Engineering sustainable roofs over our heads

The Victorian mansion on the grounds of Brucemore in Cedar Rapids was built in 1886, before the current concerns about global warming and sustainability.  But when the roof needed to be replaced, engineers from Shive-Hattery in Cedar Rapids found that going back to the original roofing materials was the most sustainable and “green” choice.

“They wanted to restore the roof to its original look,” said George Kanz, Team Leader of the Building Division at Shive-Hattery. “So we created a design using slate, and it came from the same quarry as the original roof's slate.”

One way engineers make buildings and their roofs more sustainable is to use long-lasting materials. They don't need to be replaced as often, so less material is goes to the landfill and less material needs to be created.

Slate, a natural stone found in eastern states like Pennsylvania and Vermont, is a very long-lasting roofing material.  “We worked on the Brucemore roof 10 years ago, and we expect it to last for another 60 years,” said Kanz.

But slate introduces engineering issues of its own. Its density makes it a fairly heavy material, so structural engineers determine the weight-bearing load of the building and recommend any additional support measures. Often, civil engineers are drawn into these projects as well, for their expertise on water flow.

Companies including Shive Hattery and Neumann Monson in Iowa City have also engineered roofs that reduce drainage and run-off by letting plants soak up the moisture.

These truly green roofs - roofs topped by growing plants - take a bit more engineering finesse, according to Kevin Monson at Neumann Monson. “First, a structural engineer checks the capacity of the roof to see if it can take the added weight of the vegetative material,” he said.

A roof membrane is then designed and installed to keep roots and water out of the building. A reservoir grid comes next to provide pockets for storing water for the plants, and four to 12 inches of very light-weight soil is added, providing extra insulation. Hardy plants, like sedums, often make up the green part of a vegetative roof.  “They thrive in extreme conditions and don't need much soil,” Monson noted.

The best part is that these sustainable, earth-friendly roofs are low-maintenance, too.  Neumann Monson has installed vegetative roofs on Southeast Junior High in Iowa City and the North Ridge Park Pavillion in Coralville.  The plants protect the man-made parts of the roof and are easy-care. “We've found there's not much that has to be done to keep them going,” said Monson.

By Jane Nesmith, for Gazette Special Sections