Walmart has broken ground for a new store in the Hayden Meadows region of Portland, Ore., with plans for a 40,600-square-foot green roof that will be the largest in the entire state.

The 90,000-square-foot supercenter will include the chain’s second largest green roof, surpassed only by the 70,000-square-foot EcoGarden atop a store in Chicago.

The city consulted with the Walmart design team prior to construction, and Portland Bureau of Environmental Services (BES) will keep tabs on stormwater data gathered from the roof.

“We were able to work with the partners to encourage them to do some monitoring of the performance of the roof,” says Matt Burlin, BES outreach coordinator for sustainable stormwater management. “We gave them some insight on what we’d like to learn about how the project will operate, mainly for stormwater flow and retention.”

BES offers incentives for private properties to build living, vegetated roof systems. The ecoroof program promotes stormwater management and builders receive a reward for going green. 

“Our ecoroof incentive funds up to $5 per square foot of green roof on any building except those owned by the city,” says Burlin. “Since 2008 we’ve helped fund 170 ecoroof projects with over $1.9 million for 8.8 acres of green roof.” 

The Walmart project will include two types of roof construction. “The total roof will be a combination of ecoroof and conventional roof,” says Burlin. “The ecoroof portion will be divided into sections with varying depths of growing media. The city will monitor how these different depths retain and reduce runoff quantity and how they affect runoff water quality.” 

The city’s current largest green roof, atop the LEED Gold-certified Ramona apartment complex, is a meager 31,600 square feet compared to the mammoth Walmart project in progress.

More information on Portland’s ecoroof program.