UHMW polyethelene for the top. It has a higher impact rate than lexan and a lower friction rating. It's about the same price but you get a better and longer lasting plastic with the poly. It's also more pliable which is why, if you had it on a wooden box before and left it out in the hot weather, it would warp with the expansion and contraction of the wood. Happened to mine as well but if it was lexan it could have broken it. Just take the top sheet off when winters over.