My gunsafe at about 500# is in my shop above my garage. I've never had a concern about it, other than getting it up there (took 3 of us and an appliance dolly-lots of sweat and some blood from scraped knuckles.) Of course, I can see the supports underneath as the garage doesn't have a finshed ceiling.
I'd say that it's not a worry. I'd worry more about getting it up there than whether it's too much weight.
I would guess that most modern apt's might have flooring that would take into account a waterbed's weight, so they could probably handle a safe. That is, however,
just a guess. One never knows what runs through the tiny little minds of those who determine what building codes should be.
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Shoot straight regards, Richard at The Shottist's Center
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