Why you should never put a pool on your terrace, explained with structural calculations by an architect

With the arrival of the heat and the prospects of not being able to travel as much as other years this summer, they have made many people wonder: Can I put a pool on my terrace or my balcony?

The short answer: NO.

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The long answer: join me on this exciting journey of loads and structural calculations

Why can’t I put a pool on my terrace?

The fundamental reason why you should not put a pool on a terrace is because not designed to support your weight. What happens is that with that answer we do not get an idea with how much extra weight we are overloading the structure.

Let’s do some quick numbers. A circular inflatable pool 3 meters in diameter and 76 centimeters high like this one from Amazon has an advertised capacity of 3,600 liters if it is 90% full (it is not completely cylindrical, otherwise it would be 4,836 liters).

Inflatable Pool TerraceInflatable Pool Terrace

An inflatable pool has a mass of 500 kg / m2; the terrace floor is designed to withstand 200 kg / m2.

Knowing that the density of water is approximately one kilogram per liter, that is 3,600 kg of mass (3.5 tons, which impresses more) spread over an area of ​​7.07 m2, which comes out to 509 kg / m2.

Do you know what is the calculation overhead of the Technical Building Code (page 9.) for a terrace in a residential building? 200 kg / m2. (Actually 2 kN / m2 because they are loads, not masses, but I have made the change of units – and rounded – so that it is understood).

That is, with a pool of that size we are making the structure bear more than double the load for which it was designed.

This tweet sums it up nicely using more visual units of measure

This also applies to smaller pools, because it is a matter of weight per surface. Any pool that is filled 50 centimeters will have a mass of 500 kg / m2 and, therefore, it will suppose an overload of two and a half times the predicted one.

A pool with 50 cm of water weighs 2.5 times what the structure is capable of supporting.

For the calculation we have ignored the weight of the people who would enter the pool, but let’s assume that we fill it 90% with them inside and that the density of the human body is not very different from that of water.

But what about safety factors or load redistribution?

It is true that the calculations enter safety factors, but those are there for, well, security. In case the materials do not behave as expected, to assume imperfections or defects in the construction, irregular load distributions, loss of bearing capacity due to exhaustion … In addition, they are never greater than two. Even with this coefficient, we are pushing the slab (what is under the terrace floor) and even the beams and pillars to the limit.

When it comes to inflatable pools, not even the safety factors or the load distribution are enough.

On the other hand, it is also necessary to consider that the slabs have a compression layer that redistributes the load so that this is assumed by a greater part of the structure, but this layer is more designed to distribute specific loads, such as a shelf or a bathtub. In fact, speaking of bathtubs, which are mentioned a lot in these cases: filling the bathtub is usually about 200 liters of water (aka, 200 kg), a much smaller mass that can also be easily distributed, nothing comparable with 3.5 tons of water.

So what inflatable pool can I put on my terrace?

Baby PoolBaby Pool

Continuing with the numbers that we have done before, theoretically we could put any pool on the terrace as long as we did not fill it with more than 20 cm (that gives the 2 kN / m2 maximum load). But of course, that is literally a foot of water. And there, in addition, it is necessary to add to the calculation the occupants of the pool (75 kg per adult), because we cannot consider them submerged in a handful of water.

So, keeping that in mind, the biggest pool that we should put on the terrace is a small one for babies and fill it up to 20 centimeters high at most.

Even with a baby pool the loads can exceed those provided for in the structure

For example, in a pool like this one with a diameter of 120 cm and a height of 30 cm, up to 340 liters can fit if we fill it to the top.

That is already 3 kN / m2 (300 kg / m2 to understand us), 50% more computational overhead. What happens is that, in this case, the compression layer is more effective in redistributing the load, so for calculation purposes we can consider that it is distributed over a surface greater than that occupied by the pool. Even so, it is appropriate not to fill them beyond 20 centimeters, because the pool is not the only thing on a terrace.

In the case of balconies, a linear overload of 2 kN / m is added to the calculation at the edge of the floor, that is, they are calculated to support a little more weight. However, we have preferred not to consider it for different reasons: on balconies we are more crowded, so there are more loads apart from the pool, and this linear load tends to have more effect on the calculation of the beams and columns than on the slab. , which is what tends to yield when it comes to swimming pools in places where it does not touch.

Well, my brother-in-law has one on his terrace and it hasn’t fallen

And a man who smoked like a carter lived to be 105 years old.

The terraces with pools are falling. Happens.

So no swimming pools on the terraces and on the balconies. Please. If you want to cool off, a hose or, at most, a “bathtub” or its equivalent volume on the terrace.

Terrace bathtubTerrace bathtub

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