Which type of truss is commonly used as ceiling joists in buildings with flat roofs?

The parallel chord truss is commonly used as ceiling joists in buildings with flat roofs because its design allows for a relatively uniform load distribution along the length of the truss, making it ideal for supporting horizontal loads. This type of truss features chords that are parallel to each other, which offers additional strength and stability while minimizing the overall height profile.

Using parallel chord trusses can also promote more efficient use of space, as they can be engineered to facilitate ductwork and other equipment that might occupy the ceiling area, especially in commercial buildings. Their ability to span long distances without the need for intermediate supports is particularly advantageous in flat roof designs, which typically require a straightforward deck system to support roofing materials and drainage systems.

In contrast, some other types of trusses, like bowstring or pitched chord trusses, may be better suited for specific architectural features or load-bearing requirements associated with sloped roofs. Simple trusses also tend to be used in different structural applications where loads are more concentrated and vertical.