NOFMA Tip Sheet: Proper Subflooring for Mechanically Fastened Wood Flooring
There are two basic flooring systems: 1) conventional, framed systems over a crawl space, basement, or another similar floor space; and 2) concrete slab, on grade or suspended. The two are different systems and some of their requirements can be similar and other requirements can be quite different. Each system will be addressed separately.
Conventional framed system
Some typical framing members are: solid wood joists, engineered "I" joists, and 2-inch-by-4-inch open web truss joists. These members may be spaced 12 inches, 16 inches, 19.2 inches, and 24 inches on center; most often the wider spacing is associated with the engineered members and trusses. Post and beam construction may have even wider spacing between the supporting beams with 48 inches as typical. Post and beam construction is mostly encountered in the northwest.
A framed subfloor system should be sufficiently stiff to allow minimum movement. We hear the term, L/360, as the term for the greatest allowable deflection at center span. This means that for a span of 20 feet, the deflection can be just over 5/8-inch at center span. Today in order to accommodate the wide-open areas in homes, these and longer spans are typical, thus significant deflection can occur along the length of joists. For less deflection a stiffer joist system is needed, such as L/480 or L/560 and greater. In addition, wider spacing than the traditional 16 inches OC between supporting joists allows for even more variation. These systems should all pass building codes and support the intended loads; however, with wood flooring the expected performance may not be achieved with maximum joist spacing, maximum spans, and minimum thickness subflooring.
Since wood flooring is normally placed in homes that are not designed to meet the bare minimum performance requirements, a system that exceeds the minimum is recommended by NOFMA. The APA, The Engineered Wood Association, has a recommended system called a Code Plus System that shows the use of thicker panels, a step above the minimum performance rating. NOFMA's minimum thickness recommendation for 3/4-inch solid T & G strip wood flooring is 5/8-inch (19/32-inch) plywood sheathing and 3/4-inch (23/32-inch) for OSB. The 5/8-inch thickness is the minimum fastener to wood contact for good performance of flooring. The reason for the thicker requirement with OSB is that tests have shown the thicker 3/4-inch OSB holds wood flooring fasteners only as well as 5/8-inch plywood. All panels should be performance rated and carry a stamp by the rating agency, such as APA, TECO, CAN-PLY, etc.
The "code plus" systems require 16 inches OC or less joist spacing for 5/8-inch plywood; 19.2 inches OC or less for 3/4-inch OSB or plywood; and 24 inches OC or less spacing for 7/8-inch panels. These systems are stiffer, reducing deflection and movement, thus minimizing the stress to the flooring fastener to subflooring connection.
The stiffer, code plus, systems are also generally recommended for engineered flooring, since these products are normally thinner and add minimally to the strength of the system. Many of the manufacturers have proprietary specifications, so follow manufacturer's recommendations; you've all heard this from me before and will hear it again.
Prior to the flooring installation, the house environment must be at near occupied conditions. The subflooring should be clean dry and flat ("Subfloor Prep," FCI Oct 2003). Flat for the minimal framed system may not meet the minimum requirement. For the L/360 and a 20-foot span, the 10-foot deflection allowed can be 1/3-inch, which is more than the 1/4-inch most commonly required for flatness. Further, the subfloor may be flat enough before it is loaded with flooring, furniture, and people, but not after the loads are applied. Again where a minimum system is in place, inform the customer ("Managing and Meeting Customer Expectations," Jan-Feb FCI 2004) that the final performance may not meet their performance expectation.
A less common subfloor is a solid board subfloor. You will occasionally find one in a new installation in the "Sunny South," but will often encounter them in older homes that are being remodeled. Solid board subflooring should be no wider than 6 inches and placed on a diagonal to supporting joist. The boards should also be a species of Group 1 dense soft woods or equivalent such as, SYP, Larch, or Inland Doug Fir.
For solid 3/4-inch thick flooring on a slab, the minimum thickness subflooring is 3/4-inch plywood. For 1/2-inch and thinner solid flooring and engineered flooring, 5/8-inch plywood is the minimum recommended thickness. OSB is not recommended since fastener performance as tested is less than similar thickness plywood. The 3/4-inch plywood may be installed as full sheets placed on a diagonal or as smaller units cut into either 2-foot-by- 8-foot or 16-foot-by-8-foot planks and placed perpendicular to the direction of the flooring. The full sheet diagonal placement keeps the flooring from following a seam and resulting in a panel influenced gap or buckle after installation.
The full sheets should be fastened with appropriate concrete fasteners such as concrete screws, nylon split fasteners, concrete nails, etc. Use at least 21on a 3-by-7-by-16-inch grid, 8 inches from the edge, 14 for the 2-by- 8-foot and 7 for the 16-inch-wide planks; additional fastening may be necessary for loose areas. The cut panels, 24-inch and 16- inch widths, may also be glued with cutback adhesive over the previously glued vapor retarder. They must be kerfed across the back by 1/2 the panel thickness, 3/8-inch for 3/4-inch plywood and 5/16-inch for 5/8-inch plywood (remember the 5/8-inch plywood is for less than 3/4-inch-thick flooring). The kerfing allows the panels to lay flat and also gives the adhesive opportunity to "grab" the panels as it flows into the kerf.
The 16-inch planks may also be floated, not attached to the slab, for issues such as radiant heated floors and where sound deadening is a requirement. Also, two layers of 1/2-inch plywood may be floated. The 1/2-inch panels should be fastened together with staples or screws and the bottom layer oriented on the square of the space and the top layer on a diagonal to the first.
Another subfloor system over a slab is the sleeper or screed system. The sleepers should be from group #1 species, pressure treated and dried after treatment, 2-by-4s positioned perpendicular to the flooring direction and 12 inches O.C. or less depending on flooring grade. They can be glued or fastened to the slab. The sleepers are generally cut from 18-inch to 48-inch lengths in order to lay flat. They can also be kerffed on the face and back to lay flat. Shorter flooring lengths require closer spacing to 9 inches O.C. 3/4-inch-thick solid boards and 3/4-inch-thick strips of plywood are not recommended as sleepers since the proper nailing frequency cannot be maintained unless they are placed solidly over the entire floor.
The sleeper system is commonly found in the south. Sleeper systems can also be found in other areas especially with radiant heating systems and the gypsum type slurry poured between and over the heating tubes. The 12-inch maximum sleeper spacing requirement is for 3/4-inch-thick flooring less than 4 inches wide nailed directly to the sleepers. The wider flooring, 4 inches and more, requires a subfloor of 3/4-inch plywood over sleepers, and the sleepers can be spaced 16 inches apart. Engineered flooring and thinner solid flooring products must have the sleeper system with the subfloor. Again, follow manufacturer's directions with engineered flooring for subflooring requirements.
Some important points to remember:
• Framed systems with minimum thickness subflooring on maximum spaced and spanned joists meet minimum performance requirements.
• 5/8-inch plywood is the thinnest panel recommended over joists systems.
• 3/4-inch is the thinnest OSB panel with joists systems.
• 3/4-inch plywood is the thinnest single layer panel recommended for a slab.
• OSB is not recommended for slabs and 3/4-inch flooring.
• Sleepers can be spaced a maximum 12inches for direct nailed narrow flooring, less than 4 inches.