I might opt for a more textured "paver". I can't see what's above it, but I'd be willing to bet you have an area of the roof that runs off to that spot as well. When that happens you'll NEVER get anything to grow. I know from whence I speak; we have a spot like that as well (not as large, and not with the doggie addition, but same problem). The textured material would be better suited for that runoff (IMO).