Location, location, location. Find a site that is flat, graded and has a well-drained foundation. The site should be at least 6 feet (1.8 meters) wider and longer than the structure's footprint. Then figure out how large you want the pole barn to be. Pole barns are typically built in 8-foot (2.4-meter) sections, so they can range in size from 8 to 16 feet (2.4 to 4.8 meters) to 24 to 40 feet (7.3 to 12.2 meters) in length and width. How you plan to use the barn will also determine its size [source: Fetterville].
Carefully space the post holes, using stakes to mark the center of each hole, taking care to ensure that the structure's corners are square -- exactly 90 degrees. Dig the post holes to a depth appropriate to your area and type of soil. Check to see that the poles are in alignment and vertical. Brace them, checking again that everything is level and square before pouring the concrete into the hole. Allow the concrete to set for several days [source: Burch].
Set the stringers securely at the tops of the poles, checking them with a carpenter's level. Putting up the first truss is the most difficult and may require several people, ropes, poles, braces or a backhoe. Additional trusses can be braced against the ones that are already there. Install the purlins across the length of the trusses. If you're using tin roofing, overlap the sections, using screw-in roofing nails. Caulk under the overlapped sections. Install a roof cap. Keep in mind that plywood and shingle roofs require more time, effort, and precise measuring [source: Burch].
If you're installing walls, mount girts all around the building at ground level, making sure they're level. Nail the girts to posts in several rows, close enough to attach the siding. Add plywood or metal sheeting, making accommodations for any windows and doors that will be [source: Burch].
Your cows and chickens want you to insulate your pole barn -- find out how in the next section.