Systems and methods for rig assembly and for erecting a mast of a rig and such systems with roller apparatus to facilitate movement of a mast section from a mover. In a first embodiment, the method includes erecting a rig mast section on a substructure, the substructure including a first substructure box spaced-apart from a second substructure box, the first substructure box comprising a first upper box above a first lower box and the second substructure box comprising a second upper box above a second lower box.