Last summer my inlaws put up a deck with that composite stuff and had some left over I was eyeballing for use as bed wood.
Looks like sawdust mixed with some kind of other component.
They left a few boards out in the rain with a board under them and they warped like hell.. I smacked one with a 15 lbs sledge just to see how tough the stuff was and it made a nasty dent that chipped away like particle board.
Kind of spooked me from trying to use the stuff as bed wood.
Now they just used it for the 'floor' of the deck.. and it's not real high traffic and covered so it won't see much sun or rain.
I'd be curious to see what a bed made of that stuff ends up looking like after four or five years of heavy abuse and 365 days a year exposure to the elements.
