How to Tally the Load of a Filled Tub or Tank on a Floor
Why this matters
A soaking tub on a second floor, a hot tub on a deck, a water storage tank in an attic or a large aquarium in a living room all put a load on framing that was not designed with that load in mind. The customer asks "will the floor hold it?" and the honest answer from a tech is neither yes nor no: it is a number, compared against what the floor was designed for, and a decision about whether the number is ordinary or needs an engineer. This card builds that number. It is a screen, not a structural verdict. What it does well is catch the case that obviously needs a structural check before the fixture is ordered, rather than after the ceiling below starts to crack.
The tally
FILLED-FIXTURE LOAD TALLY
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1 Water capacity ____ gal (source: ______)
2 Water weight = L1 x 8.34 ____ lb
3 Empty weight (spec sheet) ____ lb
4 Occupants: ___ x ___ lb ____ lb
5 Other (stone, gravel, cover) ____ lb
6 TOTAL = L2 + L3 + L4 + L5 ____ lb
7 Footprint: ___ in x ___ in = ____ sq in
8 L7 / 144 ____ sq ft
9 Footprint load = L6 / L8 ____ psf
10 Design live load for the room ____ psf
11 Ratio = L9 / L10 ____
12 Support points (feet, legs): ____
load per point = L6 / count ____ lb
Route: ordinary / structural check / engineer
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The steps
1. Take the water capacity from the maker, not from a guess. Tub and spa spec sheets state water capacity, usually to the overflow. For a tank, figure it from the inside dimensions; see related: How to Figure the Volume of a Round or Rectangular Tank. Write where the number came from. A capacity stated to the overflow is the right one to use, because that is the most water the fixture can hold.
2. Convert gallons to pounds with 8.34. Fresh water weighs 8.34 lb per gallon near room temperature. Hot water at bathing temperature is lighter by under 1 percent, which is not worth correcting for in a screen. The constant and where it stops being true are owned by The Water Constants 7.48, 8.34 and 231. The same conversion applied to a flood spread across a floor, rather than a fixture concentrated on a footprint, is What Standing Water Weighs on a Ceiling or Floor.
3. Add the empty weight from the spec sheet. A cast iron tub, a stone resin tub and an acrylic tub of the same size differ several times over in dry weight. A portable spa's spec sheet gives its dry weight. Never leave this line blank: on some fixtures the empty weight is a large share of the total.
4. Add the people. Use the seat count for a spa and one bather for a tub, at a planning weight per person the shop agrees on and writes down; 200 lb per occupant is a reasonable starting figure. Adding the full water and the full occupants together is conservative, because a bather displaces some water that would otherwise be in the tub. For a screen, conservative is the right direction.
5. Add anything else that sits in it or on it. Stone or gravel in an aquarium, a spa's cover, a tile surround built onto the fixture, a stand. Wet gravel and rock weigh more per gallon of space than water does.
6. Divide by the footprint to get pounds per square foot. Measure the outside footprint the fixture bears on, in inches, and divide by 144 for square feet. This is the footprint load, and it is the number you compare.
7. Compare with the room's design live load. IRC Table R301.5 (in the 2021 edition, as adopted by your jurisdiction) sets the minimum uniformly distributed live load at 40 lb per square foot for rooms other than sleeping rooms and 30 for sleeping rooms; check your edition for the figure it gives decks. A bathroom is a room other than a sleeping room, so 40 is its figure. Comparing is a screen for one reason: that 40 psf is a design load assumed to be spread evenly across the whole floor, while a filled tub concentrates its whole weight on the few square feet under it. A footprint load above 40 does not mean the floor fails. It means the fixture is outside what the uniform design figure describes, and whether the framing under it carries it depends on joist size, spacing, span and where the fixture sits relative to the bearing walls, which is a structural question.
8. Route by the shop's rule. A rule to start from, tuned to your market: any spa or hot tub on a framed floor or deck goes to an engineer, or to the spa maker's published support requirement checked by someone qualified to evaluate the framing, before it is sold. Any other fixture whose footprint load is above the room's design live load and whose total is above about 1,000 lb gets a structural check before install. Below both, it is an ordinary fixture and the install instructions govern.
Worked example 1: a 400 gallon spa on a deck
A customer wants a portable spa on an existing wood deck. The spec sheet gives a water capacity of 400 gallons, a dry weight of 650 lb, four seats and a footprint of 7 by 7 ft.
Line Item Value Unit
1 Water capacity (spec sheet) 400 gal
2 Water = 400 x 8.34 3,336 lb
3 Empty weight (spec sheet) 650 lb
4 Occupants = 4 x 200 800 lb
5 Other 0 lb
6 TOTAL 4,786 lb
7 Footprint = 84 x 84 in 7,056 sq in
8 7,056 / 144 49.0 sq ft
9 4,786 / 49.0 97.7 psf
10 Design live load 40 psf
11 Ratio = 97.7 / 40 2.44
Nearly 4,800 lb on 49 square feet, 97.7 psf, which is 2.44 times the 40 psf design figure. By the rule in step 8 this is an engineer's call regardless of the ratio, because it is a spa on framing, and the ratio says why the rule exists: the water alone (3,336 / 49 = 68.1 psf) is already 1.7 times the design figure before the spa or anyone in it is counted.
What the tech does and does not say. The tech does not tell the customer the deck will or will not hold it. The tech says the spa's filled weight is about 4,800 lb, that this is well beyond what a deck is normally built to carry over that area, and that the placement needs either an engineer's design or the spa's own support requirement checked against the deck's framing before anything is ordered. Many spa makers state a required support capacity in pounds per square foot on the spec sheet; compare it with what the deck was built for, and the engineer decides.
Hazards: do not test-fill a spa on a deck whose framing has not been checked; the test is the load. Spa wiring falls under NEC Article 680 as adopted by your jurisdiction and is work for a licensed electrician, including the disconnect and its required location. Moving a 650 lb shell is a crew lift with a dolly or spa cart and a cleared path, never a two-person carry up deck stairs.
Worked example 2: an 80 gallon freestanding tub upstairs
A remodel puts a freestanding cast iron tub in a second-floor bathroom. The spec sheet gives 80 gallons to the overflow, 300 lb dry, a footprint of 66 by 32 in, and four feet.
Line Item Value Unit
1 Water capacity (spec sheet) 80 gal
2 Water = 80 x 8.34 667 lb
3 Empty weight (spec sheet) 300 lb
4 Occupants = 1 x 200 200 lb
5 Other 0 lb
6 TOTAL 1,167 lb
7 Footprint = 66 x 32 in 2,112 sq in
8 2,112 / 144 14.67 sq ft
9 1,167 / 14.67 79.6 psf
10 Design live load (bathroom) 40 psf
11 Ratio = 79.6 / 40 1.99
12 Load per foot = 1,167 / 4 292 lb
The footprint load is 79.6 psf, twice the 40 psf design figure, and the total is 1,167 lb, over the 1,000 lb line. By the shop rule it gets a structural check before install. That is not a prediction that the floor will fail; plenty of second floors carry tubs like this. It is the point at which somebody qualified looks at the joists, their span and spacing, and where the tub lands, rather than finding out from the ceiling below.
The feet change the question underneath. Four feet carry about 292 lb each, on a small contact area. That is a load on the subfloor and the tile at four spots, separate from what the joists see, and it is what the tub maker's installation instructions address (subfloor thickness, blocking under the feet). Read them before the tile is laid, because blocking added afterward means tearing up a finished floor.
Same tub, different room. If the same tub went into a primary-bedroom suite open to the sleeping area, the design figure for that area is 30 psf, and the ratio becomes 79.6 / 30 = 2.65. The route does not change (the total is still over 1,000 lb), but it is worth knowing which number the room is designed to.
What changes the answer
- The fixture sits on a slab on grade. A concrete slab on the ground is not judged by the 40 psf framed-floor figure. A spa on a slab or a pad goes by the spa maker's pad specification (thickness, reinforcement, level), and an existing slab's suitability is a question for whoever can evaluate it.
- The footprint can be spread. A tank or aquarium on a stand bears through the stand's legs, not its own footprint, so the support points decide the load. A designed platform that spreads the load across more joists changes the arithmetic, and its design is the engineer's.
- The contents are not water. Gravel, rock and sand in an aquarium, or brine and other dense liquids in a tank, weigh more than 8.34 lb per gallon. Use the actual material's weight, from its supplier, on line 5.
- The floor is already compromised. Rot around an old tub's drain, notched or cut joists from earlier plumbing, or a sagging floor mean the design figure no longer describes the floor at all. Any of those sends the job to a structural check whatever the tally says.
How to verify you got this right
- Every weight on the tally has a source (spec sheet, label, measured dimensions), and the planning weight per occupant is written down.
- The footprint is the area the fixture actually bears on, and the support-point load is figured for fixtures on feet or legs.
- The design live load is the one for that room, with the code edition the jurisdiction adopted.
- The route on the bottom line follows the shop rule as written, checked against both the ratio and the total.
- The customer was told the number and the route, not a verdict. The job record shows who was referred and when.
References
- International Residential Code, Table R301.5, minimum uniformly distributed live loads (2021 edition, as adopted by your jurisdiction)
- NFPA 70, National Electrical Code, Article 680, as adopted by your jurisdiction
- See related: The Water Constants 7.48, 8.34 and 231
- See related: What Standing Water Weighs on a Ceiling or Floor
- Fixture and spa manufacturers' specification sheets and installation instructions