Roof Drainage Design in Coastal Climates

[Kussro]

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.

 

[bimr]

"The local code should be consulted to determine the rainfall rate that is applicable for the project location. A minimum design should be no less than 10-year/5-minute for the building roof and site unless other factors require designing for greater protection. For example, if the local code requires that the design be based on a minimum 10-year/5-minute storm but recent changes in the area’s climatic conditions have consistently produced storms that have changed in frequency and intensity (even if temporary), then it is prudent to take this into consideration. Exercise good engineering judgment and use a greater severe storm frequency and duration to design beyond the minimum."

http://www.plumbingengineer.com/sept_10/drain_feature.php

 

[cvg]

apples and oranges are getting mixed again

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.