A Pool On The Roof

What does it take to build a swimming pool on a roof?

A rooftop pool requires a structural deck designed from the footings up for the full weight of the loaded vessel, a prefabricated vessel rather than a shell poured in place, an engineer of record for the pool itself, waterproofing of the structural deck beneath the vessel, and a single crane pick to fly the finished vessel onto the roof.

Why the structure starts at the footings

A pool at grade sits in the ground and the ground takes its weight. A pool on a roof has to be carried by the building — and that load does not show up for the first time when the vessel arrives. It is accounted for in the foundation design, the column sizing, and the deck engineering, before any concrete is poured. At Leslie House the deck below the pool is the roof deck: precast concrete slabs bearing on heavy structural steel, carried down through filled-cell CMU walls and cast-in-place columns to the slab on grade — every element of it sized for the weight of the loaded vessel above. A roof that was not engineered for that load from the start cannot be retrofitted to carry it.

Prefabricated vessel, not a shell poured in place

A conventional in-ground pool is a shell formed and poured on site. A rooftop pool cannot work that way. The vessel has to be fabricated off site — in this case by Diamond Spas in stainless steel — then lifted and set as a single unit. That approach eliminates wet work on the roof deck, keeps the vessel dimensionally precise, and makes waterproofing beneath it possible. The vessel arrived crated and ready, still carrying the fabricator's mark. It went up once, in a single crane pick, and it was not coming back down.

The pool needs its own engineer

The structural engineer of record covers the building's frame. A rooftop pool requires a separate engineering engagement: Watershape Consulting served as engineer of record for the pool itself. That scope covers the hydraulics, the vessel loads, how those loads transfer to the deck below, and how the assembly behaves full of water. A full pool is a body of water that moves — dead load that sloshes — sitting directly above the rooms below. Getting that engineering wrong means a deck that cracks, or worse, one that fails. Watershape's involvement meant the pool was not an afterthought bolted onto a standing building. The structure below it was designed for the loaded vessel.

Waterproofing the deck beneath it

The structural deck below the vessel does three jobs at once: it carries the weight, it stays rigid enough that nothing in the rooms below cracks, and it stays watertight for the life of the building. The waterproofing at Leslie House was self-performed rather than subcontracted. A pool on a roof is not a situation where waterproofing failure gets caught early — by the time water appears in a room below, the damage is already done. Self-performing that scope meant one party held accountability for both the deck and its membrane.