To correctly attach recycled plastic decking to something immobile like steel, the holes through which the fixings pass must be larger than the fixings themselves to allow for thermal expansion and contraction across the seasons. This is dealt with by using large headed bolts or screws so there is no visible difference and the planks will neither warp nor buckle. While the following page deals with issues than can occur with incorrect fencing installations, much of it will be relevant to decking. https://britishrecycledplastic.co.uk/how-to-calculate-recycled-plastic-contraction-and-expansion/