Lean-to
Three walls are the charm for a lean-to structure
A lean-to refers to a roof with a single slope, or to a building that has three walls and a single sloping roof. Lean-tos can also be exterior additions that abut an existing wall and have a roof that slants downward.
The lower roofline in this image is a lean-to roof, and the upper is called a skillion. Combining the two creates a great space for clerestory windows.
Lean-tos were commonly added to existing barns for more storage space.
This barn has two lean-tos.