Measure between the trims at the top bottom and middle this will tell you if the ends of your shed side panel are square.
Gable end siding shiplap.
I started installing the shiplap cladding on the gable panel.
The peak is the tricky part multiply half of the overall wall length 12 by the vertical distance from the wall height to the peak 8 to get the gable end s square footage 12 x 8 96 sq.
Then add 20 on the gable to cover the waste of all those angle cuts 96 x 20 115 sq.
Picture of corner trim as installed.
The corner trim gives a solid edge to fix the siding up to and it also gives some end protection to the end grain.
Gable end walls exterior is finished with lp smartside prepainted wood panel siding 4 8 sheets tyvek house wrap 7 16 osb attached to the 2 12 s on 16 centers.