Roof Drainage Design in Coastal Climates
Roof Drainage Design in Coastal Climates
(OP)
What is the correct time interval to use when designing a roof drainage system (i.e. roof drains, gutters, roof conductors, etc.) in a coastal climate? The 2009 IPC states that the 100 year 1 hour storm event shall be used. However, it seems that shorter time intervals (e.g. 5 minutes) should be used for drainage elements with smaller tributary areas and short times of concentration, especially in coastal climates where short duration, high intnsity rainfall events frequently occur. Any comments would be appreciated. Thanks in advance.





RE: Roof Drainage Design in Coastal Climates
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RE: Roof Drainage Design in Coastal Climates
Rational method uses an intensity-duration relation and 5 minute time of concentration is commonly used for roofs. A one hour time of concentration would vastly underestimate the peak runoff. If you are being required to use a 100-year, 1-hour storm event, you would need to synthesize a 1-hour storm hydrograph for your rooftop in order to estimate the peak runoff. Alternatively, if you can get the precipitation data for the 100-year, 1-hour storm, pick the most intense 5 minute rainfall depth in the storm and use that.