Joist
A joist is a horizontal structural member that spans between supports and carries a floor, deck or ceiling load to beams, walls or girders.
A joist is a horizontal structural member that spans between supports and carries a floor, deck or ceiling load to beams, walls or girders.
Definition
A joist is one of a series of closely spaced horizontal members that span a gap and support a floor, roof deck or ceiling. Joists can be timber, steel, or engineered products such as open-web designs.
A plain example
In a typical floor, joists run in parallel from one bearing wall or beam to the next. Subfloor panels or a deck are fastened across their tops, and a ceiling may hang from their undersides. The spacing and span are set by the designer, not guessed on site.
Why it matters
Joists decide how a floor feels and performs: stiffness, how much it bounces, and how services can pass through. Because they are repeated many times, a small change in size or spacing affects cost, weight and the depth of the whole floor build-up.
Common confusion
A joist is often confused with a beam or a rafter. A beam usually carries joists or larger concentrated loads, while a rafter slopes and supports a roof. In steel framing, a joist is also different from a stud, which is vertical.
On a project
On a plan, joists appear as a repeating line with a label for size and spacing, and an arrow showing their direction. Find the bearing points and any openings, since framing around stairs and shafts is usually detailed separately.
Quick answers
- What is the difference between a joist and a beam? A joist is one of many repeated members supporting a floor or deck. A beam is usually a larger member that carries joists or heavy point loads.
- Can joists be steel? Yes. Cold-formed steel and open-web steel joists exist, and the right type depends on the design. Confirm with the engineer of record.
- Who decides joist size and spacing? The structural designer, based on loads and span, and checked against the local code and authority.
Related terms
Always check the project drawings and specification, and confirm requirements with the engineer of record and the local authority.
Questions people ask
What is the difference between a joist and a beam?
A joist is one of many repeated members supporting a floor or deck. A beam is usually a larger member that carries joists or heavy point loads.
Can joists be steel?
Yes. Cold-formed steel and open-web steel joists exist, and the right type depends on the design. Confirm with the engineer of record.
Who decides joist size and spacing?
The structural designer, based on loads and span, and checked against the local code and authority.
Sources and further reading
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