Economic Case Study: Green Roof ROI

by Matt Draus on Monday, March 14, 2022

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Purple-Roof DETAILS

How Detention Green Roof ROI saved a Project

The Moderne is a new 5-story condominium located in Saratoga Springs, New York. Constructed in 2021, it contains a mix of 24 high-end residential units with exterior and underground parking. It also includes a 1020 m2 (11,000 sf) rooftop amenity space with a green roof.

During the early stages of construction, issues with excessive groundwater and soil stability required additional foundation work to be completed. That work required the relocation of the underground stormwater tank. Due to the tight space constraints of the site, that new tank location would eliminate two parking spots. Local zoning requirements stipulated that losing the two parking spots would reduce the allowable number of units. That reduction would have translated into financial ruin for the project.

Where to Put the Stormwater?

The design team was immediately tasked with finding a new method of managing stormwater, and eliminating any parking was not an option. Construction had already begun, so a decision was needed quickly. The first option discussed was to install a traditional storage system inside the building. That would absorb approximately 112 m2 (1,200 sf) of interior space and cost roughly $75,000 to install. It would require annual inspections and maintenance, but most importantly, it would cost the developer a great deal of sales revenue with the loss of interior space. This option was not well received.

Fortunately, the project team found a new method of stormwater management. By upgrading the traditional green roof to a Purple-Roof concept green roof, which utilizes detention, there may be no need to lose parking or interior space.

Read more about detention type green roofs in these articles:

What is Green Roof Stormwater Detention?

Does a Detention Roof Make Sense for Your Project?

Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water

The Stormwater Plan and Green Roofs

Through careful hydrologic modeling by Green Roof Diagnostics (GRD), a Purple-Roof system was engineered and modeled to provide the specific storage and outflow reduction required, without any need for additional systems or loss of space. After confirming with the structural team and local regulators, the decision was made to implement the Purple-Roof concept and eliminate the tank system.

Learn more about green roof detention in this video:

For permitting purposes, the design storm used was a historical Type II, 100-year, 150 mm (5.9”), 24-hour storm. The required storage volume for the building was 170,000 L (6,000 cf), and the maximum allowable outflow rate was 28 L/s/ha (0.4 cf/s/a). To achieve these metrics, the green roof was upgraded by removing the traditional drainage panel and then adding 50 mm (2”) of needled mineral wool, a 150 mm (3”) honeycomb reservoir, and a 5 mm (3/16”) thick detention layer.

Overview of the 735m2 (7905 sf) vegetated Purple-Roof compliant profile. VE=vegetation, 1.5cm (0.5”), GRS=green roof soil, 11cm (4.5”), NMW=needled mineral wool, 5cm (2”), HC=honeycomb, 7.6cm (3”), DL=detention layer, 5mm (0.2”).

Overview of the 280m2 (3011sf) Purple-Roof compliant paver profile. NMW=needled mineral wool, 5cm (2”), HC=honeycomb, 7.6cm (3”), DL=detention layer, 5mm (0.2”).

Purple-Roof DETAILS

The needled mineral wool provides excellent retention and filtration properties while minimizing weight. The honeycomb detention reservoir provides a temporary storage that is always functioning, regardless of the soil saturation. The detention layer provides controlled and consistent outflow at an engineered rate over the entire green roof area.

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Green Roof Stormwater Management using Detention-Type Green Roofs

By managing the stormwater on the roof, the developer provided 180m3 (6,336 cubic feet) of stormwater storage, equal to over five shipping containers. The Purple-Roof also reduced the peak stormwater runoff by 95% and provided a 42-hour drain-down delay. The decision also kept 26 semi-truckloads of debris from being excavated, hauled, and dumped in a landfill. Finally, the Purple-Roof concept allowed the Developer to sell all 24 units, including the 112 m2 (1,200 sf) of salvaged interior space at the market rate of $550 per sf, adding $540,000 to the developer’s sales revenue.

Financial Summary

The additional cost of the Purple Roof was $196,488
The credit for no longer requiring the underground storage tank was approximately $75,000
Therefore, the actual added cost for the Purple-Roof compliant system was $121,500.

By investing $121,500 to manage the stormwater on the roof, the developer was able to sell two additional units for a total of $1,800,000.

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