Green Roof
Frequently Asked Questions
Introduction and history
In 2006, the Shakopee Mdewakanton Sioux Community (SMSC) constructed the largest green roof in the upper Midwest its new Water Reclamation Facility. The 31,000-square-foot green roof contains more than 45,000 plants, including native prairie grasses and flowers. The green roof system disperses significant amounts of water back into the atmosphere through evaporation and transpiration. During dry spells, the plant life on the roof is supported using reclaimed wastewater from the treatment process itself, yet another benefit of the high level of treatment the facility provides.
Research
The Shakopee Mdewakanton Sioux Community recognizes the research opportunity presented by the Water Reclamation Facility (WRF) green roof and its ability to answer some of the unanswered questions for Midwest green roofs. Five research projects are currently being conducted:- Looking at building heating and cooling on and inside the WRF and an adjacent building without a green roof.
- Determining plant and irrigation influence on soil moisture to optimize water retention.
- Examining influence of irrigation amount on percent cover of roof over two growing seasons.
- Establishment of Sedum by seeds, cutting, or plugs and various irrigation levels to determine optimization of upfront cost and final “green” cover.
- Competition of Sedum and native prairie vegetation under irrigation conditions after establishment.
- Exploring pollinator usage of native prairie plants used in the green roof vegetation.
Plants
A mixture of plants native to Minnesota and traditional green-roof vegetation (stonecrop) were planted on the roof in summer 2006. By the end of the first full growing season in 2007, the green roof had achieved 60% vegetative cover. Stonecrop makes up the majority of the understory, while the native grasses and forbs make up the mid- and upper- canopy. The vegetation receives water from rainfall and an irrigation system. Water from the irrigation system is the treated discharge from the WRF.Stonecrop, native grasses, and some native forbs did very well in the first growing season. Both stonecrop and native forbs were utilized by local pollinators. In addition to planted species, the green roof also hosts several kinds of volunteer plants. These surprisingly include wetland species like cattail, willow, and cottonwood.
Temperature Study
One of the benefits often attributed to green roofs is temperature regulation for the building on which it sits. Green roof advocates claim that shade and evapotranspiration produced by the vegetation keep roof temperatures lower; this in turn reduces the need for cooling during periods of warm weather. Additionally these same processes aid in mitigating the urban heat island effect which exists in developed areas.
One of our ongoing research projects addresses these potential benefits. We are currently recording air temperatures at five different heights to create a temperature profile from a height of one meter above the green roof to 1.5 meters above the floor directly below the green roof. A similar profile is being monitored in another area of the building that has a typical membrane roof with no vegetative cover. A comparison of these data sets will show whether there is a significant difference in air temperature above a green roof and whether any temperature differences are translated to the interior of the building.
Soil Moisture Study
Traditional roofs are designed to respond quickly to precipitation events and efficiently move water off of the structure; this water typically ends up in a storm drain. During a storm event this process can lead to flash flooding and erosion. With a green roof the opposite is true. One of the advantages of a green roof is its ability to retain water which can be taken up by plants and transpired back into the atmosphere or slowly released through the drainage system.
We are currently gathering soil moisture and precipitation data on a small section of our green roof. This data will allow us to quantify the response of green roof soils not only to precipitation events, but also to periods of dry weather.