Denver’s Green Roof Initiative: What Building Owners Need to Know

A growing number of cities today are passing green roof legislation that requires building owners to cover a specific percentage of their roof square footage with vegetation. In November 2017, Denver residents voted in favor of the Green Roof Initiative (also known as Denver Initiative 300). Modeled after an ordinance in Toronto, it went into effect on January 1, 2018.

Denver’s Green Roof Initiative applies to all new and existing buildings that have gross area of 25,000 square feet or more, except for some residential buildings. It also applies to all building additions (again with the residential exception) that cause the gross area to exceed 25,000 square feet. Commercial greenhouses, temporary structures, air-supported structures, and existing buildings that cannot structurally support a green roof weight load without “major structural alterations” are exempt as well.

The ordinance is “triggered” when an owner or contractor applies for a building, roofing, or reroofing permit. In other words, owners do not need to implement a green roof now if they have no plans to construct a new building or replace an existing roof. This is similar to insulation upgrade requirements for reroofing projects. However, when the time comes, meeting the requirements of a green roof initiative can be very challenging. Consequently, it can be crucial to have the guidance of an engineering firm like Pie that understands the ordinance and the most effective ways to ensure compliance.

What is a Green Roof?

A green roof is commonly defined as a vegetated surface installed over a waterproofed occupied space. There are two primary types of green roofs: extensive and intensive. An extensive green roof is one that is covered with a low-profile plant “carpet” often made up of what are called sedums. These drought-tolerant plants require shallower soil depths and lower maintenance (including irrigation) than many other plant varieties. An extensive green roof adds less weight to a structure than an intensive green roof and is less expensive.

An intensive green roof is one that may include shrubs, trees, and other types of more typical landscaping plants. This type of roof is often what is implemented if accessible space such as a patio is to be part of the green roof design. Intensive green roofs require significantly more soil depth, and consequently the structure must be able to withstand considerably more weight.

Combining Green Roof and Solar

Denver’s green roof legislation allows for some of the green roof requirement to be met by the installation of solar panels instead. However, a minimum of 30% of the roof area required by the ordinance must be green roof.

The Benefits and Challenges of Green Roofs Under Denver Initiative 300

The green roof movement has been spearheaded in part by the perceived benefits they convey to a community. For example, some studies have suggested that green roofs may help combat the urban heat island effect. They also indicate that the increase in vegetation in cities may improve air quality. In addition, green roofs may help with stormwater management.

Green roofs are also said to provide benefits to building owners. These advantages, although variable, may include increased roof durability, improved building energy efficiency, increased building valuation, and greater desirability when rooftop amenity space is added in conjunction with a green roof.

For all their potential benefits, however, implementing a green roof under the Initiative is not without its challenges. The following represents a brief list of considerations:

  • Green roofs weigh two or more times heavier than a typical roof assembly, which requires a heavier and more costly structure.
  • Green roofs cannot be readily installed on steep-slope roofs (2:12 and higher), but the initiative does not have provisions specific to pitched roofs.
  • Skylights and HVAC obstructions are not accounted for in the initiative, but have specific requirements that can make them a challenge to incorporate into green roof.
  • If a green roof/solar combination solution is implemented, a rainwater catchment system must be implemented as well. Designs incorporating solar must be carefully navigated through Denver’s permitting variance process as Colorado state water rights laws dictates that water catchment is not permitted on commercial buildings.
  • The initiative does not address roofs with shading which renders them much less capable of having a successful green roof or solar system (due to orientation, adjacent building shadowing, or shading from HVAC equipment).
  • Isolated “islands” of green roofs on a roof are less likely to survive and harder to maintain than a larger, single area of green roof, but the initiative does not prohibit this approach to providing a green roof.

Opting Out of Denver’s Green Roof Initiative With a “Cash in Lieu” Payment

Building owners may be allowed to opt out of the green roof requirement by making what’s called a “cash in lieu” payment. The Denver Planning Board must first grant the owner an exemption, which can be for a portion of green roof area or the entire required green roof/solar square footage. The initiative sets the current cash in lieu amount at $25/sf of green roof that would be required but is not being installed.

What Can Pie Do For You?

As building owners work to understand the Denver Green Roof Initiative, experienced professionals can help to ensure they are in compliance. In some cases, input from an engineering/design firm is mandated and there are many ways that Pie’s green roof expertise can be beneficial.

  • Provide a required engineer/architect-signed and sealed structural assessment indicating whether an existing building is exempt
  • Serve as the designer of record on a green roofing project (waterproofing, green roofing material, etc.)
  • Perform a green roof design review to identify areas where designs by others could be improved
  • Serve as the designer of record on a structural retrofit project to enable the installation of a green roof on an existing building
  • Provide a required engineer/architect-signed and sealed wind load calculations and wind resistance strategy letter
  • Perform required testing of waterproofing prior to green material installation and provide the required engineer/architect-signed and sealed letter detailing the results
  • Perform quality assurance observations during waterproofing and green roofing installation.

Even if you are not currently contemplating a construction or reroofing project, working to become familiar with the ordinance now can help you prepare to effectively meet your obligations down the road.

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