Although the parapet roof is recognised as a flat roof, it only appears as such. The actual roof is sloped to let the water drain away from the house. The parapet roof only looks flat. High walls around the house give the illusion of a flat roof while covering the sloped roof in between. The water drains in the parapet roof from the sloped roof to the inner corner of the high wall and is from there directed down the drain.
And therein lies the weakness of the parapet roof: for the water in any quantity rained down upon to drain away. Imagine a regular pitched roof. Heavy rain and the downpipes are blocked by leaves or something of the sort. The pipes overflow but overflow into the garden or pavement, away from the house.
If the downpipes on a parapet roof are blocked then the water has the potential to cause real damage to your home.
To avoid this as much as possible a so-called box gutter is installed. Box gutters are deeper and wider to allow for heavy rain. However also box gutters can get blocked.
Many architects and designers include overflows - a secondary downpipe at a higher location to be on the safe side of the box gutter should ever be blocked so water can run off the roof safely without damaging the house.