By David Holman, Jason Lord, Jake Gold, and Andrew Kaplan
Carleton College
All Photos by and Courtesy of Jason Lord and Dave Holman

On May 18, 2005 five Carleton College students completed building the first green roof to test 78 varieties of prairie plants native to Minnesota for their viability to the green roofing industry. This 666 sq ft roof on part of the Olin Science Building at Carleton College is also the first student-designed and built green roof in Minnesota.

The roof will provide structural, functional, environmental and aesthetic benefits to the building. The project will provide plant growth data, water monitoring and temperature data that will be used for further roof construction at Carleton and by green roof contractors. The roof can be easily viewed from the glass breezeway connecting the Olin and Mudd Science Buildings at Carleton College.

The Carleton College Green Roof Project was formed last September 2004 by David Holman ('06), Jason Lord ('06), and Jake Gold ('07) as an independent study at Carleton College in Northfield, MN. The project is advised by green architect and Carleton Director of Facilities Richard Strong, and was inspired by his class "Building the Eco-House" taught in the spring of 2004.

The fall of 2004 was spent studying green roof design and planning where green roofs could be made at Carleton. In our independent study, we set out to conduct some of our own research. To determine energy savings, we calculated the additional insulation value (R-value) that a green roof provides to an existing roof. We calculated R-values with our soil mixture saturated and dry. The team did R-value testing on various soil types during the fall and winter finding many case studies demonstrating the energy savings green roofs provide in the summer, but like other research has shown, green roofs do not contribute very significantly to a roof's R-value or energy savings in the winter. We will continue to do further testing in future years.

Last winter we planned where to build a small test roof on campus and decided what materials and plants to use. Mandi Fix ('08) and Andrew Kaplan ('08) joined the project in the spring. The team decided to try testing only prairie plants native to Minnesota. Prairies are typically so drought-resistant because of extremely deep root systems, so we expect many of our species to suffer under the low soil conditions of our green roof.

The group chose a 666 square foot roof connected to the Olin Science building. The roof is highly visible via an elevated glass walkway on the West side and is one of the strongest roofs on campus because chemicals are stored underneath. The soil substrate consists of two layers: the bottom layer is a 4" mix of 36.5% Perlite, 36.5% Vermiculite, 12% Clay Particles (a product called Turface), and 15% compost from Carleton's yard waste pile.

This bottom layer is designed to be extremely lightweight, water absorptive, and inorganic. The top layer 2" of pure compost is then slightly mixed into the bottom layer to create a heavier, more organic top layer for seed germination and prevention of soil loss via wind. The 6" mixture weighs 16.1 lbs/sq ft dry and 24.9 lbs/sq ft saturated. This unusually organic soil mixture was used to provide more lush plant growth. This compares favorably to the existing ballast, weighing 11 lbs/sf.

Our Goal in this project was to design a green roof using as many native prairie plants as possible. We used a large seed mixture of hardy prairie plants from this area. We chose species that we thought would best survive thin the hot, dry and shallow soil environment.

We planted: 54 species of prairie forbs, three types of shrub/vine species, 19
species of grasses, sedges, and rushes, and a cover crop of oats was seeded. This mix was provided by Prairie Moon nurseries of Winona, MN. To provide faster plant establishment for this highly visible roof, 250 root stocks from Prairie Moon of various prairie plants were also planted. We selected our plants with the help of Ron Bowen of Prairie Restorations, Prairie Moon Nursery and Carleton Biology and Arb faculty.

The drainage layer beneath that soil is a DBR-50 Rootbloc product manufactured and generously donated by American Wick Drain. This drainage layer provides a grid of small plastic cups to retain water while any overflow drains away.

Thanks to donation of materials and student labor, the cost of the current project was around $2.07 per square foot. The total project cost of our green roof (with free labor and drainage layer) was $1,375. We would like to thank American Wick Drain for their generous donation of 1,000 sq ft of their 1/2” drainage layer for this project! We would also like to thank Carleton’s facilities and grounds departments for their great support for this project!

Possible projects for the future include constructing another test roof which will compare plant growth using three different drainage techniques over two different soil depths. The team will be measuring plant establishment and survival over the next couple years and planning to construct a much larger project that would provide cooling benefits to a dorm. A hydrologist from Carleton will be testing the water runoff quality over time. The team will be publishing all the specifics of our materials, design and testing on this website.