It seems like at max it's 7/8 of an inch but it also drops at certain points to 3/4. It's not smooth or consistent, but maybe the piece that I have is broken. If it has paper backing throughout I think it would have to be consistent.
Maybe it is stone or concrete? How can I tell?
Did you see the little fibers in it? Like dried grass or something...
It seems like directly behind it there is some kind of wire mesh, then an air gap and then I think whatever the outside bricks are built on.
I live in Baltimore Maryland.