Roof trusses are designed to support a structure's roof solidly while using the least amount of material possible, which decreases the labor and material costs associated with the roof's construction. Because trusses are designed to carry a specific weight load in a specific way, not much can be added to their weight load without sacrificing safety and their stability.
Roof trusses 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.
Live Loads and Dead Loads
Trusses are designed to carry two types of loads: live loads and dead loads. A live load is a temporary load that puts stress on the structure. Live loads include wind, precipitation, foot traffic and all other weight that comes and goes. A dead load is carried constantly by the truss. Dead loads include the weight of the structure, roofing materials, interior materials such as drywall and insulation, furniture and stored items as well as weight hanging from the truss. Individual trusses are designed to carry live and dead loads that do not exceed a maximum limit special to each truss design; the load limits are expressed in pounds per square foot.
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. All weight hanging from the truss has to be added to the existing dead load; if the total weight exceeds the load limit of the truss, then the truss is compromised.
Compromised trusses can lead to severe structural problems such as bowing exterior walls, sagging ridge lines and roof collapse. If dead loads must be hung from trusses, then extra support can be provided by installing support beams perpendicular to the trusses, reducing the span of the bottom chords and increasing their ability to carry the weight load. Another option is to hang the weight from a beam that spans several trusses, which expands the area from which the weight hangs and decreases the per-square-foot load. Of course, the easiest and safest way to avoid potential structural problems is to avoid putting any weight load on a truss that exceeds its original design limits.