Building a shed with a flat roof is as much work as building one with a pitched roof. And flat roofs can't be totally flat, unless you're building in an area where it never rains. Flat roofs minimise the effects of strong winds, but are subject to weight from accumulations of rain, snow or ice, so they need at least some slope to shed this water. A roof with a slope of 10 per cent or less is considered flat, so even an inch or two of difference in height can work.
Lay a solid foundation and build four walls. Use concrete piers with 4-by-4-inch beams set across them for a foundation; cover the beams with a floor built of 2-by-4-inch frames and joists and covered with 3/4-inch plywood. Make walls with conventional 2-by-4 framing, studs spaced 16 inches apart, with walls nailed to the floor joists and at corners. Use a hammer and framing nails to build the frame and fasten flooring.
Design the roof slope before you erect the walls. Make end walls different heights by cutting studs one inch shorter on one wall with a circular saw or by adding a second top plate on one wall. Adjust side walls to that slope by adding triangular shims cut from 1-by-4-inch lumber atop the wall plates or just leave a slight gap at the top to be covered by siding.
Put joists across the roof, end to end. Nail 2-by-4s flat on the sides and in between with a hammer and framing nails or use metal joist hangers, which fit over the wall caps and nail to both boards, to hold vertical joists. Space joists at 24-inch intervals to conform to roofing panels.
Finish the walls with siding of your choice, cut to fill the roof slope, so the wall is covered fully. Install a door at the high end of the roof. Nail 1-by-4-inch sheathing strips across the joists from side to side. Put on corrugated metal roofing, aluminium or galvanised steel, from top to bottom across the roof; smooth metal sheds water more effectively than other roofing.
Fasten metal panels to the sheathing strips starting at the bottom of one side. Panels come in lengths from 8 to 12 feet; if possible size your flat roof so a single panel will cover end to end. Allow at least 6 inches of overhang both top and bottom. Overlap panels side by side according to the manufacturer's instructions, typically one peak and valley. Secure panels with a screw gun and aluminium or galvanised screws with plastic washer tops.
Add special drip edging, which seals the edges of the panels, if desired. Cut metal roofing as needed with tin snips or a metal saw. An alternative covering for a flat roof is a rubberised material, but it is more expensive and harder to install than corrugated panels.
Wear gloves when handling metal roofing. Edges are sharp and can cut hands and fingers. Get help handling panels; they are light, but unwieldy for one person to handle.