A lean-to is an open structure that is dependent on another structure for support. It consists of two independent supports, such as poles, and a sloped roof. The roof is attached to the exterior wall of a stable structure. The two poles support the low end of the roof. The sides and front are open to the elements. This simple structure literally leans on the stable structure.
Measure 8 feet from the exterior wall and mark the location with marker paint. Measure 8 feet in distance horizontally from the first location and mark with marker paint.
Dig two holes, one at each marked location. Make each hole 12 inches deep and 12 inches in diameter. You should now have two holes, each 8 feet from the exterior wall and 8 feet apart from each other.
Use pressure-treated 4-by-4 lumber, 8 feet in length, for the support poles. Install one pole in each hole. Use cement to support the lumber in the holes. Let the cement dry for 24 to 48 hours.
Measure 8 feet from the ground up the length of the wall and mark. Hold a 2-by-8 piece of lumber, 8 feet in length, horizontally at the mark. Line up the ends of the 2-by-8 with the tops of the supporting poles. Mark the position of the 2-by-8.
Install the 2-by-8 lumber on the exterior wall using using 4-inch lag bolts every two feet at the marks you made to secure the lumber to the wall. Ensure the 2-by-8 is square with the support poles. The poles are 7 feet in height. The piece of lumber on the exterior wall that will support your roof is 8 feet from the ground. This will create your slope.
Attach a 2-by-8 piece of lumber, 8 feet in length, to the two support poles using 4-inch lag bolts. Place the 2-by-8 horizontally across the two support poles so the crossbeam will face the exterior wall. Screw one lag bolt through one end of the 2-by-8 lumber through to the support pole. Repeat for the other end. This creates the second support for your roof.
Install joist hangers on the 2-by-8 lumber that is on the exterior wall. Start at one end and install a joist hanger every 2 feet across the lumber. You should then have five joist hangers on the lumber.
Install joist hangers on the crossbeam on the two support poles. Start at one end and install a joist hanger every 2 feet.
Install five 2-by-6 pieces of lumber, each 10 feet in length. Attach each piece of lumber to the corresponding joist hangers. This creates a series of rafters to support the roof. The roof will have a small overhang.
Install 1/2-inch oriented strand board (OSB) on the rafters to create your roof. OSB board comes in 4-foot-by-8-foot sheets. Start at one end and lay one board on the rafters. Use a nail gun to generously nail the board to the rafters. Butt a second sheet up to the first sheet and nail to the rafters. Cut a third sheet to size, butt up to the second sheet and nail to the rafters.
Cover the OSB sheets with a material appropriate to your regional climate. You can use rolled roofing, shingles or tar.