A flooring contractor called me to ask why the No. 1 common red oak floor installed six months before in a remodeled house was making slight crackling noises in various areas.
The distributor checked the wood flooring's moisture content upon delivery, and it was at 6 to 7 percent. Installation began three days later and took three days. The installed flooring remained unfinished for three months while the rest of the remodeling was completed. Approximately one month after moving back in, the homeowner began to notice the noises.
To find the source of the noise, you must figure out why the floor is moving.
When I stepped on individual strips, the only up-and-down movement was occasionally at the ends of strips. Movement there indicates that the nailing pattern is improper, or that the tongue or bottom of the groove is broken or absent. I examined the nailing pattern using a magnetic nail finder. Nails were placed on average just more than 5 inches from the ends of strips and ranged from less than 1 inch to 9 inches from the ends of strips (NOFMA recommends placing fasteners 1 to 3 inches from each end of every strip). Nails were spaced 11 inches along strips on average, and spacing ranged from 6 to 15 inches (NOFMA recommends 8- to 10-inch nail spacing along the sides of strips, with up to 12 inch spacing acceptable). I estimated that 85 percent of the nails were farther from the ends of strips, and 24 percent of the nails were spaced farther along the sides of strips, than is recommended.
The joists were 2-by-10-inch solid pine boards spaced 24 inches on center, and the subfloor was two layers of 1/2 -inch-thick plywood (NOFMA recommends that joists be spaced 16 inches on center to maintain a stable subfloor system). Joists spaced wider than 16 inches may be to code, but this means that they are installed to a minimum standard that will maintain the structural integrity of the house and protect "life, health and property" (meaning people and furniture cannot fall through the floor). This may not be good enough to maintain a noise-free floor.
Up-and-down movement was not detected in groups of strips, indicating that the movement was not in the subfloor. However, with the joists 24 inches apart, there was likely movement in the joist system or between the joists and subfloor. This would show up as movement of the entire floor system and might manifest as slight crackling noises.
As often happens with complaints, it is likely a couple of items are acting together here. First, the nails were placed a little far from the ends of strips, and occasionally nails were spaced too far apart. Proper nail placement is crucial in preventing movement and noises. Second, joist spacing determines the flexibility of the entire floor system. If the joists can flex and move, then the subfloor and wood flooring will move with them.
How to Fix the Floor
If a house has joist spacing wider than 16 inches, then it may be necessary to strengthen the underfloor system to maintain a more rigid floor system. Simply laying another layer of plywood may not be sufficient to prevent this movement; the joists may need to be braced from beneath. Also, occasional face nails may be necessary to secure the ends of strips to the subfloor.
In the Future
Proper fastener placement is imperative to adequately secure the wood flooring to the subfloor, preventing movement. Also, if the subfloor system is at minimum standards, then extra strengthening may be required for a noise-free floor.