Common lengths for floor joists range from 8 feet to 18 feet. The length, or span, of a floor joist varies based on the dimensions of the joist, type of wood used and the spacing of the joist.
Common floor joists are 2 inches thick. The width of the lumber can range from 6 to 12 inches, though joists that are 8 inches or wider are more common. Narrower joists require shorter spans to support the same load.
In most situations, the builder spaces floor joists 16 inches apart from one another. He takes this measurement from the center of an upright joist to the center of the adjacent upright joist. If he spaces joists farther apart than this, he reduces the span of the joist.
The type of wood used can have an impact on the spacing. In the United States, pine and fir varieties are the most commonly used framing lumber. Specifically, these species include Douglas fir, eastern white pine, yellow pine and spruce. Douglas fir is a highly desirable framing material due to its greater strength and aesthetically pleasing grain. Given identical spacing and lumber size, the recommended floor joist length can vary as much as almost 4 feet.