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I am going to upgrade my home safe. The safe will be on the second floor, which is the top floor. The safes I am looking at weigh 350 to 500 pounds. I could probably get by with a 300 to 350 pound safe. I mentioned this to a contractor who is doing non-structural work on my house, and he said the floor would buckle. He advised 150 pounds, max. The current safe is less than 100 pounds.

This seems odd to me, because people who weigh 300 pounds, of whom there are many, are not cautioned to avoid their second floors or to spread their feet wide apart.

This contractor has done excellent work for me on painting, carpentry, tiling, and repair of non-load bearing structure. I am not sure how seriously to take his warning.

Any other advice on installing the safe? The seller will get it upstairs and into position.

The reason I want a heavy safe: If I have home health care (hope never), I want a safe that is definitely health-aide and house-keeper proof.

What I decided, based on Answers and Comments: I cannot deal with a significant probablility that my floor will buckle or collapse. So I decided to get a lighter safe (vicinity of 100 lbs) and screw it from the inside to the walland/or floor. And, of course, use a passcode that would be impossible to infer, plus get a safe with a "three guesses and you are locked out feature". I will also sell things that I no longer use (always a good move), move a few cherished things to my safe-deposit box, and update the appraisals on a few things so I am more fully covered by insurance. That is, a mixture of strategies instead of an exclusively brute force strategy. THanks to all.

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    a 300lbs safe is no less safe from nurses and housekeepers than a 500lbs safe. It's also no less safe from locksmiths. The only additional protection a heavier safe would offer is from brute-force entry, but I hardly think that's a realistic security threat for the scenario outlined. Get a decent but cheap safe and monitor it with an alarm or camera. There's also, engineering-wise a difference between a permanent static load and a big person, and the person is usually un-avoidable, while the safe isn't.
    – dandavis
    Aug 2, 2021 at 22:25
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    A snug toggle/lag bolt from inside the safe (or even just a lockbox) would be far preferable. It's also not just that the floor would fail, but heavy weight could shift the slow settling of the foundation and cause bad things that way too.
    – dandavis
    Aug 3, 2021 at 7:13
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    There are error margins that need to be taken into account due to joist layout, but the load needs to considered for full length of the room or span. So a 3 foot wide safe in a 10x10 30psf room has only 900 pounds of capacity for that 3x10 edge of the room (if ideally located in relation to joists). Aug 3, 2021 at 18:26
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    It's worth mentioning again that bolting the safe to the floor dramatically increases resistance against being casually hauled out and weighs nearly nothing. Aug 3, 2021 at 22:11
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    Thank you for providing on update on your decision. Bear in mind that damage from fire can be more devastating than theft, and you might consider a larger safe or a second one with high fire rating to store items of high sentimental value but low street value.
    – P2000
    Aug 4, 2021 at 18:53

3 Answers 3

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Depends.

We always add 2 extra joists under refrigerators just because kitchen floors often have tile flooring and cannot sag without cracking the tile.

However, depending on the span of the joists, the floor could easily support such a load. Floors are designed for 40 lbs. per square foot (psf) so if you add up all the tributary area (the area including to the nearest joists) around the safe, I’m sure it could easily support the safe…if the floor system was originally designed correctly.

Often the bearing walls for floor joists are not spaced equally across the entire house. So, we design for the “critical” span (the longest span) and make all the joists the same size. You could easily check to see if your safe will rest on a large span or short span. (Note: If the span is extra short, they may have switched from 16” on center to 24” on center. )

The weak spot might be where the legs rest between the joists. You might add a 2x4 plate under the legs to help the subfloor.

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  • "We always add 2 extra joists under refrigerators" What about ranges? My range weighs 500lbs!
    – Glen Yates
    Aug 3, 2021 at 14:45
  • @Glen Yates Not “always”. If it slides into a cabinet or is small, we don’t worry about it. If it’s a larger unit, then yes. (It’s the concept. There’s always exceptions…)
    – Lee Sam
    Aug 3, 2021 at 14:58
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Assuming all the other advice here leads you to the conclusion that your home won't collapse from adding this safe, you should protect your floor from surface damage.

Put a piece of 3/4 inch plywood under the safe. You might get an offcut for free from a big hardware chain. This will protect the floor surface from being crushed or cracked, especially if the safe has feet or other irregularities underneath and especially if your floor is soft wood, engineered wood, vinyl, tile, or anything else that's easily crushed or cracked. If you happen to have any cracks or weak points in your subfloor, a piece of plywood will also better spread the load around the floor boards and onto nearby joists.

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I found it is interesting to answer the question of why the floor can support a 300lbs person, but not safe to support a piece of heavy furniture/appliance weights the same.

1. Effect of intermittent/moving concentrated load vs permanent concentrated load:

Do you feel the floor is bouncing when such a heavy person walks by you, and what happen when he passes over you? Isn't everything back to normal?

Simply put, a person is a lifely moving object as opposed to the fixed nature of the heavy furniture that usually stays in the same location for a long period of time. When a heavy person weighted down the floor, all the support beams/joists in the vicinity will come to help, the floor will return to the unstressed state once the weight is lifted.

However, as the heavy furniture does not move, the weight felt by the floor will always be there, which will cause permanent stress in the support members directly under it, and some built-in stress in the nearby support members, because the rebound/stress release will not occur until the heavy furniture is removed. Now, due to the built-in stress, the adjacent floor load capacity is thus reduced, and it is considered "unsafe" unless blissed by a structural engineer.

2. Size of the footprint counts

The building code requires the residential floor to be able to support a 40 psf (pounds per square feet) live load. For your safe that weighs 300lbs, it requires a footprint that covers an area of 300/40 = 7.5 SF (square feet). Your floor is fine if the footprint of the safe is equal to or larger than that, otherwise, you need to find a way to increase its footprint, which shall be determined by a structural engineer.

By the way, ideally, you shall make sure the safe is sitting on top of at least two joists, or you shall add transverse support members to better distribute the weight. Again, for any change in need of structural modification, a structural engineer shall be consulted.

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    The first half of this answer explains why you shouldn't use the live load capacity to calculate the safe dead load... which is exactly what you do in the second half of the answer. As you point out in the very first sentence, the fact that a floor can intermittently support a 300lb person does not necessarily mean it will support a 300lb safe indefinitely. Aug 3, 2021 at 19:10
  • @NuclearHoagie The first part is to explain the different structural responses to the live load (transient in nature) and a permanent dead weight, one recovers, the other does not. The second part explains the need to spread out the dead load by physically enlarge the area the dead load occupies, otherwise adding structural supports. Note that the area occupied by the dead weight should not have other load imposed on it, so the allowable floor load is in no way to be exceeded.
    – r13
    Aug 3, 2021 at 19:39
  • @NuclearHoagie Furthermore, the first part deal with overstress, say 300 lbs over 4 SF (roughly a heavyweight person occupies), it turns out a 75 psf floor load that is far above the 40 psf design load. For the second part, if the 300 lbs safe occupies a 7.5 SF area, the floor load now is identical to the design load - 40 psf. Hope you can see the difference. Good question though.
    – r13
    Aug 3, 2021 at 20:00
  • Thanks for this analysis!
    – ab2
    Aug 4, 2021 at 16:50
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    @ab2 You are welcome.
    – r13
    Aug 4, 2021 at 16:52

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