Does the roc have the mass of 4 elephants, or just the size? Wingspan would make a huge amount of its surface area. I imagined rocs as being huge, but not especially heavy, like pterosaurs.
The size I think determines the creature's weight directly, taking into account materials the creature is made of. It also determines the creatures actual strength and toughness, bigger creatures are stronger and tougher than smaller one's.
The ceilings can't be too thick if cave-ins are to occur. I'd say they're no more than an extra meter of rock.
Megabeasts pretty much have to be magic. I'm not sure what gravitational constant would allow a bird the size of 4 elephants take flight.
That we manage to carve out vast halls underground without needing support beams rather suggests that the ceilings are pretty thick.
It is quite possible for a bird the size of 4 elephants to take flight, the trouble is *only* finding a suitably size runway and being able to reach a sufficiant takeoff velocity. The former problem is a problem in everything except desert areas which would not be able to support so large a creature anyway, the latter problem is built into the nature of avian physiology; basically their humanoid build keeps them from obtaining a takeoff velocity sufficiant to allow very large birds to fly.
There have been truly massive fliers in reality before, such as certain pterodactlys and these solved the humanoid problem by actually having feet at the joints of their wings so that when they ran along the runway they would be temporarily become quadrapeds and thus they could reach a sufficiant takeoff speed. Rocs would probably solve the problem in the same manner, they would have wing-feet.