Iron, as would have been used in Roman nails is VERY reactive with oxygen, (think rust) and would have degraded any DNA smear on it's surface.
It's a very poor place to have DNA survive in readable form for 2000 years unless it was stored in a cold, dry, inert gas environment.
It's a very poor place to have DNA survive in readable form for 2000 years unless it was stored in a cold, dry, inert gas environment.