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).

Caution: The following general assumptions apply in calculating the credit for green roofs. If any of these assumptions is violated, the credit will be reduced.

• 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.

Volume credits

A green roof system acts similar to a bioretention system with an underdrain, although the underlying drainage layer may provide some volume control through storage or by retarding water loss.

Basis for volume credits

Volume credits for a green roof system are a function of the dimensions of the system, specifically the depth of the media, the storage and hydraulic characteristics of the media, the area of the green roof, and the amount of run-on from adjacent conventional roofs. The volume credit(V_s) is given by

\(V_s = A_s d_p MMWR\)

where

A_s = the surface area of the green roof(square feet);

d_p = the depth of the media, equal to the area from the bottom of the media (top of underlying drainage layer) to the top of the media; and

MMWR = moisture content at the maximum media density (cubic feet/cubic feet; often expressed as a percent volume).

The maximum media density (kg per cubic meter) is the sample weight (kg) after draining for 120 minutes divided by the initial sample volume (cubic meters). The maximum media density is used to estimate the maximum dead load for green roof assemblies. See ASTM Standard E2339-11. A green roof may be designed to meet a specific performance goal, such as the 1.1 inch Minimal Impact Design Standards (MIDS) goal for new development sites with no restrictions. Specifications for designing green roof systems, including a discussion of how to calculate the reservoir depth, are provided in the section covering design specifications for permeable pavement. Note that traditional roofs, which are treated as impervious surface, may be routed to a green roof and will therefore affect the achievment of the performance goal.

Models and calculators for calculating permeable pavement volume credits

The models and calculators discussed below are widely utilized within the stormwater community and are therefore appropriate for calculating volume credits provided the model assumptions are met and the permeable pavement is properly designed, constructed and maintained.