How to Design a Truss for a Shed

Start a truss design by determining the span or width of the roof, the pitch or angle of slope desired and the bearing point, which is the spot at which a truss will sit on a wall. Use a bottom board or chord as a bearing point, with rafters sloping over its angled ends, or make more internal room by placing the chord higher up between the rafters and using the rafters to set on the wall. The bearing point determines how to cut rafters. Refer to truss tables for size, length and strength of rafters and any extra bracing required.

Check building tables for roof loads to determine strength of truss needed. Dead load is the weight of the roof and roofing material, live load the weight of snow and ice, based on U.S. Weather Bureau statistics, and wind load the force of winds against the roof, also based on weather statistics. Tables will show loads by locales, down to zip codes. Compare local loads with strength of truss needed for the width of a roof.

Refer to lumber industry tables to find the strength of the type of lumber for a truss. Most trusses will be built of pine, which is the strongest framing lumber, but tables will show the dimension of lumber needed for various widths and roof loads. Most trusses are built with 2-by-4 boards but some sheds in severe weather locales or with wider spans may require 2-by-6 boards.

Study truss design manuals to pick a style, based on such things as the height needed inside the shed or even the amount of outside light required. Gable is the most usual style, with slopes on two sides from a peak, but gable trusses may be formed with a low bottom chord, which rests on the walls, or a higher cross chord, which offers more space. Gambrel trusses, with multiple rafter angles, make the "barn" style roof with much more internal space. Offset trusses, like clerestory, allow windows at a peak to provide light.

Select a type of gusset, the joint connector that affords a truss its greater strength. Gussets connect two boards by overlapping the sides, which are then fastened through the wide faces of the lumber; conventional rafters are nailed together through the thin edges of boards. Gussets may be metal or wood. Metal gussets are stronger and may be bought at building supply and lumber stores. Wood gussets can be made of plywood or similar material and tailored to a specific style of truss.