Obviously, with a roof-top pool/spa there's no digging, but rather creating a coffer for the pool or spa.
Coffer for the spa shown boarded around the perimeter for safety reasons.
Insulation is used to waterproof concrete outside the coffers, as well as provide rigidity for plumbing inside the coffers. It also is used to 'true up' space that needs filling. Plaster is what will waterproof the actual pool or spa.
Spa is framed and squared, just needs to be reinforced. Main drains are built.
Now it's the pool's turn for insulation.
Pool walls are foamed, nearly complete on the floor.
