Joists are horizontal elements that support structures like decks, floors, and roofs. They require beams or walls at both ends and may need additional supports along their length. The size and material of a joist depend on the span required, and manufacturers provide span tables to help consumers choose materials. Joist depth can be calculated by dividing the required span in half and adding two, but consulting a structural engineer or architect is recommended.
Joists are the horizontal elements that make up the structural framework of a deck, floor, ceiling or roof. Often made from wood or steel, they are specifically designed to provide adequate support for various structural applications. A joist is supported at both ends by a beam, wall, or other framing member. They may also require additional supports, known as “intermediate supports”, along their length. The joist span is the distance between the beams, walls or other structures that support a joist along its length.
The total joist width is the distance from one end of the joist to the other. When sizing a joist or determining its span, builders should look at 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 both ends. For example, a 10 m (3.05 ft) joist resting on a 6 cm (15.24 in) wall at both ends has a design span of 9 m (2.743 ft).
The size and material of a joist is directly related to the span. Maximum joist length is set by local building codes and varies greatly from city to city. A larger or deeper joist can safely span a greater distance than a smaller joist, although the specific distance depends on the material of the framing elements and the building design.
Most joist manufacturers have their own span tables, which provide recommendations for joist sizes based on the span required. These tables tell consumers how far a joist should deflect or move under a specific load. Each manufacturer produces a separate table for each type of joist they produce. Consumers can use these tables to choose materials based on the maximum deflection specified in the local building code. For very large joists, consumers may not be able to find joists large enough to meet the code and will instead need to add intermediate supports along the length.
As a general rule, joist depth in inches can be calculated by dividing the required span in half, then adding two. For example, a joist that is to 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.
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