Material on this deck - what, why, and how to cover?
It looks like rolled roofing to me because of the tar like substance. We normally would not walk or put furniture on rolled roofing. It may have been put down to keep water out of the walls and possibly a Sheetrock ceiling below when it rains. On the decks like this I have done, we overlaid flooring to protect the seal.
Today more people use membrane roofing to seal things like this and still add a walking surface. I would want a interlocking tile or another rigid surface so chairs won’t punch through.