Calculating credits for green roofs
Green roofs achieve reductions in stormwater volume compared to conventional roofs. Green roofs can effectively remove or reduce loads of many pollutants that are discharged from conventional roofs. Green roofs act as vegetated filters and provide temporary storage of rainwater or snowmelt. Water kept in storage may eventually be evapotranspired or "bleed' out of the system to the underlying drainage layer. Water reaching the drainage layer is eventually lost from the green roof system. Volume and pollutant reductions constitute stormwater credits that can be used to meet various goals (e.g. Total Maximum Daily loads (TMDLs), Minimal Impact Design Standards (MIDS) performance goals). Green roofs will achieve the greatest credit when they are properly designed, constructed and maintained.
Green roofs are used in the beginning of a treatment train. They may receive discharge from another roof, but otherwise do not receive stormwater runoff. Green roofs are excellent BMPs in ultra-urban settings where it is otherwise difficult to achieve volume and pollutant reductions due to space constraints. Since green roofs release water over a period of time following a precipitation event, they are most effective when discharge from the green roof is to a pervious surface, such as turf or a filter strip. Because green roofs effectively remove sediment, discharge from a green roof can be routed to any BMP (e.g. bioretention, infiltration basin, permeable pavement).
- The green roof is properly designed.
- The green roof was properly constructed, consistent with the design criteria.
- The green roof is properly maintained. The performance of the green roof should be regularly assessed.
Assumptions used to calculate credits may also vary with each calculator or model. To calculate credits it is important to ensure that your calculation is consistent with the assumptions made in the model or calculator you are using. Detailed discussions of assumptions may be found in user's manuals or other documentation for the model or calculator.
This section provides specific information on generating and calculating credits from green roofs for volume, TSS and phosphorus. Green roofs may also be effective at reducing concentrations of other pollutants such as metals. This article does not provide information on calculating credits for pollutants other than TSS and phosphorus, but references are provided that may be useful for calculating credits for other pollutants.