Premade Truss Size of a Roof

Roof Styles

• Roof style plays a role in truss size. A standard gable roof, with two flat planes requires simple truss design. A gambrel roof, a hip roof, or any roof with a directional change or dormers will add to the number of wood members needed to construct each truss.

Span and Pitch

• As the span, or horizontal distance, of the truss increases, so does the number of individual members used to build the truss. Roof trusses engineered for a flat roof feature many triangular members that give the roof strength. Trusses for a roof with a steep pitch, however, may have fewer members because the strong angle of the top truss chords bears heavier roof loads.

Lumber Dimensions

• Because roof trusses are pre-engineered, the lumber used to build them is smaller and weighs less than the heavy rafters required to stick-frame a roof of the same size. Trusses, nevertheless, offer just as much structural support. The final configuration depends on the type of roof, the size of the house and the type of roofing to be installed. In many cases, however, 2-by-4 dimensional lumber is used to construct trusses for a roof that would require 2-by-10 or 2-by-12 dimensional lumber if framed with rafters.

Attics, Lofts and Vaults

• Truss size changes when a homeowner wants an attic, a loft or just a vaulted ceiling. Ceilings in houses with roof trusses need not be flat, but the roof trusses must be altered at the manufacturer to accommodate different upper-story designs. The truss manufacturer will engineer the members in the trusses to accommodate the lack of a horizontal bottom chord.

Options and Considerations

• If the homeowner purchases a house plan with a specific type of roof, a truss manufacturing company may be able to redesign a completely different roof style that will fit just as well. Once installed, however, trusses must not be cut or altered. To do so may compromise the roof's integrity and will likely void the manufacturer's warranty.