Lean-tos are simple to construct and can be an efficient way to create a sheltered, enclosed area with minimal costs. Attaching a lean-to to an existing building maximizes the usefulness of that shelter by extending the area of the original building without incurring excessive expense. The material used to construct the original building affects the ease of attaching a lean-to. Metal buildings pose the most difficulty due to their rigidity.
Determine where the lean-to will be attached to the existing building. The preferred spot would place the lean-to under the overhanging roof of the existing building. Any water runoff will flow off of the main roof and hit the lean-to at an angle to prevent pooling and flooding in the lean-to.
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Cut the 6-inch thick lumber into blocks that run the length of the lean-to.
Measure the height at which the lean-to will be attached to the existing building. This height will be guided by the roof angle and pitch of the lean-to. Mark the height on the existing building's wall using a graphite pencil.
Drill holes for the steel screws into the lumber and attach securely to the existing metal building at the height you determined in Step 3. Generally, using one screw at one-foot length intervals is sufficient. This block of lumber will provide a foundation at which the top of the lean-to will be attached to the building.
Attach another length of lumber to the top of the roof of the lean-to at its highest edge.
Use the hammer and nails to attach the lumber on the lean-to to the lumber on the existing building's wall. The two pieces of plywood will hold the lean-to securely against the existing building.