…support a roof’s weight by transferring the weight load downward and outward to the building’s bearing walls. They do this by means of top chords, which are sloping members that extend from the peak of the roof to the top of the exterior walls at the eaves. The top chords are tied together by a horizontal bottom chord, which stretches from eave to eave and keeps the load carried by the top chords from pushing the exterior walls outward. The bottom chord also supports some of the roof’s weight via support members in the interior of the truss; the interior support members transfer weight from the top chords to the bottom chord. Weight added to the bottom chord probably will exceed the truss’ design limits and compromise the truss’ ability to support the roof.
The typical live load placed on a structure’s roof varies by the building’s use and location, and weather is the primary concern. The live loads a truss is required to bear are determined by local building codes. If the bottom chord of a roof truss functions as a floor joist, such as in an attic room, it carries a live load that also varies by use, but a typical live floor load for a residential space is about 40 pounds per square foot. The dead load on the bottom chord of a truss varies with the weight of materials attached to it, such as drywall on the ceiling; a typical load is 5 and 10 pounds per square foot. A metal or double reinforced truss will be able to give double that amount.