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Ready...Set...Grow - Denver's New Green Roof Requirement

12/01/2017 9:17 AM | CAI Rocky Mountain Chapter (Administrator)

By Justin Foy, RS, Senior Vice President, SBSA

Ready or not, community associations and all other occupancies within Denver will be required to construct green roofs, install a combination of green roof/solar energy collection, or pay a fee to be exempt from this requirement. Following in the footsteps of cities like San Francisco and Toronto, Denver voters recently approved Initiative 300. This environmental measure aims to reduce the heat island effect, naturally drain and filter stormwater, and reduce greenhouse emissions.  This Amendment to the Denver Building Code will go into effect January 1, 2018. 

The law will apply to every building in the City and County of Denver that has a height equal to or greater than four stories or 50-feet and a gross floor area of 25,000-square-feet or more. The adjacent diagram shows how the green roof coverage requirement ratchets up with incremental increases in floor area. 

The law will allow a combination of green roof and solar energy collection as long as the combination is no less than 30-percent green roof and retains or collects for reuse at least the first 1/4-inch from each rainfall or 50-percent of annual rainfall volume. 

At a minimum, the law will require every green roof to be constructed with the following assembly (in order from roof deck up): an appropriate waterproof membrane for a green roof system, root repellent system, drainage system, filtering layer, growing medium, and plants, as shown in the diagram to the left. 

Because the City wants to ensure that each building’s structure can handle a green roof system, the weight of the green roof and solar panels may require building modifications to the structural components. Consideration from the foundation to the roof will have to be made, including deflection and ponding due to permanent loads now affixed to the building. Without the original structural drawings, your community may have to deconstruct the interior finishes to determine the structural systems used. 

If the vegetation areas are not uniform in loading, the effects of non-uniform or unbalanced loads, including drift loading against the sides of the beds, may have major impacts on the existing structural systems.

A wind uplift pressure and scour report will need to be prepared and stamped by a Professional Engineer when applying for a permit. Although the overall weight of the green roof system may account for the vertical loads, individual layers must be evaluated for their ability to resist both uplift and wind scouring forces. The report will need to show how the green roof design addresses these forces.  

To ensure structural and waterproofing integrity, test protocols that can be used to validate each component prior to application of the overlying component should be conducted. These include the use of sensors or other means to document pre- and post-conditions, such as:  

  1. Flood test
  2. Electric field vector mapping
  3. Impedance test
  4. Infrared thermal imaging 
  5. Low voltage testing
  6. High voltage testing
  7. Moisture sensors 

The new law will dictate the vegetation performance and specify that the growing media be a minimum of 4-inches. No noxious weeds can be used. The plant selection and design will need to be used for urban agriculture or, within three (3) years of planting, the plantings will need to cover no less than 80-percent of the vegetated roof area. It is suggested that a landscape architect be engaged to provide recommendations and maintenance to ensure plant viability, particularly during droughts or winter.

A plan that defines the routine maintenance and necessary inspections for the green roof media and plantings to perform their required function will be required to be submitted with the permit application. Community associations will need to be aware of the operating and reserve budget impacts with the new roof system and as a new common element will require a change to the declarations under the procedures outlined by Colorado law. 

The new law provides exemptions or variances when it is proven that a community is unable to meet the green roof requirements. If two or more of the following circumstances are met, then an exemption may be permitted:

  1. The building is being used for seasonal purposes.
  2. The building is designed in such a way that it would not be possible to meet the requirements. 
  3. The building retains or collects for re-use at least the first 1/4-inch for each rainfall or 50-percent of the annual rainfall volume falling on the roof through systems that incorporate roof surfaces.
  4. The building has an Energy Star Building Rating of 80 or higher. 

A community can also make a payment of “cash-in-lieu” of construction of a green roof for the reduced or exempted area based on the average actual cost of construction of a green roof, which at this time is $25.00 per square foot. Denver will recommend changes to the cost bi-annually.

Regardless of whether a community association chooses to comply with the new law or make a cash-in-lieu payment, the impact of the new requirements will be far reaching if not impossible to meet. Communities will now need to carefully consider factors such as how to afford a new green roof system, the structure to accommodate a green roof, how it will be accessed, how it will be maintained, insurance policy updates, and reserve study updates.


Justin has project managed over $120 million in construction repair and rehabilitation projects for community associations across the United States. He has conducted over 1000 property investigations, property condition assessments, and capital reserve studies.  Justin was designated the 59th Reserve Specialist (RS) in the United States in 2001. 

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