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What is a Joist?

By B. Turner
Updated May 17, 2024
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A joist is a construction element used to support the horizontal surfaces of a home. Joists run parallel to the surface they are used to support, which may includes roofs, ceilings, or floors. The ends of each of the joists rest on beams or other structural elements. In some parts of the world, joists may be referred to locally as bandsills.

Building joists may take the form of solid beams or open-webbed trusses depending on the application. Some are cut to size during construction, while others are pre-fabricated based on building plans or design requirements. In home construction, a joist is often constructed out of wooden lumber, though plastic-laminate versions are also gaining in popularity. Most commercial contractors rely on steel joists rather than wood due to their added strength and durability.

Each home or building includes several sets of joists. Floor joists support the floor structure on each level of the home, including the ground level of homes constructed with wood. Depending on the design of the roof, the top set of floor joists may also support the roof.

Builders fasten the joists to the surrounding beams, which run perpendicular to the joists. These elements may be joined together using nails, screws, or even special clips. In masonry construction, the ends of the joists may rest on top of a brick or concrete wall. Properly-installed joists help to transfer the load of each floor to the surrounding walls, which then carry the load down to the building's foundation.

Contractors face a trade off when choosing joists, as the depth of each joist can impact the span. Generally, the longer distance each joist must span, the deeper the joist will be. Deeper joists take up more space, resulting in reduced ceiling height within the structure. In order to increase ceiling height and use a shallow joist, the contractor may have to add intermediate supports to reduce the distance the joist must span.

Many factors can impact the size of each joist. In order to calculate the correct joist sizing, contractors must consider factors such as material and total load being supported. The spacing of the beams and joists can also impact joist depth and span. By increasing the number of units and placing them closer together, contractors can create extra support for heavier floor or roof loads. This may eliminate the need for deeper joists in some types of applications.

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