These are made of some type of synthetic material. Not sure what it is but seems pretty durable. I was going to make outdoor bench seating out of these but my plans changed. I have 2 of the long ones and they measure: 76 3/4" long 11" wide The lip is 5 1/2" tall I have 1 short piece and it measures: 27" long 23" wide The lip is 3 1/2" tall Free. Must take all. Pick up in valley ridge in Calgary.