DIY Roof Truss Plans

Understand the truss elements and how they relate to a house's design and construction. Trusses have three main components: a horizontal bottom chord, which replaces a joist; two diagonal top chords, or rafters; and various internal braces, called webs. Steel plates called gussets are used to fasten the joints where chords and webs meet.

Determine the width of the house to decide the span or length a truss must cover from wall to wall. Get this from a house plan or from a designer. Select a bearing point, or the spot at which the truss will sit on the wall; this can be on the bottom chord or on the ends of the top chords. Base this decision on the amount of eave or overhang beyond the wall and the style of soffit or covering on this eave.

Choose a pitch or slope for the roof, measured in inches of rise per foot of roof. Most houses will use 5/12 or 6/12, rises of 5 or 6 inches per foot, but the style of house, locale and other factors will influence this. Roofs in areas of heavy snow typically have steeper pitches, to shed snow and ice better. The truss span and pitch are related; the length of permissible span increases as the pitch goes up.

Select a truss style, based on the loads on the roof, the interior space needed for an attic or upstairs room inside the truss brace and the height desired for a ceiling under the truss. Look through a truss catalog to pick from two dozen or so styles to fit various house widths. Some trusses have slanted or bottom chords to provide for higher internal ceilings, while others provide space for storage or a finished room.

Decide on truss spacing and material. Conventional spacing is 24 inches apart the length of the roof, but may be shortened to 16 inches in areas with heavy snow loads or strong wind forces. Most trusses up to about 34 feet wide are built with 2-inch by 4-inch lumber, but some areas may require 2-inch by 6-inch boards. Some trusses use a ridge board in the center to form the peak of the roof; others do not.

Assemble all your information — span, pitch and style — and write all the figures down. Take that information to a truss manufacturer or supplier to order the proper truss, and let computers do all the complex calculations required.