usnavdoc this is two parts.
1. If you have a truss system with a 3/4" decking and 2x6 at 12 OC this you are about 275LBS. Providing the span is not over 16 feet
2. A 2x12 at 12 OC with a 3/4" decking should be good for about 350 lbs. (NTE 16FT)
If you spread either one of the scenarios above to 14" or 16" OC then you need to reduce by 35 and 50% respectivly.
You can get some great safes with under 250LBS. I would strongly suggest placing a large safe of over 400LBS in the garage or on a concrete deck!
I retired out of Coronado California in 1996. Spent allot of time in the Americas and of course Desert Storm.