Waterproof deck - leak risk?

[dabel]

My architect sent through initial concepts for a remodel on a 2 flat property. My plan is to free up more internal space on both floors by replacing the internal stair access to the top flat with external access to a deck and then the entrance.

The deck was to sit over a bit of roof from downstairs that juts out further than the downstairs space.

My architect pointed out we'll need a waterproof deck. "Unfortunately with the relative first floor to roof levels, the proposed deck would be required to be constructed as a waterproof enclosed butynol deck. We could not span the new deck over the existing roof."

A cursory Google of "enclosed waterproof decking" dug up a lot of articles about water ingress issues they historically have caused, particularly with apartments.

Is this still a problem with today's materials and design code? Enclosed decks on buildings are all over the world so I would have assumed this is a solved problem by now but...

Any insights much appreciated.

I have done multiple searches on this topic, but most seem to concern waterproof deck over living space. We have a two level deck which is covered by a cantelever on the roof. The Upper level deck is only 6′ deep, and is completely covered by the roof. It acts as a partial roof for the Lower level deck which extends 2 ‘ further for a total of 8’ deep. This lower level deck in turn acts as a roof for a daylight basement patio below. At this point the plan is to run 2×10 joists 16″ OC sloped 1/4″ per ft. away from the building. Treated on the rims, but untreated lumber in the middle of the structures. Then: 3/4″ treated plywood Blueskin peal & stick membrane Screw down Hardie Thinset Saltillo Tile/ grout The other option to avoid the “self sealing” holes in the membrae would be to screw down the hardie, then run the blueskin/Thinset/Tile. Suggestions?