Will Your Floor Hold It?

Someone on r/preppers worked out that a 165 gallon tank would put over 200 pounds per square foot on his basement floor, against the 40 pounds per square foot he had read a house is rated for, and asked whether he was about to wreck his foundation.

He was not. But the answers he got were a mess, because the thread was arguing two different questions as though they were one, and that is true of nearly every version of this discussion online.

The two questions

Is there a space under this floor? If the answer is no, the concrete sits on the ground and the ground carries the load. If the answer is yes, the floor is a structure spanning between supports, and it has a design limit.

Those two cases have different numbers, different units and different failure modes. Both start from what the water actually weighs, which is the one figure everyone gets right. Almost all the confusion comes from applying one case’s number to the other.

If it sits on the ground

A basement slab, a garage floor, or a ground floor in a slab-on-grade house is not holding your water up. The soil is. The slab spreads the load out over that soil and keeps the surface flat.

This is why the reply “don’t garages hold cars?” is the right instinct even though it is not really an argument. A 4,000 pound car puts its whole weight through four contact patches of about 25 square inches each. That works out to roughly 40 pounds per square inch, which is 5,760 pounds per square foot. A 55 gallon drum sitting on its 3.1 square foot base is putting down about 155 pounds per square foot, or a little over one pound per square inch.

The drum is not close. Not to the car, and not to what the slab can take.

The exception is concrete you know nothing about. An old patio pad might be three inches of unreinforced mix over poorly drained fill. One structural engineer in that r/preppers thread put the working figure for an unknown outdoor slab at about 10 psi of contact pressure, on the reasoning that people put down roughly 15 psi when they walk on it and it is holding up. That is a sensible ceiling for a pad nobody has records for.

If there are joists under it

Now the numbers from the code table apply, and they are lower than people expect.

The IRC sets minimum design live loads by what the room is for. Living areas are 40 pounds per square foot. Sleeping areas are 30. A habitable attic reached by fixed stairs is 30. An attic with limited storage and no stairs is 20, and an attic without even that is 10. A garage floor is 50, in the case where it spans over something rather than sitting on the ground.

Those are uniformly distributed loads, which is the part that gets lost.

Forty psf does not mean a man breaks the floor

Someone in that thread said a grown man would go through a floor rated at 40 pounds per square foot, since he weighs 200 pounds and his feet cover well under a square foot. It reads like a good point. It is the central misconception.

A design live load is what the floor system carries spread across its whole area, all at once, with the code’s safety factors already inside it. It is not a pressure limit at a point. When you stand on a floor, the sheathing spreads your weight across several joists, and each joist takes a fraction of it. Floors are separately checked against concentrated loads, which is why a stair tread has to hold 300 pounds on a two inch square.

So the question for a drum is not “does 165 psf beat 40 psf.” It is “what does this add to the joists that are carrying it, on top of everything else in the room.”

The number that actually matters

A 55 gallon drum holds about 460 pounds of water, and another 20 or so for the drum, and it is the container most people reach for once their storage target climbs past a few dozen gallons. Put it in a bedroom, where the design load is 30 psf, and it uses up 16 square feet of that room’s allowance. A four by four foot patch.

That is usually survivable in a room that is not otherwise loaded, on joists in good condition, and much safer against a bearing wall than in the middle of a span, where the same weight does the most bending. It is not survivable as a row of four drums along an interior wall upstairs, which is roughly 1,900 pounds in about twelve square feet.

Two rules do most of the work here:

Spread it across joists, not along one. Weight lined up parallel with a single joist loads that joist alone. A sheet of plywood under the containers, laid across the joist direction, shares it.

Get near a bearing wall. Load at mid-span bends a joist the most. Load a foot from where the joist lands on a wall barely bends it at all.

Contact area is not footprint

The trap that catches people with totes: an IBC tote covers about ten square feet of floor, but it sits on a pallet, and the pallet touches the ground along a few narrow rails. The engineer in that thread measured it at a little over one square foot of real contact. Same weight, ten times the pressure.

If you are on a slab this rarely matters. If you are on an old pad or a wood floor, it decides the answer, and the fix is a sheet of plywood or a couple of 2x6s to turn one square foot of contact into eight. Container choice and long term storage are downstream of this, not the other way round.

Run your own numbers

Enter what you are storing, what it is sitting on and what the room is, and this returns the load in the units that case is actually judged in, against the design figure for that room.

It will tell you when the answer is obviously fine, which is most of the time. The figures behind it, and where they come from, are listed in the methodology. When the answer comes back close, that is the point to ask a structural engineer rather than a forum, and the arithmetic it shows is what you hand them.