The original roof at the museum’s research and collections facility, the Natural Heritage Campus, was installed in 1995; as the roof was nearing the end of its operational life, it had been leaking in numerous locations. After considering several replacement options, this summer we installed a green roof on about 20% of the existing roof area. (Read a botanist’s take on the brand-new roof). Almost three months later, things seem to be working fine.
Green roofs tend to be less popular in northern zones because the weight of the additional soil and plant material and the winter snow load significantly increase the costs of the oversized supporting steel structure. In our case, the roof membrane was already covered by heavy stone—the plant material now replaces the stone and is approximately the same weight as what was removed. As a result, no additional weight was placed on the structure.
Green roofs have a number of technical advantages over standard roofs, not the least of which is extending the life of the roof membrane it protects by 50%. The need to replace the membrane is reduced, therefore minimizing waste while lessening the risk to our collections over a longer period.
The roof can contribute to reducing heating and cooling costs because roofs are the site of the greatest heat loss in the winter and the hottest temperatures in the summer. The soils and plants keep the roof cool, which in turn reduces energy use.
Some of the disadvantages of a green roof include its initial installation cost and a more demanding maintenance program. The stonecrop plant material (species of Sedum) was specifically selected for its low maintenance needs and resistance to droughts, although of course, even these plants need water to survive and thrive. By design, the system reduces rainfall run-off by containing the rainwater so it can be mostly consumed by the plants. The plants are placed in recycled plastic containers whose principal role is to hold the water for future use, so that the plants are only watered after 10 days of drought. Weeding is required on a monthly basis, and the replenishment of dead plants is performed as needed.
Winter maintenance is limited because plants become dormant. Should shovelling be required in some locations, careful attention needs to be taken to reduce the impact on the plants. In some cases, spot re-planting of some material may be required in the spring.
The museum is closely monitoring the effectiveness of the green roof and hopes to share our experience with others who are considering it for their project. The experiment has so far been very positive, and time will tell of challenges and opportunities that may arise.