Roof Truss Information

Components

• A truss has three components: a bottom chord, which replaces a roof joist from wall to wall; top chords, which are the angled rafters that form the pitch or slope of the roof, and internal braces, called webs, which connect the chords and support the top chords. All joints are fastened with gussets that have long metal spikes which are driven into the boards from both sides with heavy pressure.

Many Types

• There are at least two dozen types of truss, to fit any style or pitch of roof. They are made to order. An architect, designer or builder gives a truss manufacturer the basic dimensions and the trusses are made to fit. The basic dimensions are width or span of the roof, from wall to wall; the pitch of the rafters and the length of the building and spacing of rafters to determine the number needed.

Fink or W

• The most common rafter design is called a Fink or W, because the internal braces form a W pattern between the bottom chord and top chords. This style was developed in 1852 by a German engineer, Albert Fink, and was first used in railroad bridges, then adapted to houses. A single Fink truss can span up to 33 feet. When the bracing pattern is repeated, a Fink truss can span up to 80 feet.

Advantages

• Trusses have several construction advantages. Because they are engineered in factories, angles and lengths may be cut more precisely. Joints with gussets are much stronger than nailed connections of rafters and joists built on site; a typical on-site rafter peak would be secured with four nails, two from each side, through the narrow part of the board. Gussets fasten on the wide edges for better holding.

Installation

• It is much easier to install trusses, which are lifted to a roof in one piece, set upright and secured to the wall tops with nails and metal brackets which act like gussets to lock two boards together. A truss roof typically can be framed in one day, less than a day for a small roof with enough help.