Which part of a truss is primarily responsible for serving as a ceiling joist?

The bottom chord of a truss is primarily responsible for serving as a ceiling joist because it is situated at the lower part of the truss structure and provides horizontal support across the room or space beneath the truss. This component connects the vertical supports and can carry the load from the ceiling structure above, allowing for the attachment of the ceiling material directly to it.

In addition, the bottom chord is critical in maintaining the overall stability of the truss design, ensuring that the loads transferred from the roof or other elements above are adequately supported and distributed. The geometry of the truss system, with its top chord, webbing, and gussets, work together to support structural integrity, but specifically, it is the bottom chord that functions as the ceiling joist in typical truss applications.