Sloped or not, each segment of the roof shall be capable to carry the loading created due to blockage of the primary roof drainage system of that segment plus the load caused by water that rises above the inlet of the secondary drainage system at its design flow rate. The event that shall be considered is an intense short-duration rainfall. This is defined differently by different codes. It is approximately equivalent to 1-hour duration rainfall with a 100-year return period. Check the governing code for your project.
Amokhta's reply covers the subject very well. Let me just add that if the roof segment has a slope steeper than 1/4 inch per foot toward points of free drainage, we do not have to check the structural strength of the roof for ponding of rainwater.
The design of the roof drains and piping is based on the rain hourly rate specified by the applicable code. Structurally, we design the roof for the water accumulated considering the deflection of the roof system.
a roof with a parapet will have roof drains and overflows - an overflow typically extends 2" above the low part of the roof (NOT the roof drain basin) - this means there could possibly be 2" of water at the deep end and based on the roof slope the weight is determined....check with the plumbing codes
I agree in 2" of water downstream be very much nice a load. Where I live it rains scarcely (220 mm a year) and you would be annoyed of the general miscare with which these things are...NOT made. Furthermore there are the inner patios where to produce the spill-over would mean plumbing, yes, this even less done than what other.
Fortunately the total load HERE is unlikely to go as high as to extenuate the structure and in fact these failures here are mostly unseen. Underpass or cellar flooding is more common.
By the way I had 20 years ago the opportunity to load with 20 cm of water a RC 25 or 30 cm deep beams/25 or 30 cm deep RC joists (half) roof in a condominium in construction and the structure made to the spanish code EH-80 passed the test without any noticeable movement.
