Our new garage roofs have a slight pitch to them, not always visible to the eye. In the picture below they are installing insulated sheets, and the thickness of the rows grows towards the center of the garage creating a slope of 1/4"/foot to theperimeters.
You may see some standing water after rain, up to 48 hours is acceptable, and if it stays longer, we are still within code, and that is OK with the city, as long as we used those graduate, insulated sheets. Due to age, roof, not my age, some of the joists sag, (mine too), and that is a "do as best as you can attitude" with the city inspector.
Burt 617 592 3368