1 Cord Firewood Shed

The header is too weak, wood gets its strength by its height. A single 2×6 is stronger and more resistant to sag than two 2×4’s put together. A single 2×8 likewise is stronger and resists sag more than two 2×6’s put together. etc. It looks like you have a 2×4 as the header for the 8 foot opening. If so, it’s more than 4x too weak if it’s double 2×4’s it’s 3x too weak. An old timer rule when determining what size the header should be for a large opening is to use the length to determine the 2x’s. For example, a 6 foot opening you’d use two 2×6’s (with a 1/2″ plywood inbetween so it’s 3.5″ wide) for the header of the opening. Opening 8 feet wide? Use two 2x8s and 1/2 Plywood sandwiched. 12 feet wide? Use two 2×12’s & 1/2 Plywood sandwiched. If that make sense, use the width of the opening. Since yours is 8 feet wide the header above that wide opening should be two 2×8’s and 1/2″ plywood sandwiched.