Scientists in Hong Kong have demonstrated a new ultra-white ceramic material that can drastically cool buildings by reflecting sunlight and heat at record highs. The beetle-inspired material gets its ability from its nanostructure, stays tough to the elements and should be relatively easy to scale…
Ultra-white ceramic cools buildings with record-high 99.6% reflectivity::undefined
I can guarantee that a Rooftop Terrace garden cuts down almost 40% to 60% heat ever reaching the ceiling. If you have enough cover with smaller plants under larger bushes/shrubs/small trees then there will be a cool breeze around the terrace, provide nesting places for small birds and animals, a pocket of nature in an otherwise concrete heat jungle.
The problem is who can afford to maintain the Terrace garden is the bigger challenge. Constantly checking soil, composting, watering, maintenance and just time+expense is usually beyond a lot of folks.
Trees? Not many. Grasses, herbs, wildflowers, and shrubs? Tons of them.
And you can pretty easily retrofit over an existing sloped roof.
And the weight is no more than a tiled roof.
Well of course you'd use waterproof base layer, typically you'd use several even without the plants. It's tried and tested with multiple cases of failing with age. That's the issue. Even just flat roofs have been a failure point even without the plants but soil and plants are a definite concern for builders when talking long term.
Are you a builder?
Do you have any experience installing and maintaining green roofs?
Your assertion than you'd typically use several waterproofing layers suggests not.
I have experience building these systems in the real world and the documentation to support their use.
BTW - flat roofs aren't a thing. Expect in traditional building in desert areas. "Flat" roofs aren't flat.