture in the crawl space and subfloor. This works fine as long as you don’t exceed more than 65% of the floor.

Q: What is the best method to fasten underlayment panels?
The best method is to lay out one course of underlayment panels at a time. I will chalk line my starting course and lay out each panel of that course. Then I will fasten each panel completely before I proceed to the next one.

The reason for doing one panel at a time is I do not want to nail any fullness into a panel. I always work from the closed side to the open side. Once I completed a panel I will go to the next one.
I try to fasten the joints first, then work on the interior fastening. I prefer the mallet-driven fastener because it tends to draw the panel down better than a pneumatic. Pneumatic nailers tend to overdrive the fastener, which can weaken the long-term hold.

I would like to thank my readers for asking some great questions. If you are ever stumped on a resilient flooring installation, feel free to contact me.