How much does a hot tub weigh with water?
A hot tub for 4 to 6 people usually holds 1,000 to 1,800 litres of water, that is 1 to 1.8 tonnes, plus the shell's own weight (250 to 450 kg depending on the model) and the bathers. In total we are talking about 2 to 2.5 tonnes over 4 to 5 m2, or 400 to 600 kg per square metre. The design imposed load for homes is usually of the order of 2 kN/m2, about 200 kg/m2, and a penthouse terrace is normally not designed for that.
That does not make it impossible. It means an engineer decides: he checks the slab structure and calculates whether the slab holds, whether the load needs spreading with a steel frame or whether the tub should sit over a beam or a column. It is not a paper exercise but a calculation so that the slab does not deflect.
Can a hot tub go on any terrace?
No, and structure decides, not the view. Three key questions: what is under the terrace (a neighbour's room, a staircase, your bedroom), what kind of structure it is (concrete slab, steel joists with brick vaults, an old timber floor) and where the beams and columns run. Over a load-bearing line you can put far more than mid-span.
In old Eixample buildings, where the floor is often steel joists, the calculation matters especially: deflection over someone else's room cannot be corrected afterwards. If the slab does not hold, alternatives remain: a small tub with less volume, a separate steel frame that bears on load-bearing walls, or a solution without water in permanent use.
Is an engineer's project needed and who signs it?
Yes. Adding such a load to a structure requires a technical project signed by an architect or competent engineer and a slab calculation. Without it, the insurer, the licence process and the comunidad have grounds to refuse, and in case of damage you are left without cover.
Contents: structural calculation, a waterproofing design, a services layout (pumps, electrics, drainage), a noise assessment. On permits: a permanent pool is processed as building work, not minor repair, and permit support is a separate service. Who signs is explained in the article on who signs the technical project.
What waterproofing goes under a rooftop pool?
Double. The first line of defence is the vessel itself: monolithic (stainless steel, fibreglass, concrete with membrane) or prefabricated. The second: waterproofing beneath the vessel and a counter-fall to a tray draining to an outlet, plus leak sensors with phone alerts. A leak from a rooftop pool means hundreds of litres onto a neighbour's ceiling in an hour.
The tray under the vessel is deliberately made larger, with upstands and drainage, so any leak goes to the outlet and not downwards. It is not excess caution: repairing someone else's flat costs many times more than sensors and a tray. Emptying for maintenance is planned to the sewer, not onto the terrace.
Which system to choose: overflow or skimmer?
On a roof, a skimmer system or a small overflow vessel is more often chosen. An overflow pool gives a smooth, handsome water surface but demands a balance tank and more complex hydraulics: extra water volume, weight and space for a plant room. A skimmer is simpler, lighter and cheaper to maintain, and for a small pool meant for cooling off and a few strokes it is enough.
If you want the edgeless water line, discuss it with the engineer from the start: edge, channel, tank and pumps must coexist with weight and noise limits. At height wind and evaporation also act, raising water use, and they belong in the maintenance plan.
Where to put pumps and filters without bothering neighbours
The plant is placed away from neighbours' bedrooms and your own: in a separate room, an acoustically insulated niche or a ventilated technical module. Pumps go on anti-vibration mounts, pipes on rubber-lined clamps, and variable-speed pumps are used, almost silent at low revs. At night the equipment runs at minimum or is switched off.
Plan space for the filter, dosing, heating and the electrical board in advance: on a penthouse terrace there is little room and the equipment needs service access. The water heat pump also makes noise, and its position is coordinated with the other outdoor units. This is the part of the project that cannot be seen and that decides whether neighbours put up with you.
Where to start and how BCN YARD can help
Start with a question: what is under my terrace and what is on record about the slab? Then a technician's visit with measurements, an engineer's consultation, an assessment of possible schemes and a combined budget. If the slab will not do, we talk about a smaller tub or another position, not about forcing the calculation.
BCN YARD runs these projects in Russian and Spanish, with one person responsible for the site: visit, measurements, coordination with the engineer and subcontractors, a phased estimate. Permits and documents can be handled through our permit support service.
Frequently asked questions
Does a hot tub on a terrace need comunidad consent?
Almost always you should notify and obtain consent: the load affects common structure, noise and possible leaks concern neighbours, and the building's insurer may take such changes into account. A permanent pool needs a technical project and a licence. A portable tub holding a tonne of water is not harmless either, and the calculation is still needed.
Is an inflatable or portable hot tub an option?
It is easier to install but does not remove the calculation: the mass of water is the same and the base must carry it evenly. For permanent use in a premium home you need solutions with waterproofing, drainage and quiet equipment, not a temporary tub standing on tiles.
What does it cost to maintain a rooftop pool?
No figure can be given without a project, but the cost items are known: electricity for pumps and heating, chemicals or a disinfection system, water and filter replacement, a service visit and sensor checks. Include them in the project from the start and choose automation that lets you monitor the water if you do not live in Spain.