Installing Truss Bracing
Measure the dimensions of the roof from end-to-end and from the bottom edge to the peak. If it's a gambrel roof, measure from the bottom edge to where the angle of the roof changes, and from that point to the peak of the roof.
Cut 2-by-4 boards to fit the length of the shed from end-to-end with a circular saw. If the roof is too long for a single board, divide the length of the roof in half and cut the boards to fit that smaller length. The 2-by-4s used for the bracing are paced no more than 4 feet apart between the peak and the bottom edge (or the angled sections on a gambrel roof), so cut enough braces to fit your needs. Shed roofs are so small, however, that you'll most likely need only one brace for each roof section. Wear safety goggles when sawing the wood.
Mark the location of the braces by snapping a chalk line. Place the chalk lines on the underside of the rafters, spacing them so that there are no more than 4 feet between the peak and the bottom edge, or the gambrel corners and the peak or bottom edge.
Lift the 2-by-4 bracing up against the underside of the rafters with an assistant's help. Center the brace over the chalk line. Drive two nails through the 2-by-4 and into each rafter. If you need to use multiple braces due to the length of the roof, lift the first brace into position so that it runs from the back wall of the shed to the center of the shed. Position the brace so that it runs along the top edge of the chalk line, and nail it to the rafters. Install the second brace from the front wall of the shed to the center of the shed, aligning it along the bottom edge of the chalk line. The two braces will meet on the center rafter.
Installing Collar Ties
Measure the distance from the peak of the roof straight down until you reach the top edge of the wall. One way to do this is to run a string tightly across the shed between two opposing walls and measuring the distance from the peak to the string.
Divide the distance from the peak of the roof to the string by one-third (.33). This determines the height of the collar ties.
Snap chalk lines across the sides of the rafters at the height needed for the collar ties.
Measure the distance across the shed from one side of the roof to the other at the location of the chalk lines.
Cut 2-by-4 boards that match the width of the shed along the chalk lines.
Lift the 2-by-4 boards into position against the bottom edge of the chalk lines with an assistant's help. Place the 4-inch edge of the boards against the sides of the rafters. The top corners of the boards should come into contact with the underside of the roof.
Drive two nails through each end of the collar ties and into the sides of the rafters.