What material is it made of.

All the one's I've ever seen nowadays are Aluminum. Sometimes I will use stainless steel, pan head screws, instead of the spiral aluminum nails, depending on what they're going into and especially if it's over carpet that you may need to pull up some day. However, I would recommend finding a piece of stainless steel for outdoors near a pool. I'm not sure what effect the chlorine will have on the bare aluminum. But I'm not a chemist, so.....