This project has 40,000 square feet of roof area and no non-roof site area. The base roof area is 20,000 sf and the tower roof area is 20,000 sf as well. The design team will be collecting storm water from the 20,000 sf of tower roof area. When the annual rainfall averages for this area are calculated with the size and demand of the 12,500 gallon stormwater retention tank, the project is able to determine that, annually, 70% of the stormwater falling on the base roof area will be collected, and will be used for irrigation and cooling tower make-up. These calculations are based on one -year (twelve hour) storm information and the average annual rainfall on the site. Since 70% of the average annual rain water is collected of the tower roof area, the design team is assigning a run off coefficient of 0.30 for the tower roof area (based on the LEED Reference Guide\'s recommendation to assign an appropriate run-off coefficient if one is not included in the LEED Tables or the LEED Calculator, and also based on a similar LEED CIR, dated 1/17/2002). When this run-off coefficient of 0.30 is combined with the run-off coefficient of 0.95 for the base roof, the project shows a 34% decrease in the imperviousness of the site and thus meets the requirements for this credit. Will the USGBC accept this methodology in determining that the project has decreased the rate or quantity of stormwater run-off by at least 25 %?
Yes, your methodology of determining the rate of stormwater is acceptable and shows that your project should meet the required decrease. Please include your calculations for the base roof area collection quantities to verify that the coefficient used is justified, as well as all other required submittals for this credit. Applicable internationally.