The Society of Public Health Engineers has published a technical bulletin on rainfall intensities and drainage design for flat roofs. It is designed to provide a basic introduction to the design of flat roof drainage and internal pipework.
It covers rainfall intensity vs duration of a storm; interface with underground drainage, how to design rainfall; catchment areas; overflow provision; and roofs which reduce flow.
The bulletin is available to download for free at on the CIBSE Knowledge Portal cibse.org/knowledge