Before building a lean-to, research the local zoning laws, and choose construction materials that are appropriate for the local weather. For shorter, lightweight roofs, use pressure-treated southern yellow pine supporting poles, while longer or heavier roofs require 6-inch-by-6-inch timbers or steel columns for adequate support. To manage indirect moisture, use a sheath of CDX plywood on the wall of the adjoining structure.
Design lean-to roofs at an angle to prevent rain from pooling and snow accumulation. Use 2-inch-by-6-inch nominal framing for roof spans of 10 feet or longer spans and southern yellow pine for roof framing and rafters of smaller constructions. Use 2-inch-by-6-inch nominal southern yellow pine for the nailing rafters, and attach with the anchors required by local building codes.
Generally, lathing strips made from 1-inch-by-4-inch lumber will support the weight of the roofing, but 2-inch-by-4-inch lumber is easier to attach and provides extra support. Use nails long enough to join lumber components securely, and attach metal roofing or siding to lumber using screws.
Nailing rafters may be joined directly to the side of the support posts, but doing so places the entire weight of the structure onto the fasteners. Notch the tops of the supporting poles, and rest the nailing rafters in the grooves for more support.