Joists support horizontal surfaces in buildings and can be solid beams or open mesh trusses. They are attached to surrounding beams and can be made of wood, plastic laminate, or steel. Proper installation is important for load transfer. Joist size depends on factors such as material, load, and spacing.
A joist is a building element used to support the horizontal surfaces of a house. The joists run parallel to the surface they are used to support, which can include roofs, ceilings or floors. The ends of each of the joists rest on beams or other structural members. In some parts of the world, joists may locally be called window sills.
Building joists can take the form of solid beams or open mesh trusses depending on the application. Some are cut to size during construction, while others are prefabricated based on building plans or design requirements. In home construction, a joist is often constructed of wood lumber, although plastic laminate versions are also gaining popularity. Most commercial contractors rely on steel joists rather than wood due to their greater strength and durability.
Each house or building includes several sets of joists. Floor joists support the floor structure on each level of the house, including the ground floor of log-built homes. Depending on the roof design, the top set of floor joists may also support the roof.
Builders attach the joists to surrounding beams, which run perpendicular to the joists. These elements can be joined together using nails, screws or even special clips. In masonry construction, the ends of the joists may rest against a brick or concrete wall. Properly installed joists help transfer each floor’s load 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 affect the span. Generally, the more distance each joist has to cover, the deeper the joist will be. Deeper joists take up more space, resulting in lower ceiling height within the structure. To increase ceiling height and use a shallow joist, the contractor may need to add intermediate supports to reduce the distance the joist has to span.
Many factors can affect the size of each joist. To calculate proper joist sizing, contractors must consider factors such as material and total load to be supported. The spacing of the beams and joists can also affect the depth and span of the joists. By increasing the number of units and placing them closer together, contractors can create additional support for heavier floor or roof loads. This can eliminate the need for deeper joists in some types of applications.
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