It will require some careful angled cutting but its not too complicated. There are two slopes to the tops of the walls but the roof is a plane with one slope; one corner is lowered which raises the opposite corner. The slope of the roof will be perpendicular to a level line between the tops of the two 11 1/3 ft high corners.
The pitches in the original post are for the tops of the exterior walls; the pitch of the roof will be about 2 1/4 ft in 12 ft. shown below with the 12 ft long red triangle.
perspective
elevation
elevation
roof plan
Q
Entrance
Q