Q: The flooring article (“Installing Prefinished Strip Flooring”) in the November 2015 issue showed the boards being installed tight to the baseboards. Shouldn’t there be an expansion space around the perimeter?
A: Howard Brickman, a wood-flooring contractor and consultant based in Norwell, Mass., responds: Over many decades of installing wood flooring as well as studying the science of building materials and wood technology, I have come to realize that many of the “rules” regarding wood flooring are based on a deeply ingrained (pun intended) mythology and not necessarily on fact.
The first myth is that wood is a “living and breathing” thing, so wood flooring needs plenty of expansion space. The underlying truth to this myth is that wood shrinks and swells when it loses or gains moisture. But this movement does not define life.
The second myth, which builds on the first one, is that wood and water don’t play nicely together. But water is an integral part of all wood. (And once the use of water-based finishes became widespread, this myth lost a lot of its mojo.) Here’s a fun fact: A 10-foot-square oak floor at 8% moisture content (MC) contains 2.7 gallons of water. For a more robust explanation of the relationship between wood and moisture, read The Wood Handbook (free online from the Forest Products Laboratory) or Bruce Hoadley’s book, Understanding Wood (not free, but more fun to read).
PREVENTING MOVEMENT IN WOOD FLOORING
Setting the myths aside, there are three things we can do to prevent solid tongue-and-groove wood flooring from moving.
The first is to control the moisture in the house. In the U.S., most single-family residences are wood-frame construction with plywood or OSB structural subflooring fastened to solid-wood or engineered joists. The wood flooring is then blind nailed with cleats or staples directly to the structural subfloor. Because flooring can swell if exposed to excessive moisture, it’s critical not only that the building interior is dry when the flooring is installed, but also that the subfloor is dry. In a perfect world, wood flooring with an 8% MC performs best when installed on a subfloor that also has an 8% MC. If there is a difference in MC, the bottom of the flooring and the top of the subfloor will start to swap moisture as soon as the flooring is nailed down. The solution is not to stop the swap, but rather to dry out the subfloor before the installation. Every percentage of difference in MC increases the risk that the floor will not stay flat over time.
The next strategy involves fasteners. The blind nails that you drive into T&G flooring stay in place by the friction between the nail surface and the surrounding subfloor material. As you know from experience, pulling a nail out gets easier once it starts to move. Any lateral movement of the wood flooring will loosen the nails, resulting in a loose and noisy floor. So the second way to prevent lateral movement is to use lots of fasteners. I recommend driving nails every 4 to 6 inches.
OK, we’ve dried out the subfloor and nailed the heck out of the flooring, but dang, the surface of the new floor still became cupped and wavy—not smooth and flat—even though no visible water had gotten on the floor. This is because water exists as a gas suspended in the atmosphere, described with the term “relative humidity” (RH). If you expose wood to a higher RH, given enough time, its MC will increase and the flooring will swell. Conversely, if you expose wood to a lower RH, the MC will decrease, and the flooring will shrink. But there is a “Goldilocks” MC for your climate zone: halfway between the winter low and the summer high. In the Northeast, that’s 7.5%. It’s a bit higher in the South and a bit lower in the arid, high-altitude regions of the West. So the third factor is to make sure the flooring is at the right MC for your part of the country.
EXPANSION SPACE OR NOT?
So what about the expansion space? The angle of the nails keeps the flooring from pushing back toward the starting wall, so no space is needed there. Wood doesn’t shrink or swell much along the longitudinal axis, so no space is needed at the ends where they butt up against the wall. And if you use enough fasteners to prevent lateral movement, then little or no expansion space is needed along the finishing wall. A dry subfloor plus correct flooring MC plus a lot of fasteners equals the best wood-floor performance. The simple fact is that when wood flooring is exposed to excessive moisture, the floor will cup—even if left with a quarter of a mile of expansion space.