Now You See ‘Em, Now You Don’t: Green Roofs

Some of the projects I work on experience many design changes between initial concept and completion of construction. On healthy projects, the most dramatic changes that I see occur after the schematic design phase, before the beginning of the construction documents phase. A few of these projects of mine have included “green roofs” (vegetated roofs) at the outset of the project. But those were gone before the construction documents phase started. I’m not sure who’s talking whom down from the roof, but it’s outta there, over and over again. Who brought the idea to the project? Was it just something that added a splash of color to the architect’s renderings? Had the owner always wanted to be able to walk in a garden while simultaneously enjoying a great view from a rooftop? Was everyone on board with a green roof until the contractor’s preliminary pricing came in?

Denver voters just passed the Green Roof Initiative last month, mandating vegetated roofs for certain buildings within the City limits. I wrote a commentary about this for the newsletter of our Denver Chapter of the Construction Specifications Institute. It includes a link to the text of the ordinance, and touches on the specifics of the initiative, some green roof risks for owners, and the loopholes in the ordinance. Here’s part of that:

“Denver voters faced Initiated Ordinance 300, the Denver Green Roof Initiative, in our recent election, and voted it in, by a small margin. What does this mean for building owners, developers, architects, engineers, contractors, roofing distributors and product representatives?” Continue reading…

Green roofs have benefits, mostly for the people who get to look out windows and see plants instead of roofing materials, but they can also mitigate urban heat island impacts, and help to improve the quality of stormwater before it hits municipal systems. Perhaps the most idealistic of the selling points made by the supporters of the Green Roof Initiative is the dream of rooftop urban farming. From the mission of the supporters: “Buildings are permitted to use the rooftop space for urban agriculture. This allows for the building to rent out their rooftop space to urban farmers who can then supply their goods around the city.” However, crops need a roof that meets more than just the minimum mandated by the ordinance. Here’s a basic overview of the 3 main types of green roofs. Some require beefier supporting structure, and more complex irrigation systems, than others.

Most owners who are merely looking to build a commercial building in the Denver area will do the minimum green roof, use a loophole to get out of building a green roof altogether, or build outside the City limits.

As I wrote in my commentary, “Building owners will have costs for their green roofs beyond the design and construction phases. They will have risks that owners without green roofs don’t have. Green roof plantings need to be maintained, cared for, and watered, forever. Insurance riders for green roofs will increase the costs of building insurance, but regular insurance policies usually exclude problems stemming from green roofs, so these will be necessary costs. Problems with the waterproofing components of green roof assemblies, discoverable only after moisture intrusion into buildings, are more difficult to diagnose and repair than problems with non-green-roof assemblies. Moisture intrusion is the most common cause of damage to buildings, and roofs are the parts of buildings that are most prone to moisture intrusion…”

Aesthetically, a green roofscape is a lovely goal for Denver, but building owners should not contribute to this blindly. They need to know what risks they are taking on. Sometimes people start projects without knowing a lot, then learn more as design goes on. (And then the green roofs on my projects aren’t there anymore.) In my work as a construction specifications consultant to architects, I expect to see a little bit more of the same pattern I’ve been seeing for my projects – green roofs that are there at schematic design, and gone before construction. Now you see ’em, now you don’t.

green grass

(Not a green roof. Just green.)

 

 

 

 

3 thoughts on “Now You See ‘Em, Now You Don’t: Green Roofs

  1. Liz:

    Do the “green roofs” have to be permanent planted roofs, whether extensive or intensive? Or can they use pre-planted trays placed on a roof membrane and protection board? (I like to call these trays “chia pavers”).

  2. Good question, Dave. I think that if the intent of the Standard is followed, trays wouldn’t be allowed.
    Some excerpts from the elected “Denver Green Roof Construction Standard” that make me think that trays would not be consistent with the intent of the Standard.
    B. Denver Green Roof Construction Standard: mandatory provisions.
    1. Green roof assembly.
    A green roof assembly shall, as a minimum, consist of a root repellent system, a drainage system, a filtering layer, a growing medium and plants, and shall be installed on a waterproof membrane of an applicable roof.
    8. Waterproofing
    (a) The design and construction shall include the installation of a root barrier in all vegetated roofing systems.
    11. Vegetation performance.
    In order to support plant survivability:
    (a) When structurally possible, the growing media shall be at a minimum 4 inches; or
    (b) The applicant shall provide a report confirming that the engineered system as designed provides plant survivability comparable to that of an un-irrigated system with growing media at minimum 4 inches.
    13. Irrigation.
    Adequate measures shall be provided to permit irrigation necessary to initiate and sustain the vegetation during the service life of the green roof.

    https://www.denvergov.org/content/dam/denvergov/Portals/778/documents/VoterInfo/SecondSamplePetition_GreenRoofs_2017_2_23.pdf

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