With a little ingenuity and some know-how, you can construct a lean-to shed against the outside wall of a stable barn, garage or outbuilding. This type of shed has an entry door of its own. When constructing a lean-to against an existing building, construct its exterior to look as if it's always been in place. A lean-to shed also describes a simple, stand-alone structure enclosed on three sides with one side totally open. This type of lean-to is built for stacking firewood or storing yard equipment for easy access.
Design three shed sides against a building facade, which will form the fourth side. Plan to construct the shed 240 cm (8 feet) wide and 480 cm (16 feet) long with the shortest wall 240 cm (8 feet) high. Construct the higher wall, the one resting against the building, approximately 300 cm (10 feet) tall.
Build a concrete perimeter for the shed walls to rest on. Dig footings roughly 200 mm (8 inches) wide and 200 mm (8 inches) deep for a concrete pour. Plan to construct framing to rest on the concrete once the footing has dried for 24 hours. Don't worry about framing the perimeter of a footing of this size. Just pour the concrete directly into the ditch line. Plan to add small pebbles to create a floor inside the shed after construction is complete.
Cut 50 by 100 mm (2 by 4 inch) boards with a circular saw to create each facade of the shed. Build each side 240 cm (8 feet) high with studs nailed into a bottom board, called a sill plate. Lay the framing out on a flat surface. Mark the sill plate with a pencil to install the studs exactly on 400 mm (16 inch) centres. Nail them into place from the bottom of the sill plate and replicate this with the top plate, marking the top plate every 400 mm (16 inches). Allow an opening for the door in one wall framing.
Secure a header board along the side of the existing building roughly 300 cm (10 feet) high, so that roof rafters will slope at least 30 degrees. Use a circular saw to cut 50 by 100 mm (2 by 4 inch) rafter boards on the proper angle to create the sloping roof. Secure the rafters with 100 mm (4 inch) nails into the header board on 400 mm (16 inch) centres. Install 12 mm (1/2 inch) plywood over the rafters with nails or screws. Nail tar paper and asphalt shingles over the plywood. Enclose the exterior with plank boards or exterior house siding.
Enclose the exterior of the lean-to framing with plank boards or exterior house siding to match the existing building the shed rests against. Roll or brush two or three coats of exterior-grade paint or stain on the building. Install a guttering system around the overhang area of the roof line to skirt water away. Add a downspout and splash block at the corners of the lean-to building.
Install a landscaping border to keep the shed neater. Use boards to frame a border about 600 mm (24 inches) wide around three sides of the lean-to. Fill this framing with pebbles or mulch to keep weeds from growing. Add shelving inside the lean-to for stacking firewood. Avoid placing wood directly on the ground, since this can attract termites. Create a vapour barrier with plastic sheeting between stored wood and the ground.