Decking on child's playset
My son-in-law is building a playset outdoors for his children. He is using left-over wood from his attic flooring (I think it's pressed wood -- has wood chips showing) for the deck of the playset. Will this be safe left unfinished? It's about 4ftX4ft with the only bracing being around the edges. It will currently hold his weight -- over 200 pounds, so he says it will be fine for the little guys.
I thought that type of wood was meant to be covered up, protected from the elements, but the older I get, the less sure I am of anything, so I would like some advice from those who use this product. Thank you so much.