Types of Roof Trusses
Roof trusses are prefabricated structural frames that support the roof covering and transfer loads to bearing walls. The triangular geometry of trusses provides inherent structural efficiency, using less material than conventional rafters while spanning greater distances. Common truss types include the king post truss for spans up to 16 feet and the queen post truss for spans up to 30 feet. Fink trusses, also called W-trusses, are the most common type for residential construction and can span up to 40 feet.
Scissor trusses provide a vaulted ceiling effect by sloping the bottom chord parallel to the roof slope. Howe trusses with diagonal webs toward the center are efficient for medium spans. Parallel chord trusses are used for floor construction and flat roof applications. Each truss type is optimized for specific span ranges, loading conditions, and architectural requirements.
Truss Design and Loads
The design of roof trusses follows engineering principles codified in the Truss Plate Institute standards. Each truss is designed for specific loading conditions including dead loads, live loads from snow, and wind loads appropriate for the geographic location. truss geometry and member sizes. corner bead protects outside corners. house wrap with drainage channels. Snow loads range from 10 psf in mild climates to over 100 psf in mountainous regions.
The metal plate connected wood truss industry uses sophisticated design software that optimizes truss geometry and member sizes. The design process considers the lumber grade, species, and size for each member, as well as connector plate placement. The deflection of the truss under design loads must not exceed L/240 for trusses with brittle finishes.
Installation and Bracing
Truss installation must follow the approved truss placement diagram. Permanent bracing consists of lateral bracing perpendicular to the truss plane and cross bracing. Continuous lateral bracing is required at the top chord, bottom chord, and at intermediate points along compression web members.
Roof sheathing transfers lateral loads to the trusses and provides the base for roof covering. Minimum sheathing thickness is 7/16 inch OSB. The connection between trusses and bearing walls must resist both gravity loads and uplift forces from wind. Hurricane clips or truss hold-down connectors are required in high-wind regions.