Summer Fun and Structural Integrity
June 27, 2014 8:31 AM   Subscribe

So I'm the proud owner of a new above-ground swimming pool. I set it up in the corner of my building's rooftop and filled it with water. Everything's perfect, but there's one nagging thought: should I be worried about the floor not holding up?

I live in Tel Aviv, Israel. My building is from the 1930's or '40s.
The pool is 10 ft. (3 meters) in diameter, and the water's about 2 ft. (60 cm) deep. So according to my calculations, the thing weighs around 4 metric tons right now. On one hand, that's as much as... well, a 4-ton truck, which sounds like a lot; On the other hand, it's also about as much as the weight of 50 people, which seems trivial.
I don't know the layout of the apartment underneath. For obvious reasons, I'd rather not consult the neighbors who live there... or my landlord, for that matter.
Yeah, I guess I should have done more homework before actually doing this... But now it's up, everything seems okay and I'd obviously like to be able to enjoy it for the rest of the summer.
So... How much weight are roofs of this kind designed to support?
On a scale of one to ten, how worried should I be?
posted by Silky Slim to Grab Bag (25 answers total) 3 users marked this as a favorite
I think you are doing a dangerous and irresponsible thing. There is no way that building was designed to hold a SWIMMING POOL on it. Do you want to be responsible when it caves and kills people underneath?

The fact that you don't want to consult the landlord shows that you know perfectly well this is dangerous, irresponsible, and absolutely not ok.
posted by fingersandtoes at 8:34 AM on June 27, 2014 [26 favorites]

Response by poster: Addition: It's a 3-story building.
posted by Silky Slim at 8:35 AM on June 27, 2014

Well, you're not doing the roof any favors, that's for sure! You could be damaging it, even if it's not going to collapse.

Roofs aren't designed for heavy loads, like a foundation, or the floors/ceiling between floors of a building, hell, if you want to put an AC unit up there, you have to reinforce.

So, that said, what you're doing is dangerous and kind of dickish.

Remove the water, take the pool to the courtyard, or your patio, and use it there.
posted by Ruthless Bunny at 8:36 AM on June 27, 2014

Yeah, generally roofs aren't designed to hold swimming pools, even small ones. I don't know how it works in Israel but I've had leases before that specifically stated "no water-filled furniture." They meant waterbeds and fish tanks, but I'd imagine swimming pools and hot tubs were also a no-no.
posted by bondcliff at 8:37 AM on June 27, 2014 [1 favorite]

Best answer: You clearly know the answer already that this is unsafe and a bad idea, so I officially give you Internet Permission to say oops, take it down immediately, and have a chat with an engineer before repeating the experiment.
posted by Dip Flash at 8:38 AM on June 27, 2014 [11 favorites]

I don't think you want to get that phone call where this plan goes awry. Drain that thing NOW! :)
posted by ian1977 at 8:39 AM on June 27, 2014

To follow up - I am looking at installing a large BATHTUB in my house and have been advised that the floor will need to be reinforced if I do that. Floors are not designed to support weight of pools. Even if your math is right and I don't think it is, 50 people visiting for a party is not the same stress on the materials as the constant weight of a massive amount of water.

And that's without even considering the titanic structural damage to the building that would occur if anything happened to rip/puncture/fold down an edge and cause the water to pour out.

I have lots of relatives in Tel Aviv and I can tell you I would call the police if I knew your address. You are endangering your neighbors.
posted by fingersandtoes at 8:43 AM on June 27, 2014 [1 favorite]

We have a deck on our roof. It is also quite heavy.

We reinforced the roof with steel beams to hold it.

Did you reinforce your roof?


Then take it down.
posted by Oktober at 8:44 AM on June 27, 2014 [2 favorites]

This is an incredibly dumb thing to do. If I were your landlord I would seriously try to evict you for this.
posted by ryanrs at 8:48 AM on June 27, 2014 [3 favorites]

Response by poster: Okay, draining it now...
posted by Silky Slim at 8:50 AM on June 27, 2014 [6 favorites]

I'm not sure how things work over there, but here in the U.S. such a situation would be a huge legal liability. If the roof did collapse, you'd certainly get sued for the damage and you could go to jail for any injuries or deaths that resulted.
posted by Ostara at 8:50 AM on June 27, 2014

I hope you find a nice place to put it on the ground.
posted by mareli at 8:53 AM on June 27, 2014 [3 favorites]

So I'm the proud owner of a new above-ground swimming pool. I set it up in the corner of my building's rooftop and filled it with water.

This is the point where I let out an audible gasp and yelled out, Ohmygod!

Okay, draining it now...

posted by Room 641-A at 9:00 AM on June 27, 2014 [3 favorites]

Response by poster: Okay folks. Thank you for steering me right.
posted by Silky Slim at 9:03 AM on June 27, 2014 [1 favorite]

Okay, draining it now...

I think you did the right thing to drain it, Silky Slim. Back when I lived in Brooklyn and shared a huge rooftop with a few neighbors, one of my neighbors set up a jungle of giant potted plants on his 10x15 space (when you were inside, total privacy!). It was very, very cool but the roof started to buckle into the store below us and he had to take everything down. We were all surprised and shocked that it could do that kind of damage.

We were on the second floor of a three-story building.
posted by mochapickle at 9:11 AM on June 27, 2014 [1 favorite]

A gallon of water is 8.34 pounds.

Amazon says this pool is 1100 gallons.

1100 gallons of water is 9,174 pounds.
posted by jenkinsEar at 9:14 AM on June 27, 2014 [3 favorites]

A=3.14159*(5 ft)^2=78.53975 square feet
V=(78.53975 ft^2)(2 ft)=157.0795 cubic feet
Weight=volume * density
density of water = 62.4lb/ft^3
Weight=(157.0795 ft^3)(62.4 lb/ft^3)=9801.76081 lbs

(9801.76081lb) / (78.53975 ft^2) * (1 ft^2 / 144 in^2) = 0.86 lb/in^2
posted by spikeleemajortomdickandharryconnickjrmints at 9:14 AM on June 27, 2014 [2 favorites]

Response by poster: While the pool drains, let me give you a bit more information that I left out for simplicity's sake:
My apartment is actually on the 4th story of this building. This apartment covers only a part of the roof. That means the "roof" of the building actually holds the weight of two whole additional apartments. So it probably has been designed and/or reinforced to support a lot of weight.
To reiterate: I am draining the pool as I'm typing this.
posted by Silky Slim at 9:20 AM on June 27, 2014

Response by poster: jenkinsEar - that's about the same math I did. Around 4 tons if filled all the way, which I didn't.
posted by Silky Slim at 9:22 AM on June 27, 2014

Any justification that starts with "probably" is no justification at all.

Thanks for draining the pool.
posted by ocherdraco at 9:23 AM on June 27, 2014 [2 favorites]

Best answer: I recently had to do a whole bunch of reading on building code, to design my living roof workshop. I am not a structural engineer, the city made me pay a real structural engineer before they'd sign off on the drawings, but...

In the United States, modern second stories are engineered for about 40 lbs/sq.ft. (there are dynamic load and static load numbers here that I'm glossing over), sloped roofs are engineered for as little as 15 lbs/sq.ft. (depending on climate and expected snow load).

Water is about 62.5 lbs/cubic foot. So at 2' deep, you're loading the area over that pool to 125 lbs/sq.ft. More than 3x the likely design load of a modern building.

The issue likely isn't the strength of the walls holding the vertical loading, that part of a wall assembly is rarely a problem, the issue is the joists underneath the surface and their deflection, and shear strength of the supporting walls should there by seismic activity.

And then the fact that that water's going to be a moderately dynamic load, and... yeah, glad you're draining it. Talk to a structural engineer. You may be fine, where that pool is on the roof may totally influence how the things below it are loaded, but have that conversation with someone who can give you a legally binding sign-off on it.
posted by straw at 9:25 AM on June 27, 2014 [7 favorites]

Oh, and: My living roof is engineered for 120 lbs/sq.ft. I didn't have to change the roof sheathing (3/4" ply on 2' gaps) or the wall studs (2x4s), I did have to have follow particular nailing schedule on the wall sheathing, and beef up the roof trusses. And from an engineering standpoint, the primary reason for beefing up the ceiling is to not worry about flex in wallboard or plaster attached to it (my ceiling is open, but...).

The elements that needed consideration aren't necessarily what I thought of as the obvious ones.
posted by straw at 9:29 AM on June 27, 2014 [1 favorite]

Response by poster: Thank you straw, sounds like sound advice.
And thank you all as well. This has probably not been my finest hour, and you did not steer me wrong.
posted by Silky Slim at 9:30 AM on June 27, 2014 [4 favorites]

thanks for draining :) hope you can find a cousin with a ground floor cottage or something who would be happy to set it up and have you enjoy it there.
posted by fingersandtoes at 9:34 AM on June 27, 2014 [1 favorite]

You made a stranger on the internet gasp and say ohmygod out loud! It's totally your finest hour.

If you are able to get a structural engineer to evaluate the roof with pool, please come back and let us know.
posted by medusa at 8:02 PM on June 27, 2014 [3 favorites]

« Older Cable Internet Issues -- bad cable modem?   |   I need to write some incredibly tedious essays for... Newer »
This thread is closed to new comments.