Abstract: A composite insulated roof board structure comprising a light-weight insulated roof board member having good thermal insulation properties and a protective top layer which overlies the insulated roof board member. The insulated roof board member comprises a main portion of rigid coherent solid insulating material, such as polyisocyanurate or expanded polystyrene, and a top portion comprising a plurality of spaced blocks above the main portion and integral therewith. The blocks form therebetween a network of interconnected channels which are at the same level for horizontal venting of water vapor to all peripheral edges of the roof board member. The blocks are typically square and the channels are typically in the form of two sets of mutually perpendicular channels. The roof board composite is typically formed as a panel of rectangular shape.