Solid board subfloors play a great role in the history of wood flooring.
There are three primary types of wood subflooring materials that are considered acceptable below a wood floor installation: plywood, oriented strand board (OSB), and solid boards.
Plywood is a board or panel made of cross-directional veneers and/or layers of wood for dimensional stability. According to the Engineered Wood Association (APA), the first patent for plywood dates back to 1865, although it wasn’t until the late 1930s and early 1940s when plywood subfloors became recognized for use as a general building material. Plywood then was declared “an essential war material” during World War II. At the end of the war, during the postwar economic boom, plywood became a standard subfloor material for new construction, and remained the standard for the next several decades.
In the late 1970s and early 1980s, the growing concept of reconstituting
wood fibers in order to produce new building materials led to the development of OSB. Instead of solid sheets of veneer, OSB is made of small wood strands that are glued together in cross-laminated layers. Today, most of the subflooring used in new construction is made almost exclusively from OSB panels. During recent years, many of these OSB panels have evolved into what is known as a “high performance” alternative to the older commodity OSB products from 30-50 years ago.
Prior to the plywood and OSB sheet goods we are most familiar with today, solid board subflooring was the go-to material used as wooden subfloors. This type of subfloor has had a long, diverse history.
Homes dating to colonial America commonly have solid wood subflooring made of very old, wide-plank materials, measuring upwards of 24” in width. In the days they were produced, they actually were not referred to as “subfloors,” they were simply, “the floors.” These floors were made of long, thick, and wide planks that were cut from the abundance of native trees from the land where the building was being built. In time, the supply of local old growth trees depleted, and the floorboard widths narrowed.
Common species of wood used for these floors included cedar, Douglas-fir, eastern hemlock, eastern white pine, Idaho white pine, longleaf pine, ponderosa pine, red pine, redwood, southern cypress, southern yellow pine, spruce, sugar pine, tamarack, western hemlock, western larch, white fir, and other regional species.
Converting the timber from the land into usable dimensional lumber was a herculean process. The predominant method used to cut dimensional lumber for the home was to pitsaw the logs into planks. A pitsaw is a large saw with handles on each end that requires two people to operate it; one stood in a pit beneath the log, while the other was perched above it. Working together, they pushed and pulled at opposite ends of the saw, carefully following a straight line that indicated the direction of the cut.
The edges of these rough-sawn planks were squared, laid side by side, and face-nailed into the floor joists or beams with hand-forged nails. The carpenters often flattened the floors by chiseling the planks from the underside to fit the uneven joists. Once installed, these floors were usually left bare, and became naturally worn through years of use.
With the invention of the side-matcher in 1885, wood flooring began to be mass-produced. It was around this time that the solid boards became the substrate to these mass-produced wood floors. The subfloor planks that were used may have been square-edged, shiplapped, or even tongue and grooved (see image below).
TOP, RIGHT TOP, AND RIGHT BOTTOM: Variations in hammer chisel marks provide clues about the workmen and the ways they used their tools. Often flooring was chiseled to fit joists.
ABOVE: With the invention of the side-matcher in 1885, solid boards became the substrate to these mass-produced wood floors.
A paper written by the University of Virginia, and published through the United States Department of Agriculture (USDA), Forest Products Laboratory, dating back to 1961, began to define proper uses and installation methods for solid board subfloors. The following is an example of some of the guidance that was provided in this paper for these types of subfloors:
- A solid board subfloor served the following functions:
- It brings the tops of the joists to a common level
- It helps stiffen the floor
- It blocks off cracks that may develop in finished flooring
- It serves as a safe working surface for workmen during the construction process
- It provides a level base for finished floor and partition soleplates
- Joist spacing requirements for these solid board subfloors were required to be no greater than 16” on center (OC).
- With square-edged or shiplapped boards, all of the end joints were required to land on a joist. Boards may have also been laid on diagonal or perpendicular to the joists.
- These boards normally were laid with spacing between planks for drainage purposes (when exposed to rain during construction). In fact, it was not uncommon to feel a draft or to see the basement lights showing through the boards.
- Boards were face-nailed with a minimum of two eight-penny nails on each joist, unless the planks were 8” wide, where they would require three per joist.
- For the subfloor materials that were tongue and grooved and end-matched, it was required that their length must be long enough so that each board would rest on at least two joists, and installed so that the end joints of adjacent boards would not occur between the same pair of joists. The tongue and groove flooring is nailed straight to the floor joists through the topside of the tongue (same as a wood flooring install).
- When these subfloors were used over beam framing rather than joists. The planks were required to be 2” nominal thickness material, with tongue and grooved, or splined edges. They were not to exceed 8” widths.
According to NWFA Guidelines, solid board subflooring is typically ¾” x 5½” (1” x 6” nominal), Group 1 dense softwood, No. 2 Common, kiln-dried material. NWFA Guidelines also state that solid-board subflooring should be installed on a 45° angle to the direction of the joists, with all board ends full bearing on the joists and fastened with minimum 8d rosin-coated or ring-shanked nails, or equivalent.
These solid board subfloors are not the only style of solid board subfloors that we run across, so what happens when you run across a subfloor that doesn’t fall within this definition?
First and foremost, try to talk the owner into simply refinishing the “treasure of a floor” they are living on. There is nothing more jaw-dropping than an old restored wooden floor. However, some homeowners don’t like the basement drafting through the gaps, or seeing light shine through the floorboards. In these cases, follow the NWFA Installation Guidelines for installing over an existing wood floor that has been installed directly to the floor joists.
As NWFA Guidelines indicate, the only wood flooring you can install directly to a solid board subfloor or an existing wood floor is ¾” solid or engineered wood flooring. These types of floors can be installed perpendicular to or on a diagonal to the direction of the existing flooring.
With solid or engineered flooring less than ¾” thick, parquet flooring, or end grain blocks, you will need to overlay the existing floors with a minimum 11/32” subfloor panels. When overlaying the existing floor, do not fasten the wood panel material to the joists; only fasten it to the existing floor. For more information about proper installation methods of wood flooring, refer to NWFA Installation Guidelines.
The treasures that are uncovered in historic buildings are priceless. Solid board subfloors play a great role in the history of wood flooring. Considering that an old wood subfloor that was once a part of the old growth forest from the local land should give us peace in knowing that those floors still exist, and continue to provide a role in our everyday lives. The next time you are asked to restore one of these relics, consider it an honor to be able to bring the wood back to its glory. Or, if you are asked to install a new wood floor over the top, just know the solid wood subfloor is a critical component of how the new floor will perform.
Brett Miller is the vice president of technical standards, training, and certification for the National Wood Flooring Association in St. Louis, Missouri. He can be reached at firstname.lastname@example.org.