Philadelphia PWD Stormwater Calculator

PWD Green & Blue Roofs: Background

The Role of Green Roofs in PWD Compliance

Green roofs are a preferred Stormwater Management Practice (SMP) by the Philadelphia Water Department because they mimic natural hydrologic processes. Under PWD regulations, a properly designed green roof is considered Disconnected Impervious Cover (DIC).

• Water Quality Volume (WQv): The primary benefit is that the area of the green roof itself is considered managed. It does not contribute to the site's Directly Connected Impervious Area (DCIA) that requires treatment. The growing medium captures and treats the required 1.5 inches of rainfall for the Water Quality storm.
• Flood Control: While green roofs satisfy the WQv requirement for their own footprint, they still produce runoff during larger storms. For site-wide flood control modeling, runoff from the green roof area must be calculated using a PWD-specified NRCS Runoff Curve Number (CN) of 86.

Key PWD Design Standards for Green Roofs

• Minimum Medium Depth: A minimum of 3 inches of growing medium is required for a roof to qualify as a green roof SMP.
• Receiving Runoff: If a green roof receives runoff from another, separate impervious area (e.g., a higher roof level), its medium depth must be increased. The required depth is calculated as: Min. Depth (in) = 3 + [3 * (Tributary Impervious Area / Green Roof Area)]. This tool automates this check.
• Plantings: PWD recommends that at least 50% of the plants be varieties of sedum or other drought-tolerant, perennial vegetation.

The Role of Blue Roofs

Blue roofs are non-vegetated systems designed to temporarily store (detain) stormwater and release it slowly over time. They are primarily used for rate control, not volume reduction or water quality treatment in the same way green roofs are.

• Function: They use flow control devices (like weirs or roof drain restrictors) to pond water on the roof and discharge it slowly, reducing the peak flow rate to the sewer system.
• PWD Requirements: The key PWD design standard for blue roofs is the 72-hour maximum drain down time. This means the outlet control must be designed to release the stored volume over a period of up to three days.
• Use Case: They are an acceptable practice for managing Channel Protection and Flood Control requirements, especially in combined sewer areas where infiltration is not feasible.