The first thing is not to use a higher safety factor then necessary, second to consider how to deal with the occasional high rainfall rate that generally will not last for more than 2 minutes (NOT 1 HOUR).
Solutions:
1) Design the primary system to a realistic return rate, 108mm/hr has proved to work well in most locations in the UK with several 2 minute occurrences annually.
2) If a high rainfall rate is the result of calculation, deal with the excess above 108mm/hr by some other means:
a) Charge it via overflow weirs to the outside of the building
b) Install an internal gravity overflow system
c) Install a secondary syphonic system, which only operates when the water level builds up on the roof to a predetermined level.
d) Allow the water to build-up on the roof for a short period; the high intensity rate will only last for 2 minutes, following which the system will remove the excess quickly.
