Comparison of Roof Trusses

Common Trusses

• A common roof truss is used to build a gable roof. In this system, two members called top chords slope up from the eave line and meet at the roof peak; the roof sheathing is attached to these chords. A bottom chord stretches horizontally between the top chords at the eave line. The points where the bottom chord meets the top chord are where the truss rests on bearing walls and transfers the weight of the roof to the walls. A vertical king post rests on the bottom chord and supports the peak of the truss, and a web system of short members helps to transfer the load from the top chords to the bottom chord.

Scissor Trusses

• A scissor truss is similar to a common truss, but in this design two bottom chords slope upward from the eave line to the king post. In a common truss, the bottom chord functions as a horizontal ceiling joist for the room below, while in a scissor truss, the slope of the bottom chords form a cathedral ceiling. Other variations on the common truss can be used to form a studio or tray ceiling.

Gambrel and Hip Trusses

• Other truss types are used to form different exterior roof profiles. A gambrel or barn-style roof is made using trusses with a more complex top chord design than that used in a common truss. Chords extend from the peak with a relatively shallow slope to a point between the peak and the eave. From that point, another set of chords extends to the eave line with a steeper slope, creating a roof with a double pitch. A hip roof slopes from all four walls to a central ridge line, unlike a gable roof which slopes only from two parallel walls. A hip truss has two top chords that slope up from the eaves until they meet a horizontal chord that forms the roof ridge. The roof's perpendicular slopes are formed by half trusses called mono trusses that also slope up to the ridge of the hip truss.

Room-in-Attic Trusses

• In a room-in-attic truss, the horizontal bottom chord functions as a floor joist for an attic room. The system of web supports is designed to allow space for the room within the truss, and wall sheathing is attached to vertical web members.