I have never seen this type of material used as roof decking. Looks a little like sheet rock. The home is from 1960’s. Any thought would be appreciated.
I have seen it at least once but have forgotten what it is called. I am sure someone here will know and let us both know.
It’s some kind of fiber board. I doubt it’s rated as sheathing, so there must be a rated plywood/osb on top of that. Did you notice the roof leak in the background?
It is one of the fiberboard products, possibly Insulite or Celotex.
It was rated for roof decking and worked…until it got wet.
In the neighborhood I grew up in, several homes had it covered with tar and gravel.
I would be very careful walking on it…ask how I know! 
Somewhat common on older homes for wall sheathing, but roof decking…hmmm. Here is one I found.
That’s what it is, I had a cabin with it on the roof