---
title: extensive and intensive are not the same roof
canonical_url: https://ensurance.app/guide/extensive-and-intensive-are-not-the-same-roof
markdown_url: https://ensurance.app/guide/extensive-and-intensive-are-not-the-same-roof.md
subtitle: six inches of media is a different living layer than a roof garden
category: ecosystem-services
---

# extensive and intensive are not the same roof

*six inches of media is a different living layer than a roof garden*

People often say “green roof” as if it is one uniform thing. In practice, different living layers behave differently, ask different things of a structure, and are credited differently in programs. The New Jersey Stormwater BMP Manual recognizes two types by media depth: extensive and intensive. Understanding that line is the first step to right-sizing expectations, tests, and maintenance.

A green roof lives in a layered assembly: vegetation rooted in growing media above a drainage layer, over a root barrier and waterproofing. It manages rain that lands on its own vegetated surface and its walkways; it is not a catchall for other roof areas. In New Jersey, a compliant design counts as green infrastructure and for runoff quantity. Groundwater recharge is not allowed, and runoff quality is not allowed.

:::johnson
**an extensive roof and an intensive roof are different living layers.** choosing the right type sets realistic performance, maintenance, and structural expectations before drawings go out for review.

[a sedum mat is not a meadow](/guide/a-sedum-mat-is-not-a-meadow?from=guide)
:::

## what an extensive green roof is
An extensive green roof uses a thinner growing medium and a plant palette that can thrive in that limited depth. The New Jersey manual frames the assembly as vegetation in growing media over a drainage layer, above filter fabric, root barrier, and waterproofing. It reduces runoff by storing water in the media and by evapotranspiration from the plants. The result is a living layer that holds water in the media and returns some of it by evapotranspiration, within the limits of that depth.

A compliant extensive assembly follows material guidance so the layer drains, breathes, and resists degradation. The manual states growing media generally include 80–90% lightweight aggregate by weight and 10–20% stable organic matter, with a pH between 6 and 8.5. Permeability must be at least 1 inch per hour by ASTM E2399 or FLL. The slope of the roof must be 20% or less, and the vegetation must meet a minimum vegetation density of 85%.

Plant selection responds to depth, wind, shade from panels, and any slope. Lawn grasses are generally not acceptable. Many extensive roofs use sedums, and natives are preferred when they will establish in the conditions provided. Walkways can be part of the vegetated roof footprint and are managed as part of the system that treats rain where it lands.

A short way to visualize the assembly is as follows:
- Vegetation
- Growing medium
- Filter fabric
- Drainage layer
- Root barrier
- Waterproofing

The plants, the growing media, and the water that media holds exist on the roof whether or not anyone buys a certificate. Ensurance funds that living layer. It is not the green-roof product. In this guide, certificate means an ensurance certificate.

## six inches is the cutoff in this manual
The New Jersey Stormwater BMP Manual draws the line by growing media depth, which is a practical way to differentiate the living layer. Extensive means 6 inches or less of media, intensive means 6 inches or greater. That simple threshold changes the expected plant palette, irrigation expectations, maintenance access, and the dead and live loads a structural engineer has to check, using the definitions in ASTM E2397 rather than a guessed weight.

Other jurisdictions also define and regulate vegetated roofs, and their thresholds may differ because they serve different program goals. For example, Denver’s Green Buildings Ordinance requires a cool roof and one additional green-building option for new buildings 25,000 gross square feet and larger, including additions and campuses. For the vegetated option, Denver’s guide notes professional qualifications, irrigation, plant lists, a maintenance plan, and a minimum of 5 inches of growing media with 80% coverage within 3 years. Do not equate Denver’s 5 inches with New Jersey’s 6-inch cutoff; they come from different programs with different scopes.

Local incentives can add another layer of review. In Philadelphia, zoning 14-702 offers a green roof density bonus, and the Philadelphia Water Department reviews the design with an operations and maintenance agreement as part of the path. These processes sit alongside the technical facts in the New Jersey manual rather than replacing them. If a project must satisfy multiple authorities, write the specification to meet the most stringent relevant requirement and submit the same layered logic to each reviewer.

Because intensive roofs start at 6 inches of media and often include taller, more demanding plantings, verify structure before selecting intensive on an existing deck. ASTM E2397/E2397M defines dead load as the drained weight including retained water and live load as transient water when the drainage layer is full. Those definitions help the design team and structural engineer speak the same language while they evaluate whether the deck can support the chosen type. Align that conversation with the program limits: New Jersey allows a slope up to 20%, while ASTM E2777-14 provides guidance for vegetative roofs up to 15% slope as a design guide.

| aspect | extensive (≤ 6 in media) | intensive (≥ 6 in media) |
| --- | --- | --- |
| media depth | 6 inches or less | 6 inches or greater |
| media water retention band (mix selection only) | 35–65% by volume | 45–65% by volume |
| planting character | Low-profile, drought-tolerant; sedums are often chosen; lawn grasses generally not acceptable; natives preferred when they will establish | Can support larger perennials, shrubs, and small trees where structure allows; broader palettes; shade and wind shape the list |
| accessibility and use | Typically limited access, with walkways as designed components | Can include programmed use areas if detailed and protected accordingly |
| irrigation expectation | Often minimal or seasonal, depending on climate and plants | Often includes regular irrigation for more demanding plantings |
| structural implication | Generally lower weight; confirm loads and details | Higher weight; structural check is required on existing decks |
| slope limit in New Jersey | Up to 20% | Up to 20% |
| assembly layers | Vegetation, media, filter fabric, drainage, root barrier, waterproofing | Same assembly logic; thicker media and more robust detailing as needed |
| program credit in New Jersey | Counts as green infrastructure and for runoff quantity; not for groundwater recharge or runoff quality | Same program credit notes |

## a tray does not pick the type
Modular trays are one construction method for building the layered system, not a type by themselves. A tray can be filled to an extensive depth or to an intensive depth, and trays can appear alongside built-in-place zones in the same project. The scrutiny still follows the manual: verify slope is at or below 20%, confirm minimum vegetation density of 85%, and specify media that meets permeability, pH, composition, and water-retention criteria.

Leak detection belongs in the maintenance plan, and a flood test alone is not enough to find small leaks. Trays do not absolve the team from planning for inspection zones, protection boards, and access for maintenance. A compliant roof also needs the deed notice so the living layer is not later altered or removed without review. Keep drainage boundaries clear: manage rain that falls on the vegetated roof and its walkways, and do not drain other roof areas onto it.

When trays are used on an existing building, the same structural due diligence applies. Use ASTM E2397/E2397M’s dead and live load definitions to frame the calculation and review with the structural engineer. Because modularity can make change seem easy, protect the system’s integrity in the specification and maintenance plan so unplanned swaps do not undercut performance or code compliance. In all cases, assemble the layers so the plants root in stable media and not in saturated fines.

## the retention percent is not the storm
The manual points to ASTM E2399 or FLL as acceptable test methods to determine media permeability and maximum media water retention. Those water-retention percentages are for selecting a media mix, not for calculating how much runoff the next storm will retain. The manual defines runoff retention as field capacity minus the wilting point of the media, and it assumes the media dried to the wilting point between storms. That means a back-to-back storm will not see the full calculated volume because the roof is already holding water.

Permeability of at least 1 inch per hour by ASTM E2399 or FLL keeps the layer draining so plants do not sit in water and so transient ponding clears to the drains. The selected retention band matters, but it is only one part of how the layer behaves; plant uptake, wind, solar exposure, and shading by panels shape evapotranspiration through the season. If you choose an intensive system with deeper media and a higher retention band, coordinate with structure using ASTM E2397/E2397M’s load definitions so the drained weight and transient water load are both accounted for. The goal is a living layer that performs within the manual’s assumptions without overstating storm-by-storm outcomes.

ASTM E2777-14 is a guide for vegetative roof systems up to 15% slope; it does not fund projects and it does not set New Jersey’s slope limit. The New Jersey manual’s slope maximum is 20%. Those two numbers can coexist because one is a design guide and the other is a jurisdictional limit; design the roof to satisfy both where they overlap and follow the manual where it is more specific. Likewise, FLL is an accepted test body alongside ASTM for the purposes the manual names; it is not a payor and does not replace program review.

## frequently asked questions

### what is an extensive green roof?
An extensive green roof is a layered system with vegetation rooted in a thin growing medium over a drainage layer, with filter fabric, a root barrier, and waterproofing below. In New Jersey, extensive means 6 inches or less of media, on roofs up to 20% slope, with a minimum vegetation density of 85%. Media is mostly lightweight aggregate with stable organic matter, pH 6 to 8.5, and at least 1 inch per hour permeability by ASTM E2399 or FLL. It is designed to manage rain on its own vegetated surface and its walkways and not to accept runoff from other roofs.

### how is an intensive green roof different?
An intensive green roof begins at 6 inches of media and often supports a broader plant palette, including larger perennials, shrubs, and small trees where structure and detailing allow. With greater depth comes higher drained weight and more retained water, so the structural engineer should verify capacity, especially on existing decks. Maintenance access, irrigation expectations, and protective detailing also tend to increase. Program recognition in New Jersey is the same category of green infrastructure that counts for runoff quantity but not for groundwater recharge or runoff quality.

### can you retrofit either type?
Extensive is the usual retrofit, because the media is 6 inches or less and the unit weight is lower. Intensive may not be suitable on an existing deck. Do not pick intensive until a structural engineer has checked the deck using ASTM E2397/E2397M’s dead and live load definitions. That check is useful for an extensive roof as well. Plan for leak detection in the maintenance plan, because a flood test is not enough for small leaks, and commit to the deed notice so the system is not removed or altered later without review. Keep within the 20% slope maximum and design the roof to manage only the rain that lands on its vegetated surface and its walkways.

### does a modular tray change the type?
No. Modular trays are a construction method that can be filled to either an extensive or intensive depth. A tray-based system must still meet the same media composition, permeability, pH, and water-retention criteria, and it must satisfy the same slope, vegetation density, and maintenance planning requirements. Treat trays as one way to build the layered assembly, not as a substitute for type selection.

## the series
For a grounding in the living layer itself, see [what a green roof actually is](/guide/what-a-green-roof-actually-is?from=guide). To understand why plant carpets are not the same as diverse plantings, read [a sedum mat is not a meadow](/guide/a-sedum-mat-is-not-a-meadow?from=guide). You can also continue through the full series here: [what a green roof actually is](/guide/what-a-green-roof-actually-is?from=guide), [extensive and intensive are not the same roof](/guide/extensive-and-intensive-are-not-the-same-roof?from=guide), [a sedum mat is not a meadow](/guide/a-sedum-mat-is-not-a-meadow?from=guide), [a green roof standard is not a living roof](/guide/a-green-roof-standard-is-not-a-living-roof?from=guide), [who pays to keep a green roof living](/guide/who-pays-to-keep-a-green-roof-living?from=guide).

## sources
New Jersey Stormwater BMP Manual, Chapter 9.4 Green Roofs, January 2026. The chapter defines extensive and intensive by 6-inch media depth, limits slope to 20%, requires minimum vegetation density of 85%, specifies media composition ranges, pH 6 to 8.5, permeability of at least 1 inch per hour by ASTM E2399 or FLL, and cites maximum media water-retention bands for mix selection (35–65% by volume for extensive; 45–65% by volume for intensive). It clarifies that green roofs count as green infrastructure and for runoff quantity but not for groundwater recharge or runoff quality, that roofs manage rain on their own vegetated surface and walkways, and that modular trays are a construction method that can deliver either type. It also requires a maintenance plan with leak detection and a deed notice, and notes that a flood test is not enough for small leaks.

ASTM E2777-14 is a guide for vegetative (green) roof systems, intensive and extensive, addressing roofs up to 15% slope as design guidance. ASTM E2397/E2397M-15 defines dead load as the drained weight including retained water and live load as transient water when the drainage layer is full. ASTM E2399 is the test the New Jersey manual cites for measuring media permeability and maximum media water retention. FLL, Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau, is named alongside ASTM as an acceptable test body in the manual.

Denver Green Buildings Ordinance green space guide (Option A) summarizes how new buildings 25,000 gross square feet and larger, including additions and campuses, must provide a cool roof and one additional green-building option. The vegetated roof path calls for professional qualifications, irrigation, plant lists, a maintenance plan, 80% coverage within 3 years, and at least 5 inches of growing media; if a later amendment removes that green space, the project must replace it or pay cash-in-lieu. The guide does not make a cool roof the same as a green roof, and its 5-inch note should not be equated to New Jersey’s 6-inch cutoff.

Philadelphia zoning 14-702 includes a green roof density bonus pathway. The Philadelphia Water Department reviews designs, and an operations and maintenance agreement is part of the path. These local processes complement, rather than override, the technical criteria summarized above for New Jersey.