A plank is disclosed. The plank has a cellular thermoform plastic substrate, a groove formed in at least one planar surface of the cellular thermoform plastic substrate, and a fiber reinforced thermoset plastic backing on the at least one planar surface. The fiber reinforced thermoset plastic backing includes a fiber chord positioned in the groove, a fiber matt placed upon at least a portion of the one planar surface, and a cured strip thermoset resin on the one planar surface in which the fiber chord and the fiber matt are embedded. Planks for siding of buildings and a method of forming the plank are also disclosed.