What Is the Spacing Between Typical Basement Rafters?

Standard Installation

• The most common spacing for the joists that form the ceiling of the basement is 16 inches on-center. On-center refers to the distance from the center of one joist to the center of the next joist. Taking into account the width of the joist, which for standard 2-by-10s or 2-by-12s, is 1 1/5 inches, the space between joists installed on 16-inch centers, is 14 1/2 inches. Standard 4-by-8 sheets of subflooring fit uniformly from the center of one joist to the center of another joist when the members are on 16-inch centers.

Trusses

• Floor trusses are pre-engineered at a truss manufacturing company and delivered ready-to-install to the jobsite. The wood used to build trusses is not a large as the dimensional lumber used for stick-frame joists, but trusses are every bit as strong because they contain cross members between the top and bottom chords. Trusses that form the ceiling of the basement generally sit on 24-inch centers. A 4-by-8 sheet of subflooring fits on 24-inch centers as well as it does on 16-inch centers.

The Black Diamond

• If you’ve used a carpenter’s tape measure, you might have noticed small black diamonds here and there on the tape. These diamonds are not an accident and they’re not spaced at random. They are exactly 19.2 inches apart. Like 16-inch centers and 24-inch centers, 4-by-8 sheets fit on-center when joists are 19.2 inches apart. The only difference is that the sheets must be positioned perpendicular to the direction of the joists. Architects sometimes choose 19.2–inch spacing increments when the floor will bear a heavier-than-usual weight load, or when a smaller-than-usual dimensional members are used to frame the floor.

Basement Rafters

• Rafters, by definition, are the structural members that form a roof. The only time a basement has rafters is when the basement ceiling is also the roof of the structure. This occurs in some berm-type houses and in basement additions that do not extend above-grade. When a basement ceiling serves as a roof, the rafters are generally on 24-inch centers.