Hi. Backer board is a concrete board reinforced with fiberglass mesh that is manufactured for use as a substrate upon which to lay tile floors. I love the texture, color and striations in it... and the cost is right. If used outside, it's wise to seal it with a concrete sealer. We built this bridge out of 2x6 pressure treated wood, then covered it with the backer board. It comes in 1/2" and 1/4" thicknesses. We used 1/2" for this but, for vertical projects, I prefer the 1/4".