Why DEP Keeps Asking for a Green Roof: The SMP Hierarchy Explained

For most new construction in NYC, DEP's stormwater rule effectively requires a green roof. The rule doesn't name one; it requires vegetated retention to the maximum extent practicable, but on a lot where the building covers most of the ground, the roof is usually the only practice left that can satisfy it.

What the rule actually requires

15 RCNY §19.1-03.4 sorts every stormwater management practice (SMP) into a three-tier hierarchy:

Green roofs are considered an example of a Tier 1 vegetated retention SMP,[1] and the rules state:

"Vegetated retention practices must be used to the maximum extent practicable to meet requirements."[2]

Only when documented site constraints rule that out does the design move to Tier 2 non-vegetated retention, and only after that to Tier 3 detention (in combined sewer areas) or Tier 3 treatment (in separately sewered areas).[2] This has been the rule citywide since February 15, 2022.[2]

Why DEP typically requests a green roof

DEP requires Tier 1 practices to be used to the maximum extent practicable or shown infeasible. What makes the green roof so common in practice is that the five site constraints that commonly rule out ground-level SMPs like bioretention or rain gardens (i.e. poor soil, shallow bedrock, contamination, paving mandates, tight setbacks) rarely apply to a roof.[1] So on a dense NYC lot, the green roof is often the one Tier 1 practice that survives a constraint analysis, at least partially.

What a green roof has to do, once it's chosen

• Counts 100% toward water quality and runoff reduction volume. 0% toward sewer operations volume; CSS sites still need detention for that.[3]
• Can only manage rainfall on its own footprint, at a 1:1 ratio. Extra depth doesn't help other parts of the site.[3]
• Needs to use a minimum required depth of growing media.[3]

Where this gets missed

Roof space is typically shared with mechanical equipment, bulkheads, and/or amenity decks, and every square foot lost has to be justified in the SWPPP.[4] This SWPPP rule is separate from the Department of Buildings' "sustainable roofing zone" rule, which accepts solar alone.[5] Satisfying DOB's roof rule does not satisfy DEP's, since solar isn't an SMP on DEP's hierarchy at all.[1] In fact, DEP allows green roof systems beneath solar panels.[4]

Bring the SWPPP engineer in at schematic design, while equipment layout can still move. AMC prepares SWPPPs and SMP hierarchy documentation for DEP stormwater construction permits; see our services.

This article describes the general rule and does not replace DEP's project-specific determinations.