NYC Green Roof & Blue Roof Design and Compliance Tool
Disclaimer
This tool is for preliminary informational and educational purposes only. It is NOT a substitute for detailed site-specific engineering calculations, professional judgment, or official NYC DEP review/submission.
This tool helps practitioners design green roofs (vegetated roofs for stormwater retention) and blue roofs (rooftop detention systems) in compliance with the NYC Stormwater Manual (Feb 2024).
Enter project details below to calculate required storage volumes and release rates, and check compliance with Water Quality Volume (WQv), Runoff Reduction Volume (RRv), and Sewer Operations Volume (Vv) criteria. The tool also provides guidance on key NYCDEP requirements.
The runoff volume from the 90th percentile storm (1.5 inches in NYC) that must be managed for water quality treatment (typically via retention/infiltration/ET or specific treatment practices).
Equation:
WQv = P × Rv × A / 12
Where: P = 1.5 in, A = contributing area (sf), Rv = 0.05 + 0.009×(% impervious)
Runoff Reduction Volume (RRv)
Minimum volume from new impervious area that must be retained (infiltrated, evapotranspired, or reused).
Equation:
RRvmin = P × S × Aic / 12
Where: P = 1.5 in, S = soil-specific factor (HSG A=0.55, B=0.40, C=0.30, D=0.20), Aic = new impervious area (sf)
Note: Aic = A × (% impervious / 100)
Sewer Operations Volume (Vv)
Detention storage volume required for larger storms (peak flow control).
Equation:
Vv = RD × Cw × A / 12
Where: RD = rainfall depth (varies: 1.85" CSS/site, 1.50" CSS/house or MS4/site, 1.10" MS4/house), Cw = weighted runoff coefficient, A = area draining to practice/outlet (sf)
Maximum Release Rate (QDRR)
The allowed peak discharge to sewers, per acre. NYC: q = 0.1 cfs/acre (CSS) or 1.0 cfs/acre (MS4).
Equation:
QDRR = q × A / 43560
Where: A = area in sf (same area as Vv calculation). Flow control must not exceed this rate for the Vv design storm.
All parameters must be determined for each practice and the site. NYC requires a retention-first approach: meet WQv with retention (which usually achieves RRv), then use detention for Vv. In combined sewer areas (CSS), detention may be used to manage WQv volume if retention is infeasible. In MS4 areas, WQv must be retained or treated (detention alone is insufficient).
Permit Applicability Summary
This summarizes when NYC stormwater rules apply and how green/blue roofs fit into compliance.
Triggering Criteria (Unified Stormwater Rule)
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Applies to projects disturbing ≥20,000 sq ft of soil OR creating ≥5,000 sq ft of new impervious area ("covered development").
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Requires management of WQv (treatment/retention) and RRv (retention).
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Additional No Net Increase (NNI) requirements may apply in MS4 areas discharging to impaired waters if disturbance ≥20,000 sq ft and impervious cover increases.
Sewer Connection Permits (Vv & QDRR)
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Required for new or modified connections to the city sewer (House or Site Connection).
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Requires management of Vv (detention volume) and QDRR (release rate) for the area draining to the connection.
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Most new buildings trigger both Stormwater Permit (WQv/RRv) and Sewer Connection (Vv/QDRR) requirements.
Hierarchy of Practices
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NYC DEP mandates a retention-first approach.
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Maximize retention (green roofs, infiltration, harvesting) to meet WQv and RRv first.
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Use detention (blue roofs, tanks) for remaining volume control needs or Vv peak flow control.
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Green roofs are a preferred retention practice, especially on dense sites. Blue roofs are useful for detention, particularly when combined with a green layer or when only Vv control is needed.