Answer: Unfortunately, a windy rooftop in full sun (including the hot afternoon sun) like this can be quite difficult to plant successfully because the plants (and soil in the relatively uninsulated, exposed and limited container space) will dry out very quickly -- plants lose extra moisture through their foliage in a windy spot.
You will have to experiment and see what works best for you. If at all possible a windbreak would be really helpful in reducing overall stress on the plants. Generally speaking you will need plants that are tolerant of heat, wind, and somewhat dry soil. You might look at junipers for your evergreens along with possibly ornamental grasses. Lilac and burning bush might be possibilities as might smaller cultivars of crabapple. Keep in mind that it is not unusual to need to replace woody plants annually in this type of growing location.
Your local professional nursery staff may have some additional suggestions, or if you have a neighbor who is gardening in a similar location/exposure then you could compare notes and see what is working well for them.
The soil mix should be a soil-less mix formulated for container plants. These are somewhat lighter in dry weight compared to a soil based mix, however once you add the necessary water to keep the soil evenly moist (not saturated, but damp like a wrung out sponge) then it does become quite heavy. For this reason I would suggest you check with a building engineer or architect to verify how much weight you can safely place there. Good luck with your project!
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