Shadow Lines are Horizontal Grooves on the Face of Steel Garage Doors
Some garage doors have shadow lines on them. These shadow lines are horizontal grooves along the bottom and top of the sections. They are there add strength, rigidity and straightness to the face of steel doors. These grooves are separate (and in addition to) the seams between the garage door sections.
Shadow Lines Are Often Only on 21″ High Sections
If doors do have shadow lines, they may or may not have these grooves on 18″ high sections and might only have them on 21″ high sections. 7′ high doors will generally come with (4) 21″ sections and so all sections would have shadow lines. 6′ 6″ high doors will generally have (2) 18″ intermediate sections without shadow lines.