What is a joist?
A joist is one of a series of parallel horizontal members that support a floor or ceiling, spanning between beams, walls, or foundations.
Floor joists carry the subfloor and everything on it; ceiling joists carry the ceiling below and can help tie a roof together. They're spaced at a regular interval and sized to the span and load they carry.
Joists can be solid lumber or engineered products like I-joists. How far they can span depends on the species, size, spacing, and load, which is why span tables (and bearing requirements at the ends) matter.
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