Apply for a permit. Check with your local city or county building and zoning authority and obtain a permit if necessary. You may need to provide a plan showing the dimensions of the building and its location relative to your home, other buildings, and your property boundaries. Some authorities also require a materials list. If you do not get a permit you could be fined.
Prepare the building site and materials. Clean up all vegetation and use a shovel or backhoe to level and smooth the ground. Choose a location where water will run off and not pool. Cut the posts or poles to create a sloping roof. The front posts should be at least 18 inches taller than the posts to be installed at the rear of the building so the rainwater will run off at the rear.
Dig the post holes. Make certain the building will be square. Use an auger or post-hole digger to make holes at least 18 inches deep. Uprights should not be spaced more than 10 feet apart; since most roofing and siding sheets are eight feet long, spacing uprights at 8-foot intervals will simplify construction. Plant the posts in the holes and use rocks to brace them or nail scrap pieces of lumber lightly to make braces. Use a plumb line or level to be sure they are not leaning.
Pour the concrete. If you are making a concrete floor, set up the forms so that the floor will extend to the exterior of the posts. Make certain they are level and square. Spread scrap metal and rebar inside the floor area to reinforce the concrete. Pour the concrete for the floor and into the holes around the posts. Leave it to cure for 72 hours. If you plan to have a dirt floor, pour concrete into the post holes only.
Attach the beams to the tops of the posts or poles. Use stainless steel bolts, washers and lugs. Poles should be notched to make flat and stronger joints. Install the rafters across the beams and attach them with galvanized screws. They should not be spaced further than 16 inches apart if you live in an area where there will be a snow load on the roof. Add battens between the rafters for additional roof support.
Attach the roof. Lay the roofing sheets across the beams and rafters. Make certain the channels run toward the lowest part of the roof at the rear of the building to facilitate rain and snow runoff. Attach the roof with roofing nails. Overlap the sheets to prevent leakage and cut the sheets to fit if necessary.
Install the siding. Hammer or bolt horizontal poles or beams across the bottom, and center of the posts. If you intend to keep large animals like horses or cows in the shed, these should be strong enough to contain them. Use galvanized nails to attach metal roofing sheets to these and the upright posts as siding. Leave a doorway wide enough for the livestock to enter and leave. Install a gate in the doorway using hinges and screws or bolts.