A roll (16) of structural material for use in the manufacture of large composite structures such as wind turbine blades is described. The roll (16) comprises an elongate stack of structural layers (10) fastened together and wrapped around a reel (14). The reel (14) comprises a plurality of mutually spaced supports (50) about which the stack is folded into a rolled stack. Sections of the rolled stack between the supports are substantially unsupported by the reel and are held in a slack state in order to prevent wrinkles from forming in the rolled stack.