Determine the key figures for shed roof trusses: the pitch of the two slopes and the run of each side, which is the length each rafter must support. Make the pitch the same on both sides to simplify truss construction. Figure the run from the peak to the outer wall on each side. For a 10-foot wide shed, for instance, make one side 8 feet, the other 2 feet.
Make long side rafters first. Lay a 2-by-4-inch board on a flat surface with the 4-inch side up. Put the point of a framing square at the bottom on one end, with the thin tongue across the board. Align the 12-inch mark on the wide blade with the top of the board. Put the pitch mark on the tongue, 5-inch for a slope of 5 inches per foot, for example, at the top of the board. Mark the angle that forms along the tongue for the top or plumb cut.
Find the "length of common rafter" table on the blade and check under the pitch mark for the difference -- 13 for a 5/12 foot or 13 inches for every foot of rafter run in this example. Multiply that by the run, eight times 13 or 104 inches. Measure that distance down the bottom of the board with a tape measure and mark a vertical line 1 inch into the bottom of the board. Measure 3 1/2 inches back toward the plumb cut from the bottom of that line, then connect that point and the top of the vertical line for a triangle birdsmouth to fit on the wall cap.
Measure the amount of overhang desired, typically a foot, and mark another angled line. Figure this like the plumb cut, but with the point of the square at the top of the board. Cut all those angles with a circular saw. Make all rafters needed for the length of the shed, 6 for a 12-foot shed with trusses spaced 24 inches apart, for example.
Start with a shorter rafter board and repeat the calculations for short side rafters. Use the 2-inch pitch mark to mark the plumb cut. Put the birdsmouth at 26 inches, two times 13, and add the overhang, which might be the same as on the long side or either longer or shorter.
Lay one long and one short rafter on a flat surface. Align the plumb cuts and measure between the birdsmouths to ensure they will align on the wall caps, 10 feet apart in this example. Place a 2-by-4-inch board horizontally between the rafters a foot down the short side from the peak. Mark the angles of the rafters on that board for a cross tie, to connect the rafters. Cut six cross ties.
Put pieces for one truss together, rafters aligned and cross tie perfectly horizontal. Make gussets of 1/2-inch plywood to fasten the three joints where boards meet. Use rectangular gussets for the side joints; make the top gusset straight across the bottom to connect the two rafters, but angled at the top for the peak. Fasten gussets with 1 1/4-inch galvanized screws driven with a screw gun. Put a gusset on each side of each truss.
Mark the shed wall caps for truss locations with a tape measure, marker and square. Measure 1 1/2 inches from the back end wall and draw a line across the cap. Measure 23 1/4 inches from the end wall and mark the outside of the second truss; use the tongue of the square to mark the second truss inside line. Measure 24 inches from that line and mark another truss. Repeat that to the end of the roof; the last space may be under 24 inches.
Lift the first end truss into position. Set it plumb with a level and nail the birdsmouths to the wall caps with three 16d framing nails, driven diagonally with a hammer, two on one side of the truss end, one on the other. Raise all other trusses, set them plumb and brace them temporarily with 1-by-4-inch boards nailed between the cross ties.
Cover the trusses with 5/8-inch oriented strand board (OSB) nailed to rafter sides with 8d galvanized nails. Put the rough side of the OSB out. Use as many full 4-by-8-foot panels as possible. Nail full panels in place at the peak on the long side and cut the excess with a circular saw. Let the panel on the short side overlap the top edge of the top panel on the long side.