DIY Barn and Shed Trusses

Build the basic gambrel truss with two rafters of equal length on each side of the roof, cut at 22.5-degree angles on each end. Design a truss on graph paper. Start with a horizontal line scaled to the width of the roof. Make a half circle above that line. Use a protractor to draw lines of equal length at 22.5-degree angles from the peak to the side of the roof and then down to the wall. Measure those lines with a tape measure to determine the length for each rafter element.

Mark a set of pattern rafters using the "common top cut" scale on a speed square. Put the "pivot point" of the square at the top of a rafter board and align the 22.5-degree mark at the bottom of the rafter. Reverse the angles at each end so they will form a 45-degree angle when matched with another rafter. Cut one set of four rafters as a pattern, using a circular saw or a miter saw; miter is better because it can be set at the precise angle.

Put the test rafters on a flat surface with the angled ends together. Use a tape measure to check the distance between the bottoms of the rafters, which should be the same as the width of the roof. Make any adjustments and use those rafters as patterns to cut all others. Figure the number of trusses needed by dividing the length of the building by the rafter spacing: 24 inches for a shed, sometimes 16 inches for a barn, which will have a loft or other storage suspended from the trusses.

Lay a set of rafters on a flat surface. Fasten the joints with metal or wood gussets, which are plates that overlap the joint and secure the rafter on each side. Buy metal gussets, which have spikes to drive into the rafters and holes for nails driven with a hammer. Make wood gussets of plywood, with one straight edge across the joint and edges cut with a circular or miter saw to conform to the rafter angles. Cut wood gussets with a circular saw or a miter saw. Fasten wood gussets with screws and a screw gun.

Add braces one of three ways for a larger roof: cut rafter boards to fit vertically between the knuckle joints and the wall top; install horizontal cross braces between the knuckle joints; or put braces from the center of the bottom rafter to the center of the top rafter. Choose the bracing depending on the width of the roof and the internal space needed. Secure any braces with gussets.