I have watched this video before and I have always been amazed at their skill and expertize. Like you said, it was not his 1st time, . I was wondering if they go back after the mortar dries and re-mortar the outside to fill in the gaps and help seal everything together to avoid leaks. The mortar mix that he uses looked very watery to me, I was surprised by that. It must make it stickier somehow. Very impressive roof structure.