The small, 5-inch-by-5-inch scuppers in the parapet wall were inadequate to the task of draining this poorly sloped roof.
Receptors for the scuppers should be mounted as low as possible on the outside of the parapet so that water can't back up onto the roof if the receptors become obstructed.
Leaves can collect around the base of a drain, even one covered with a leaf basket. The leaves then keep water from draining off the roof.
Drains are often positioned poorly, as in the corner of this gutter, where there is no room for a clamp ring and no room to solder.
Though this scupper was about 4 feet wide, it was only 4 inches tall and was quickly blocked by accumulating leaves.