Pre-cast Beam System for a Flat Accesible Roof

[KCRatnayake]

Hello All,
Recently I encountered this issue. Is pre-cast beams with 2.5" thick overlay suitable for a flat accessible roof top? Will water-proofing work on this? Building is 100' long and 50' wide. Ther will be insulation and water proofing.

Many thanks,
KC

 

[Teguci]

First - The precast should be sloped to drain
Second - Even with a slope, leaks will form at the joints between precast members. You can tool the joints and caulk them but they will fail in a few years

For occupied space below precast, a plaza system works best. This involves the precast structure with a 2" topping-waterproof membrane system - drainage space - slip material - topping slab.

If the roof were inaccessible then you could just get away with a waterproof membrane system (EPDM or hot applied).