Will a typical house floor carry a 300 lb. safe?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.