Building a lean-to shed requires basic knowledge of framing, but aside from having the tools, materials and helpful information it is a job that can be done by people with little woodworking experience. Because it “leans” up against an existing structure, an additional foundation is not necessary. A true lean-to is built up on the side of a garage or a house. It is the ideal place to store just about anything that would otherwise be stored in the garage.
Step 1: Foundation and Floor Joists
Since the lean-to is supported on one side by an existing structure, the only foundation required will be two parallel 4x4 pieces of pressure treated lumber. These will lay parallel to the side of the structure you build the shed against. Start by constructing the floor. Attach 6-2x4s cut at 45 inches–the floor joists–spaced evenly between 2 pressure treated 2x4s cut to 6 feet in length–the rim joists. Cut 2-6 foot lengths of pressure treated 4x4 lumber for the foundational skids. Attach these to the floor frame parallel to the rim joists 1 ¾ inches from the edge.
Step 2: Plywood Floor
On top of the floor joist frame, cut and lay plywood cut to 4x6 feet. This should require one piece of plywood because you buy it in 4x8 foot sheets. Set the completed floor frame in place against the structure.
Step 3: Back Wall
The back wall is 8 feet high by 6 feet wide. Using a 2x4, cut a bottom plate to 6 feet in length and a 6 foot long top plate. Cut 4 studs to 8 feet in length and miter the tops of them to 18 degrees. Nail the studs to the bottom plate at their flat ends and the top plate to the mitered end. Make sure they are spaced evenly across the 6 foot plates. Erect the back wall frame, attach it with nails and temporarily hold it in place.
Step 4: Front Wall Specs
The front wall will have a 4 foot wide door in the middle. With 2x4 lumber, cut a 6 foot top plate and two 1 foot bottom plates. The front studs are 81 ¼ inches in length–this number is based on the 18 degree miter of the back studs. Cut 4 to this length. The width of the door is 4 feet or 48 inches. To this add the width of two 2x4s–3 inches. Thus, cut two 2x4s to 51 inches in length for the doubled header. Attach these to each other. The jack studs give extra support to the header, and they should be 73 inches in length. Cripple studs give support between the top plate and the header. Cut two to 6 ¼ inches.
Step 5: Build Front Wall
Build the front wall in much the same way. To the two bottom plates attach two studs on either end. Next, attach the other ends of each to the top plate. Attach the jack studs along the inside edge of the two inner studs. Atop the top end of the jack studs run the double header across and attach it. The cripple studs will align with the rafters and should be spaced evenly between the two end studs. Use 16 penny nails for all joining and attach the front wall to the floor frame and brace it.
Step 6: Side Walls and Rafters
Each side has a 41 inch long top and bottom plate. There are also 3 studs for each side cut to 81 ¼ inches. Build each wall section separately and position them in between the front and back walls. Secure them with screws. Cut 4 rafters from 2x4 lumber. They will rest atop the front and back top plates. Miter the rear end to 72 degrees and the front end to 18 degrees. The top length of each rafter – from mitered end to mitered end should be 50 and 9/16 inches. The bottom of the front end on each rafter should lay flush upon the front top plate. To make it fit, measure 3 and 5/16 inches in from the front mitered end on the bottom of the rafter. From that point, make an 18 degree cut perpendicular to the front miter. Now the rafters should both be flush against the structure and atop the front plates. Space them evenly across the plates, matching up the two inner rafters with the cripple studs on the front wall. Cut and install one gable stud between each side wall top plate and the end rafter.
Step 7: Siding and Back
Attach ½ inch CDX on the back from the bottom of the rim joists up to the top of the rafters at the back end. On the front and sides, install siding from the bottom of the rim joists all the way up to cover the sides of the rafters. Use one piece per side if you can, cutting it at an 18 degree angle.
Step 8: Roof
Lay ½ inch CDX sheathing atop the rafters for the roof sheet. On top of this, roll out enough 15 pound roofing felt or tar paper to cover the sheathing and nail on asphalt shingles with roofing nails.
Step 9: Door
Use siding framed with 1x3 inch strips of wood to make the door and attach it to the front wall with hinges.
Your lean-to is built. Now you can trim it to cover any exposed edges and paint it like you wish.