A quick horse barn that provides shelter from the elements is easy to build. Having a place for your horse to get out of weather is vital to its well-being. All you really need are three sides, four if you prefer, and a roof. A barn that closes in too tight is not good either. Adequate ventilation without draft is important to your horse's health and happiness. A lean-to is fast and easy, provides great shelter and, best of all, one person can erect it in a single day.
Make sure the area you are building is flat and level. Choose your location wisely. Pick an area with good sun and cross breezes if possible. A shade tree or two is nice, but not necessary. However, if you choose a location with trees, make sure they are not directly surrounding your build site. Tree limbs can make digging to sink posts difficult, roots can interfere with underground posts as they grow, and overhanging branches can tear up a roof.
Mark a square area with four corners 10 feet apart. Use a post hole auger to make a four-foot hole for each corner post. Insert the two 12-foot four-by-fours in the front corner holes, and the 10-foot four-by-fours in the two rear corner holes. With four feet in the ground, that will make your horse barn eight feet tall in the front, and six feet tall in the back. Use a level to make sure they are straight upright and even in the hole as you back-fill dirt around them and pack them in tight.
Nail a 10-foot two-by-four from side to side at the base of the left side of the shed. Continue around the back with another two-by-four and then attach a third on the right side of the shelter. Do the same around the center of the corner posts, and then at the top of the six-foot corner to the six-foot mark of the front corners.
Attach a 12-foot two-by-four on each side with the broadside flat against the side of the outer edge of the corner posts from back to front at an angle determined by the posts to create a roof support. Attach one 10-foot two-by-four at the back edge of the roof, one at the center, and one across the front of the shed roof supports. Attach 12-foot two-by-fours from front to back, at one-foot increments across the roof supports.
Cover sides, back and roof with ¾-inch plywood. Cover roofing plywood with metal sheet and attach with metal screws to prevent leaking and water damage to the wood. Paint sides with weatherproof barn paint to protect wood from rot.