Building a lean-to on a garage provides instant space for storing firewood, outdoor tools or hay. A lean-to is basically a structure with a roof sloped in one direction either attached to another building or free-standing. While many lean-tos are enclosed with walls, often they're left open in at least one direction, sometimes more. The versatility of the structure allows it to be used for a variety of purposes, even as an extra carport or an office space off the garage.
Plan your lean-to. Decide how far from the garage it will extend and how long it will run down the side of the garage. You may choose to have it run the length of the garage, but if you don't plan on needing that much space, you can certainly make it smaller. Think about your intended purpose for the lean-to when deciding on its size. Check to see if any building permits are necessary for your structure.
Dig post holes 4 feet apart along the length of what will be the outer edge of the lean-to. Check with your local builders association or the building code to see what the frost line is in your area and make sure the post holes are dug below the frost line to prevent the structure from shifting in cold weather. In most places, this is between 1 and 4 feet.
Mix the cement in a mixer or with a wheelbarrow and shovel. Set the tubes in the post holes, so they stick about 6 inches above the ground. Pour the mixed concrete into the tubes and put the rebar into the concrete, leaving 4 inches exposed at the top of the tube. Let the concrete set.
At the bottom of each 4 by 4 post, drill a 4-inch hole to fit over the rebar. The hole should be snug, so the post is set firmly on the concrete pier. Cut the posts to the appropriate height and mount them on the piers.
Screw a piece of 2 by 6 to the outside of the posts and to the side of the garage running the length of the lean-to. Attach rafter brackets with screws to the inside of these 2 by 6 beams, then hang 2 by 4 and 2 by 6 rafters between the beams using screws. If you're planning an enclosed lean-to with a floor, screw floor beams to the outside of the posts and the wall of the garage as well. Add joist brackets and hang joists.
Nail OSB to the roof of the structure. If you're planning walls, frame the walls out of 2 by 4s and attach them to the roof and floor frame with lag bolts or large screws. Nail OSB to the outer walls and floor as well.
Staple or nail tar paper to the roof and walls. Nail shingles to the roof. On the walls use siding that will match the garage's siding.