How to Design an Attic Truss

Design an attic truss using graph paper and a scale like 1/8 inch to the foot. Start by drawing a horizontal pencil line the width or span of the roof, the distance between wall tops. Use a ruler to measure this on the graph. Decide on the size needed for the interior attic space, the width of the room and the height; storage attics require only 4 or 5 feet, others need 8-foot walls like standard rooms.

Mark the height needed with the ruler and draw another horizontal line, parallel to the bottom chord line. Draw vertical lines at the width needed to mark the outside edges of the attic space. Connect the outside of the bottom chord line and the edge of the outside attic wall lines with a triangular line, which will form a peak where the slopes on each side meet.

Calculate the pitch or slope of that angle, in inches of rise per foot of run from wall to peak. Measure the equivalent of one foot horizontally, then measure the distance between that point at the rafter chord; if that represents 7 inches, for example, the roof pitch will be 7/12, a rise of 7 inches per foot. Measure 1/2 inch, the equivalent of 4 feet and multiply the graph height by four, if the 1/8 scale is too small.

Locate and mark the braces. Put one in the center from the bottom of the peak to the center of the attic top. Add braces on each side of the attic room, depending on the width. Put a single diagonal brace from the bottom of the attic wall to the center of the rafter chord for a narrow roof. Use a three-triangle style for a longer span, a vertical web halfway between the attic wall and rafter and an angled web from the top of that brace to the bottom of the attic wall support.

Convert the scale to full dimensions in feet and inches and roof pitch and provide the information to a truss builder. Entering those basic figures in a truss design computer program will produce the specific lengths and angles needed to cut the boards and fasten joints with steel gussets.