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Running With the Joists

Q: When is it safe to lay hardwood floors against conventional wisdom?

The This Old House crew
Photo by Webb Chappell

We want to add solid oak flooring to several rooms in our home. The subfloor is ¾-inch tongue-and-groove plywood, which has been screwed and glued to floor joists that are 16 inches on center. Flooring contractors we've talked to say that because of the subfloor system used, we can orient the hardwood flooring parallel to the floor joists, rather than the standard method of running them perpendicular to the joists. Is that true? And would there be any advantage to gluing the tongues into the grooves (not to the subfloor) to keep the hardwood from separating during seasonal changes?
— Judy and Kurt, Gurnee, Ill


Pat Hunt replies: The reason for running wood flooring perpendicular to the joists in conventional construction is that it provides a stronger, stiffer floor system. But because your subfloor is beefier than conventional construction, I'm not surprised that some flooring contractors would feel comfortable running the flooring parallel to the joists.
Even so, the major flooring trade associations take a more conservative approach. According to the National Wood Flooring Association, for example, finish flooring that runs parallel with the joists should have a layer of ½-inch plywood underlayment on top of the existing subfloor (with the long dimension perpendicular to that of the first layer), or 2x6 blocking between the joists on 24-inch centers. Also, check with your flooring manufacturer; running a floor this way may void the flooring's warranty.
I don't think you'll find anyone who recommends gluing solid wood floorboards together. Doing so would prevent the floor from expanding and contracting as it must when the humidity changes, and that in turn will eventually lead to cracking and other problems.
(Flooring contractor Pat Hunt, of Hunt Hardwood Flooring in Lexington, Mass., has installed new wood floors in several This Old House projects.)


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