Regardless of what type of flooring you install, it often comes down to wood. Installers have been using wood underlayment for generations because it is a renewable resource that is relatively inexpensive, and highly adaptable. Simply put, wood is uniquely well suited to the task of supporting almost any type of flooring you install on top of it. But out of sight does not mean out of mind. You may not see the underlayment once the floor is installed, but it will surely affect the way that floor looks and performs over the long term. So, with that in mind let’s take a look at some of the questions that pop up when you are laying down wood.

What are the proper fasteners for securing 1/4” wood underlayment?
The Engineered Wood Association (formerly the APA) indicates the proper fastener to be a 3d ring- or screw- shank nail, minimum 12-1/2 gage (0.099”) shank diameter for underlayment 1/4 ” to 1/2 ” in thickness. 4d for panels thicker than 1/2”.

How close should the nails or staples be from the edges of the underlayment panels?
The distance from the edges of the underlayment joints is generally not to exceed 3/8” back from the joint. This stops any type of peaking of the underlayment panel that would show through the finished floor.

How far should the underlayment joints be off set from the subfloor joints?
The general consensus is 6 to 8”. The farther the joints are offset the less chance of movement being deflected from the subfloor joint to the underlayment joint. This should apply to both the side and end joints.

Should the fasteners for underlayment extend down into the floor joists?
No, the subfloor should be fastened to the joist system, but the underlayment should be fastened to the subfloor. Underlayments should not be glued to the subfloor unless the goal is to increase the subfloor flexural strength. Once a fastener extends through the subfloor panel it tends to pull the grain with it weakening the fastener’s holding power.

Should the underlayment joints be butted tight, lightly or spaced?
That depends on the manufacturer of the underlayment. Most manufacturers recommend the joint to be butted lightly, while others recommend the joints to be spaced. None of the manufacturers recommend the underlayment joints to be butted tight. Most manufacturers warn against fastening any fullness into the panels. This means when the fastening is done it should be done by starting next to the adjoining panel working to the open side of the panel. Nail only one panel at a time. Tacking down all or the panels and coming back and fastening them all at one time is a complaint waiting to happen.

What are the patching techniques use for underlayment joints?
The best way is to butt the joints and sand the joints to smooth and use no filler unless a specific joint needs to be filled. If you do have to fill the joints keep the compressive strength of the patching compound as high as possible. To achieve this you must follow the patch manufacturer’s water to powder recommendations to the letter. You will note that the patching compound will be a little thick and is limited in its coverage; however if you use a power mixer you will find the speed in which the patch is whipped will break the surface tension of the water used in the mix, producing a smoother and thinner mix to work with. To aid in the spreading of the patch, slightly dampen the joints with a damp rag to eliminate the absorbency of the underlayment, making the patch easier to spread. Do not add more water to the patch.

How important is acclimation to underlayment panels?
Underlayment panels should be protected from moisture prior to their installation. Many installers of underlayment believe that underlayment stored in a dry warehouse is adequate for acclimation. Not true. In order to acclimate underlayment the panels should be removed from the skid and stored on edge in the room(s) where they are to be installed for several days prior to installation. Underlayment in skids can take weeks to acclimate. If you cannot get the underlayment to the job site at least get it out from the skid and allow it to acclimate in the shop, providing the shop environment is close to the job environment.
What is the main cause of squeaks in a wood floor system?
There are two main causes of squeaks in wood floors and both are related to movement. It could be that the edges of wood panels are rubbing against each other. Or, it could be that a piece of wood is moving “up and down” on the shank of the fastener. Some squeaks are installation related. Usually it is a result of the drying of the wood, which allows it to shrink and create the movement.

What causes tunneling of a sheet vinyl directly over an underlayment joint?
Tunneling is a de-bonding of the sheet vinyl and/or patching compound directly over the underlayment joint. The cause is an excessive movement of the underlayment at the joint. There are several causes for the movement: moisture, improper fastening, over watering of the patching compound, low compressive strength patching compound, improper fastening of the underlayment joints and in some cases the edge treatment on the wood panel edges.

What causes nail-heads to show through the surface the sheet vinyl?
Occasionally, fasteners may “pop” or “back out.” In such cases, fastener heads sometime rise above the underlayment surface and “telegraph” as bumps through resilient floor covering. The best way to prevent nail popping is to use the proper fasteners. Ring and screw-shank nails have a higher withdrawal resistance than a staple, but be sure to use a fastener length approximately equal to the total thickness of the underlayment and subfloor and assure that the subfloor is dry. Installing underlayment over a damp subfloor allows the wood around the fastener to shrink. The fastener appears to have backed out, but it is the wood around it shrinking. This shrinking of the wood will also push up any patch that was used to fill any fastener heads.

Again big thanks for the info from JJ Haines.