Floor Joist Size Code Requirements

Wood

• One factor affecting the maximum size of floor joists is the species and grade of the wood comprising them. For example, assume 2-by-6 floor joists that are spaced at 12 inches and supporting a dead load of 10 pounds per square foot. If they're made of grade 1 Southern pine, these joists can span (or have a length of) 12 feet, 3 inches. At grade 2, the span drops to 12 feet and at grade 3, it goes down to 11 feet, 10 inches. Changing the wood to spruce limits the span to 11 feet, 3 inches for grades 1 and 2, and 9 feet, 8 inches for grade 3.

Spacing

• Increasing the joist spacing from 12 inches reduces the maximum size of joists. Using the same standards as the previous example, but with 16-inch spacing, Southern pine spans at grade 1 are 10 feet, 11 inches; at grade 2 they’re 10 feet 9 inches; and at grade 3 they drop to 9 feet. With spruce, grade 1 and 2 spans reach 10 feet, 3 inches, while grade 3 spans become 8 feet, 5 inches.

Space Type

• Changing the supported space to a residential living area reduces the span because the floor experiences more movement and greater shifts in weight. All things being equal to the previous example, grade 1 of Southern pine spaced at 12 inches can reach 10 feet, 11 inches. At grade 2, it can be 10 feet 9 inches, and at grade 3, it drops to 9 feet, 4 inches. Spruce at grade 1 and 2 can span 10 feet, 3 inches, while at grade 3, the joist can span 8 feet, 8 inches.

Consideration

• Many other considerations can affect joist size requirements. They include the maximum weight it supports; the size of the joist (such as 2-by-8 or 2-by-12); and whether they're supporting cantilevers or balconies. Changing the joist material from wood to cold-formed steel also affects joist size. For example, a steel joist with the 550X162-33 designation and the ability to support up to 30 pounds in each square foot can have a minimum joist span of 11 feet, 7 inches, with 12 inches of spacing, and a minimum of 10 feet, 7 inches with a spacing of 16 inches.