Most houses today are built with roof trusses made in factories. These prefabricated trusses are lighter, stronger and easier to install than conventional joists and rafters. A builder can make his own trusses to save money or if he works in places where delivery of factory trusses is difficult. Making a 5/12 truss, for a roof slope of 5 inches per foot, is not complicated once you know the span the truss must cover.
Span
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The style of truss will depend on its span, the width from wall to wall, and the amount of space needed inside the truss, for storage. A 5/12 truss is a medium slope and will not accommodate a roof with a lot of interior space, even on a very wide building. Most basic truss designs will span 20 to 36 feet.
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Basic Design
All trusses are made with a bottom chord, which goes from wall to wall, two rafters, which form the slope from the wall to the center peak, and some internal bracing between the chord and rafters. The bottom chord could rest on the wall, like a conventional roof joist, with the rafters extending beyond the wall or the rafters can be the bearing point on the wall, with the bottom chord inside the walls. Either can be used with a 5/12 truss.
Common Styles
The most common truss styles for roofs between 20 and 36 feet are fink or W, howe or K and fan. Fink has internal braces connecting the peak to the bottom chord with one brace and the bottom chord to the rafter with another; the braces are shaped like a W. Howe has a center post from peak to bottom chord with one upright and one slanted brace on each side that resemble the letter K. Fan is similar to fink except with an upright brace at the bottom of the W sides.
Rafter Basics
Truss construction starts with cutting rafters. Use a framing square, with a thin tongue and a wide blade connected at a 90-degree angle. Set the square on a rafter board with the 5-inch mark on the tongue and the 12-inch mark on the blade at the top of the board, with the point at the bottom. That forms the top or plumb cut angle for the peak. A table on the blade shows how to calculate the length of rafter needed to reach the wall.
Truss Assembly
Once rafters are cut, trusses are assembled one at a time by setting the rafters on flat surface with the braces in proper places. Joints are secured with gussets, which are metal or wooden plates that overlap the joint and are fastened to the rafter or brace on each side. Plywood gussets are best for homemade trusses. They need to be cut flat on the bottom and angled to the slope of the rafters on the top and be secured with galvanized screws. Gussets go on both sides of every joint.