A lean-to describes a roof using one slope, or to a construction that has three walls and one sloping roof. Lean-tos can likewise be exterior additions that abut an present wall and have a roof that slants downward.
The reduced roofline in this image is really a lean-to roof, and the upper is called a skillion. Combining the two creates a fantastic area for clerestory windows.
Blackburn Architects, PC
Lean-tos were added to existing barns to get more storage area.
This duplex has two lean-tos.