Joists are used in floors and as column support. The joist (also called Bandsill) supports the load which the floor is built to bear. It is like the skeleton a building. Trusses are of many types; the simplest is a planar truss which is used as a roof support system.
|Use||Used in floors and as column support. The joist supports the load which the floor is built to bear.||A truss is used to support the roof.|
Joists are usually supported by beams and repeat every few feet in the structure of the building. The depth required for the joist depends upon the span between the supporting structures. A rule of thumb for calculating the depth of a wooden floor joist for a residential property is half the span in feet plus two inches; for example, the joist depth required for a 14-foot span is 9 inches.
The depth of a truss, or the height between the upper and lower chords, is what makes it an efficient structural form. A solid girder or beam of equal strength would have substantial weight and material cost as compared to a truss. For a given span length, a deeper truss will require less material in the chords and greater material in the verticals and diagonals. An optimum depth of the truss will maximize the efficiency.