Abstract: An extruded plastic decking plank for mounting to an underlying support structure, the plank having a rigid foam core, a resilient outer plastic shell, and a clamping portion for securing the plank to the support structure. The top surface of the plank can be provided with a non-slip surface. The invention also includes an attachment system for securing such decking planks to a support structure by engaging the clamping portions of the decking planks onto clamps or hold down blocks which are secured onto the support structure, and which permit relative motion between the planks and the structure in the planks' lengthwise direction to prevent stress and buckling caused by uneven expansion.