Types of Flat Roof Wood Trusses

Parallel Chord

• The most basic style of flat-roof truss is called a parallel chord. It has top and bottom horizontal boards, called chords, with strong braces between them. Some styles use a combination of vertical and diagonal braces; others use only diagonal braces. The chords are the same length and are parallel to each other. The depth of the truss is determined by the length, or span, typically a depth 7 percent of the span in inches.

Warren Style

• A Warren-style flat truss has parallel chords with all diagonal braces, which form a sort of constant W pattern. This style has a vertical post at each end. The first diagonal on each end slopes from the bottom of the vertical post to the top chord. The next diagonal slopes from the top chord back to the bottom. This pattern is repeated across the width of the truss. Depth will vary with length.

Slopes

• There are at least two styles of sloping flat-roof truss (a roof with a pitch of less than 3 inches per foot is considered flat). One style has parallel chords, both of which slope at the same angle, with a vertical post at each end. This has a vertical post in the center and a combination of vertical and diagonal posts on each side. Another style has a horizontal bottom chord but a top chord that slopes. The internal bracing has a center post, and vertical and diagonal braces between. Slopes and brace spacing vary with length.

Special Types

• Special styles of flat-roof truss include a top chord-bearing truss and a cantilever end truss. A top chord-bearing truss has a top chord that extends beyond the end of the bottom chord. This style is fastened to the wall tops with the top chord; other flat trusses are installed with the bottom chord bearing on the wall. A cantilever-end flat truss has a vertical post at one end, a vertical center post with a combination of vertical and diagonal braces on each side, but slopes on one end from the top of the last vertical post to the bottom chord.

Limits

• Flat trusses are limited in length to the lumber available and generally do not span more than 20 feet if made of wood. They may be combined for longer spans but require posts for support where trusses meet. Commercial and industrial buildings with longer-span flat trusses use steel rather than wood. The stresses on the braces or webs of flat trusses are much greater than those on pitched-roof trusses. Flat-truss joints are secured with steel plates called gussets, like all prefabricated trusses.