Spline is a challenging subject to write an extensive article on. For starters, spline plays one exclusive role within the installation, which is never seen nor appreciated by the end-user. Spline is also one of the most critical components of any properly installed wood floor. This article details when spline is required, proper installation methods of spline, perspective from a spline manufacturer, and a few other industries that utilize spline in different applications.
Spline is known by many names, depending on geography and depending on what you were originally taught to call it. Some common names include slip-tongue, false-tongue, loose-tongue, tongue-spline, and probably many others. In this article, it will be referred to as “spline.”
The singular purpose of wood floor spline is simply to allow two pieces of wood flooring to be joined together to make a single flat surface, while maintaining consistent tongue and groove engagement through the entire flooring installation.
For most wood floor installations, spline should be a regular component of what is delivered to every jobsite. Spline is required in each of these situations:
- Anytime the wood floor installation requires direction changes.
- When reversing installation direction, such as starting in the center of a layout or back-filling.
- At all headers or flush transitions.
- When installing or assembling any parquet pattern where two pieces of wood adjoin.
The singular purpose of wood floor spline is simply to allow two pieces of wood flooring to be joined together to make a single flat surface, while maintaining consistent tongue and groove engagement through the entire flooring installation.
One of the most important elements of spline is how it matches the specific flooring profile. Not all splines fit the same groove profile of all flooring. ¾” spline is readily available at most flooring distributors.
Todd Hooper, the third-generation owner and president of Log Power, notes, “The dimension of the spline is the primary concern we focus on. Our priority is to ensure it fits. All the spline that we produce is milled precisely to meet the NOFMA groove configuration for ¾” flooring.” Hooper has transitioned 100 percent of his company’s energies into producing spline for the wood flooring industry. They currently are producing more than one million feet of spline per year.
Manufacturing spline can be dangerous and tricky. The small profile of the material requires precision and a keen eye for quality. Hooper exclusively uses a kiln-dried, prime-grade poplar to produce their spline. He said they do this for a couple reasons: 1- it is the cleanest wood to mill into such a small profile (oak splits too easily), and 2- it is an inexpensive species (pine mills easily but is expensive).
Some manufacturers also may create some spline for 5/8” flooring and some for ½” flooring, but this is not common. The reason is because not all groove profiles are identical for all flooring, especially when you get into the engineered flooring tongue and groove profile variances. In these cases, you may need to make your own spline. This can be done by using a table saw to rip down some quality plywood or flooring material to a dimension that allows the spline to snugly fit both grooves of two adjoining boards.
With wood flooring, spline is a crucial component of the entire installation, with the ultimate purpose of joining two boards together. The term “spline” is used in other industries as well (although not always identical in purpose).
In cabinetry and other fine woodworking applications, spline is considered a type of joinery, similar to biscuits and dowels. Like what we use in flooring, they are a thin piece of wood placed within matching grooves of two pieces of wood that are to be joined together, commonly at the miters, to reinforce the joint (Image 1).
Structural insulated panels used in some residential and light commercial construction utilize splines to connect panels together and, in some cases, to increase the overall load capacity of the panels. The splines are designated as surface splines, dimensional lumber splines, and engineered lumber splines. (Images 2 and 3).
In suspended acoustical ceilings, a spline (also known as slip-tongue) is a strip of metal that is inserted into two matching slots between adjacent acoustical tiles, forming a concealed mechanical joint (Image 4).
In the window screening business, spline is a vinyl cord that holds the window screen material into the screen frame by using a very specific spline tool.
In mechanics, a spline is a type of gear. The spline gears have a ridge or tooth on a drive shaft that matches with a groove in the mating piece which transfers torque from one member to the other. The most common types of spline gears are parallel key splines, involuted splines, and serrations.
Spline also exists in the computer-aided design (CAD) and computer graphics fields and in mathematics. This type of spline refers to a piecewise polynomial (parametric), which generally is used to approximate the overall surface curve in an effort to impose a smooth surface. (Only Lenny Hall understands these types of splines.)
Historically, spline was a naval architectural shipbuilding term referring to a long strip of flexible wood bent into the curved shapes commonly used to scribe the smooth cross sections of the boat hull, a technique called “lofting.”
Not spline, but slip-tongue, is a term that dates back to the old logging days. The “Slip-Tongue Log Skidder” was a horse-drawn wagon with very large wheels used to drag logs through rough terrain to logging railroad staging areas.
Spline is a necessary component in many products and plays a similar role with each. You could argue that the term itself is a universal term used for similar purposes regardless of which industry. The history and many applications for spline make this unseen product one of the most crucial to any installation.
Brett Miller is the vice president of technical standards, training, and certification for the National Wood Flooring Association in St. Louis. He can be reached at brett.miller@nwfa.org.
One of the little known and seldom taught installation methods for solids in high humidity markets requires the use of a spline. Solids attempt to expand and when doing so they move in the direction of least resistance. That direction is toward the tongue or head of the fastner. It’s easer to push in that direction than the opposite as the fastener has difficulty bending.
There is far more movement of the flooring in wall to wall installations than center- out. Center-out is where the spline is needed.
Gyms are almost universally installed that way.