Abstract: Apparatus for mounting continuous length sheets of thermo-expansive transparent plastic includes longitudinal rods for affixation along the longitudinal edges of each sheet to form a thickness enlargement thereat and mounting members having channels formed therein adapted to receive and retain the enlargement. The channel has an interior area larger than the rods and a restricted outward opening smaller than the rods but larger than the sheet thickness whereby in assembly space exists in the receiving area unoccupied by the rods and in the restricted opening unoccupied by the sheet permitting shifting movement of the enlargement and the sheet therewithin for relatively free thermo-expansion of the sheet. Each sheet is supported in flexed condition along its lengthwise edges by two such mounting members. Plural sheets may be mounted in side-by-side arrangement to form a roof or other structural or architectural surface. Sheets may be mounted by this system to form overhead skylights, canopies or the like.