Which of the following best describes a joist?

The best description of a joist is a wood framing member used to support floor or roof sheeting. Joists are horizontal structural elements that span open spaces and provide support for the surfaces above them. They are critical in transferring loads from the roof or floor sheeting down to the walls or beams below. The use of joists helps in creating level surfaces and allows for the distribution of weight in a building.

In this context, the other options do not accurately describe a joist. A series of triangles refers more to trusses, which are engineered components often using a framework to improve strength and structural integrity. A beam that carries other beams pertains to larger structural members, typically called girders or beams, which serve a different function than joists. Lastly, a heavy timber structural component suggests large timber pieces used primarily in post and beam construction, which can be different from the commonly used lightweight materials associated with standard joisting systems.