A covered roof includes a roof deck, an insulation board, and a coverboard. The insulation board includes a polyurethane, a polyisocyanurate, or a mix of polyurethane and polyisocyanurate cellular structure, has a density that is less than 2.5 pounds per cubic foot, and has a first planar surface and a second planar surface, each having a facer positioned adjacent thereto. The coverboard includes a polyurethane, a polyisocyanurate, or a mix of polyurethane and polyisocyanurate cellular structure, has a density greater than about 2.5 pounds per cubic foot and less than 6 pounds per cubic foot, an iso index of at least 175, and a first planar surface and a second planar surface, each having a facer positioned adjacent thereto.