Trusses attach to the top of a home's walls to provide a solid peak and support for roofing materials. Though they come in a variety of styles to accommodate things like vaulted ceilings, attic rooms or a range of roof types, all trusses follow the same basic principles. In the 21st century, most are prefabricated in a style that fits your plans the best.
A common pitched truss consists of a bottom chord, with two top chords attached at the ends and connecting at the top with metal gussets to create a triangular roof peak. These chords are further stabilised with truss webs that splay from a gusset in the centre of the bottom chord to the top chord junction, as well as positions near the centre of each top chord.
Common trusses are usually built entirely of 2-by-4s, 2-by-6s or 2-by-8s. The length of each chord and truss web piece depends on the measurements of the particular roof being built. Larger metal gussetts, or connector plates, are used at the peak and along the bottom chord than at other single connection locations, such as along the top chords. The more connections that must be made in one location, the larger the connector plate.
Trusses vary in shape and construction according to the design of the planned roof and attic space. Added truss web pieces, top chords and bottom chords can give the roof a variety of different shapes. See the first References link for a diagram of different truss styles.
Common diagrams for roof trusses also contain load-bearing information. For a standard triangular truss made of 2-by-4s, the top chord will have a maximum live load of 13.6kg. per square foot; the bottom chord will have 4.54kg. per square foot. The total load-bearing potential for this type of truss is 22.7kg. per square foot.