Abstract: Moisture entrapped within roofing systems of the type wherein a layer of normally low-permeance, cellular plastic insulation board (e.g. foamed polystyrene) is employed in conjunction with at least one layer of moisture-bearing construction material (e.g. lightweight insulating vermiculite concrete) is made able to be vented from within the system by providing the board with a plurality of openings therethrough and further preventing the fluid construction material when placed upon the board from entering and filling the openings. Such prevention of filling of the openings may be accomplished by making the dimension of the openings such that the fluid construction material will not flow therein yet moisture will pass, or by covering the openings with a material (e.g. paper) which will prevent such filling, the material however being permeable by any moisture later entrapped within the roofing system.