How to Build & Plan Roof Trusses

Assemble the key dimensions for truss planning -- the length of the house to determine the number of trusses which will vary with spacing, the width to decide the span or space the truss must support and the pitch or angle of slope of the roof. Use the span and pitch to choose a basic truss design; the most common styles are Fink or W, which can span up to 33 feet, and Howe or K, which can span up to 36 feet.

Calculate the dead load or weight of the trusses and roofing material based on the span of the truss and type of roofing material; clay tiles will be heaviest, steel or aluminum lightest, asphalt or composition shingles somewhere in between. Figure the square footage of the roof and use the weight of material per square foot for dead load.

Use U.S. Forest Service and Weather Bureau tables to figure live load and wind load. Live load is external weight, mainly snow and ice accumulations; weather tables will show the projected dead load by locale. Wind load is the force of wind on a roof and is a major factor in areas of high winds such as tropical storms.

Check truss span tables, available from building supply stores, truss contractors or online, for specific spans based on pitch and snow loads. Allowable spans will vary with the roof pitch and size of lumber used in the rafter chords and bottom or horizontal chords. Spans increase as pitch goes up; steep roofs will have wider allowable spans because they shed snow better. Most trusses are built with 2-by-4-inch lumber but some use 2-by-6-inch, either for all members or for either top or bottom chords.

Design a truss once loads have been determined and a style is selected. Calculate the height above plate, that is, the bearing point at which a truss sits on an outside wall. A raised heel truss sits higher on the plate and allows more space for insulation. Make sure the height and bearing point accommodate the type of interior ceiling planned; some special types, like vaulted, require special truss design.

Translate all the calculations into a plan on graph paper, showing the width of the bottom chord, slope and length of the top rafter chords and spacing and size of the internal braces called webs and all the connection angles. Use a framing square to figure angles for top and end cuts, and to calculate the length of truss rafters and such features as overhangs for eave soffits.

Cut all truss members with a circular saw according to the dimensions of your plan. Build your own trusses, if allowed, with 1/2-inch plywood gussets to secure the joints where boards meet. Lay truss components on a flat surface, like a driveway, with all the components properly aligned. Make rectangular gussets to join bracing webs to top and bottom chords. Make top gussets with a wide horizontal bottom to connect both rafters and an angled top to conform to the pitch of the roof peak.

Fasten truss joints with 1 1/4-inch galvanized screws and a screw gun to secure gussets to all joints. Put gussets on both sides of each truss. Lay completed trusses flat or stack them vertically; trusses have very little lateral strength and will be damaged if twisted or bent after construction.