Q. Instead of glue and nails, my framing subcontractor used screws to fasten the sheathing to the I-joists of a new 14-foot-wide addition. While there are no interior walls bearing on the floor system and no excessive notching in the I-joist flanges — and the joists fall well within the APA’s allowable span guidelines — my clients say their floor is excessively bouncy. Does the use of adhesive contribute to the stiffness of an I-joist floor system? Would it help to add midspan blocking?
A. Bryan Readling, P.E., senior engineer at the APA/Engineered Wood Association, responds: Midspan blocking — typically used with sawn lumber joists — isn’t normally recommended for I-joist floor systems. In a floor framed with sawn lumber, blocking helps joists share loads so that a weaker joist may be reinforced by stronger joists on either side. Blocking also helps to vertically align twisted joists, improving their performance. But since I-joists are already straight and uniform in strength, blocking doesn’t add significantly to their performance.
Construction adhesives prevent movement of the floor sheathing relative to the joists, eliminating a common source of squeaks: vibrations created as fasteners rub up and down within the floor sheathing. Glued floor sheathing also increases each joist’s effective cross-sectional area and stiffness. The enhanced bond allows the joist and sheathing to act together in response to short-term loads, like foot traffic. In fact, not using glue with either nailed or screwed-down floor sheathing could reduce the allowable span of a residential I-joist by as much as 12 inches.
It’s unlikely that your floor joists are overspanned. The span tables for most I-joist systems are based on an L/480 deflection, stiffer than the building-code minimum of L/360. (The tables also assume glued-nailed sheathing.)
Deflection of the floor sheathing between the joists shouldn’t be contributing to the problem, since 23/32-inch APA-rated Sturd-I-Floor can span up to 24 inches, according to code. With joists 16 inches on-center, this floor sheathing has a “code-plus” allowable live load of 240 pounds per square foot. And using a liberal amount of floor-construction adhesive (I like to see squeeze-out on both sides of the joist) can stiffen the floor sheathing even further by limiting rotation at the connection.
The easiest way to improve a bouncy floor is to add furniture and carpeting. Extra mass dampens floor vibrations, while carpet and pad tend to soften footfalls and make vibrations less apparent.