The present invention relates to a composite fiberboard, which is specifically adapted for use as a coverboard in roofing systems. The formulation and physical characteristics of the composite fiberboard of the invention have increased the flexural strength of the board relative the flexural strength of current commercial fiberboards of similar type. In turn, the composite fiberboard is able to meet the Class 90 wind uplift requirements as measured by Factory Mutual FM 4450 Approval Standard for Class I Insulated Steel Deck Roofs.