The short answer
A rooftop pool is built only after a civil engineer confirms the slab can carry the weight of the water. A prefab pool has the edge here, because its walls are light and need no tonnes of concrete. In an apartment block the co-owners must also agree.
Weight is the first question
One cubic metre of water weighs one tonne. A small pool one metre deep loads the slab with a tonne per square metre, far more than an ordinary roof was designed for. So the engineer first checks the building drawings and, if needed, suggests strengthening or a shallower depth.
Often the answer is to sit the pool over beams or columns, where the slab is strongest.
Why prefab is lighter
A concrete pool adds the weight of its walls and floor to the weight of the water. Steel walls weigh very little, the membrane even less. The pool is supported from outside with braces and cladding, without heavy walls.
Waterproofing and drainage
The PVC membrane keeps the water inside the pool. Under it, the roof must have its own waterproofing in good condition. We also plan a drain point for when the pool must be emptied for service, and an overflow for rain.
Apartment blocks: what is needed
In an apartment block the roof is usually common property. If it is not yours for exclusive use, you need the co-owners' agreement under the building's rules. Even on a penthouse with its own roof, inform the building manager.
From our experienceOn roofs in central Athens wind and sun are stronger than in a garden. A shade sail or pergola makes the pool far more pleasant at midday.
