A truss is a roof or floor support structure built from a connected series of box- or triangle-shaped elements to distribute load to a building's walls or foundation. These are often pre-fabricated and sent to a construction site. A girder truss is designed to support other secondary roof supports. Buildings constructed in L or T shapes use girder truss systems where the roof changes direction. Using these structures in roof design eliminates the need for a load-bearing wall below, resulting in open floor plans.
Hip roof designs, where the roof on each side of the building slopes up to the top, incorporate girder structural members where the hip roof elements connect to the main roof. The girder truss on each end of the main roof must support the weight of the hip roof element on each side. Hip roofs can provide structural strength advantages over gabled roofs in hurricane zones.
Wood is a common construction material for girder systems. The girder truss can be built by joining multiple single support elements. High-strength bolts are required to connect the elements, because the girder system will carry a much higher load than a single roof truss. A structural engineer or architect can calculate the design loads, girder size and fastener requirements to safely carry the load.
When a building roof changes direction, the second roof line intersects with the first. Cutting the roof supports or simply nailing the new roof line into the original roof supports can quickly overload the roof structure. A girder truss is installed where the second roof line begins to intersect with the first roof. The two roofs are then joined with connecting members that can safely be hung from the girder.
Improvements in construction materials have led to improved building designs. Beginning in the 1980s, engineered lumber using combinations of wood and polymer adhesives resulted in structural wood products much stronger than standard wood. These engineered systems permit wider spans without using load-bearing walls. Ceilings can be raised without adversely affecting the roof strength, since the engineered structures can carry higher loads.
Girder trusses can be integrated into other useful roof designs. Gables can be incorporated into hip roofs by lowering the side roof rafters to create small gables. These elements not only add character, they can incorporate vents to reduce attic heat loads without sacrificing strength. For occupied spaces such as second floor rooms, end gables can incorporate windows without having to add reinforcing walls.
Designs for girder trusses do not have to resemble standard sloped roofs. Flat roofs for industrial or warehouse operations can use level girder supports. When the building changes direction, addition of an engineered support system at the intersection can result in an open room design favorable to commercial or warehouse operations.