Joists are the horizontal members that make up the structural framing of a deck, floor, ceiling or roof. Often made from wood or steel, they are engineered specifically to provide adequate support to various structural applications. A joist is supported at either end by either a beam, wall, or other framing member. They may also require additional supports, known as "intermediate supports," along their length. Joist span is the distance between the beams, walls, or other structures that support a joist along its length.
The total joist span is the distance from one end of the joist to the other. When sizing a joist or determining its span, builders must examine the design span rather than the total span. The design span is the length of the joist measured from the face of the supports at either end. For example, a 10-foot (3.05 m) joist resting on a 6-inch (15.24 cm) wall at either end has a design span of 9 feet (2.743 m).
The size and material of a joist is directly related to the span. The maximum length of joists is set by local building codes, and varies widely from city to city. A larger or deeper joist is capable of safely spanning a greater distance than a smaller joist, though the specific distance depends on the material of the framing members as well as on the design of the building.
Most joist manufacturers have their own span tables, which give recommendations for joist size based on the required span. These tables tell consumers how far a joist is expected to deflect, or move when subjected to a specific load. Each manufacturer produces a separate table for each type of joist it manufactures. Consumers can use these tables to choose materials based on the maximum deflection specified in the local building code. For very large joist spans, consumers may not be able to find large enough joists to meet code, and will instead have to add intermediate supports along the length.
As a general rule-of-thumb, joist depth in inches can be calculated by dividing the required span in half, then adding two. For example, a joist that must span 10 feet (3.05 m) should be at least 7 inches (17.78 cm) deep. While this type of calculation is useful for making preliminary estimates, homeowners or builders should consult a structural engineer or architect to confirm the correct span and depth for each application.