Aqua Shield Boxwood Stair Treads, 8.5 by 30-Inch, Medium Grey, Set of 4 Unique ridged construction effectively removes and traps dirt and moisture Molded design will not crush or breakdown over time or use Anti-Static, 100-percent premium polypropylene fiber face will not mildew, mold or rot Made with commercial grade SBR rubber backing Waterguard stair treads are certified slip resistant by the national floor safety institute