A lean-to horse shelter is a simple structure consisting of three walls, a pitched roof, and one open side. It's a very basic way of providing protection from the elements for your horses without having to bring them in from the pasture and stable them every time there's a storm. With a little skill, building a lean-to can be done in less than a day.
Choose a location for your lean-to. Make sure the open side is facing away from prevailing winds and is in an area with well-drained soil. Level the area as well as possible.
Determine the size of your lean-to. A basic size would be 10 by 10. Any structure larger than this would require more supporting posts for stability.
Dig four holes for the four-by-four posts. The foundation holes should be at least two feet deep. While pouring concrete isn't necessary, it can add extra stability to the structure, and some might choose to do this.
Cut the posts at the desired height. Angle the top cuts to coincide with the desired pitch of the lean-to roof.
Use lag bolts to attach two-by-fours to the posts as a frame. Depending on the height of your lean-to, you'll want at least two, but likely three levels of two-by-four. If the lean-to is longer than 10 feet, put one or two extra two-by-fours in place to support the roof.
Screw the plywood to the frame with 1 1/2-inch screws.
Measure and attach tar paper to the lean-to roof. Nail on shingles with roofing nails.