For my metal shed door, the best wood is framing lumber. Framing lumber is made of a 2-by-4 or 2-by-6 inch wood shed that is cut into specific lengths and used to form the frame of a house or other building.  The lumber is also treated with chemicals to resist insects and rot. This makes it an ideal choice for the door of a metal shed, as it will be durable and long-lasting. On top of that, framing lumber is relatively inexpensive, making it a cost-effective option for this project.