According to one aspect, a roofing system comprises a plurality of insulation boards adapted for overlying a roof deck to form a layer of insulation. Each of the plurality of insulation boards has opposing planar surfaces and has a facer on at least one planar surface, and each of the plurality of insulation boards comprises polyisocyanurate foam having a first polyisocyanurate foam density. The roofing system further comprises a plurality of cover boards adapted for overlying the layer of insulation. Each of the plurality of cover boards has opposing planar surfaces and has a facer on at least one planar surface, and each of the plurality of cover boards comprises polyisocyanurate foam having a second polyisocyanurate foam density greater than the first polyisocyanurate foam density. The greater polyisocyanurate foam density provides for a compressive strength to resist deformation.