Corrugated metal makes a good, inexpensive roof for a shed. It is either aluminium or galvanised steel; either material may be obtained in a variety of colours, usually some sort of powder-coated colouring for durability. There are many styles and patterns to choose from, with the corrugations -- the ridges and valleys -- varying in depth and spacing. Both materials also come in many thicknesses, with the choice depending on the strength required for the roof. Installation is similar for both materials.
Select a style and determine its coverage area. Measure the roof with a tape measure to determine the number of panels needed. Standard corrugated panels are 2 1/2-by-1/2 inches -- the tops of the ridges are 2 1/2 inches apart and the valleys are 1/2-inch deep. These panels are 26 inches wide with a coverage area of 24 inches, to allow for overlapping seams. Other styles will vary in width and coverage. A standing seam style, for instance, has very wide valleys with vertical seams that interlock rather than overlap.
Install panels starting at the bottom of one end of the roof, the end away from any prevailing wind so wind forces will hit the top of an overlap. Fasten the panels to roof joists with a screw gun and galvanised steel or aluminium screws, depending on the metal selected, with plastic washers at the heads. Put the first screw at the bottom-outside end of the panel, but not right at the roof edge. Secure the other side of that panel, then the tops of the panel, then add screws in between. Use about 20 screws per panel, into the joists.
Overlap the second panel the recommended distance, typically one ridge and one valley, and secure it with a screw through both panel edges. Fasten the opposite side, then the tops, then fill in between. Drive screws in until the plastic washer cap makes a firm watertight bond against the metal. Secure standing seam panels by locking the flat tongue edge of one panel into the grooved edge of the next.
Start a second row with a half-panel, cut lengthwise with tin snips or a metal saw, so the seams do not align with the bottom row. Install the same way as the first row -- corners first, then screws in between. Cut panels to fit at the top of the roof with tin snips or a metal saw. Panels come in lengths from 6 to 12 feet. Use full panels where possible to avoid cutting -- single 12-foot panels for a 12-foot roof, two 8-foot panels for a 16-foot roof, and so on.
Add ridge caps to overlap two panels at the peak of a gable shed roof, which slopes on two sides. Install edge caps, which are similar to ridge caps, around the perimeter of the roof; these fit over the corrugated panels to seal the edges against water penetration. Use edge caps at the top of a single-slope shed roof. Follow manufacturer's directions for installation of these caps; most will be screwed on but techniques can vary.