I believe it is 4mm (@3/16″)ply. It bends into the mold easily but use caution on the number two panels. Don’t grab them by the edge while bending them into the mold especially about 1/3 of the way from the front: I cracked one at its edge – had to take it out and repair it (glass patch over crack in outside surface. Can’t really notice it in the beautiful in the finished trailer. Don’t be shy; its an easy fix.