Trusses are structural elements that are used to support the roof of a building. They are designed to withstand the three types of loads associated with a building: live loads, dead loads, and environmental loads. The bottom chord of a truss supports the dead load of ceiling materials and the live loads of people or objects. It also serves as a rafter tie and keeps the roof from spreading apart. The top chords of a truss are in compression, while the bottom chord is in tension to resist being pulled apart. Trusses are typically built in a factory rather than at the job site, which makes them less expensive and can cut roof framing costs by as much as 50% .