Clear roof has Vikings' new stadium at forefront of NFL trend

MINNEAPOLIS -- Before its glass doors have opened for a Minnesota Vikings game, or the first load of snow has skittered off the roof on a Saturday night to prevent a catastrophe on Sunday, U.S. Bank Stadium has made the transition from architectural curiosity to stadium trendsetter.

The reason for the quick turn? The roof of the new $1.1 billion facility in downtown Minneapolis, which is designed to let in natural light, keep away snowdrifts and sunburn and make one of the NFL's coldest outposts feel like it's open to the outdoors -- without being exposed to the elements.

Approximately 60 percent of the stadium's roof is covered in ethylene tetraflouroethylene (or ETFE), a transparent polymer similar to Teflon that will allow natural light to flood the stadium. The German-developed material is lighter than glass, and is fritted to block out harmful UV rays. It's so thin the Vikings can still hold flyovers before their indoor games, and reflects enough sound that the team expects its new venue to match the ear-splitting volume of the Metrodome.

Snow, which felled the Metrodome's roof in 2010 following a 17-inch storm, also won't stick to the roof, sliding off its steep pitch into catch basins and heated gutters that funnel it back into the ground. U.S. Bank Stadium won't be the first sports venue covered in ETFE (remember the "Water Cube" in the 2008 Beijing Olympics?), but it will have the largest ETFE roof in the world. The stadium's roof has already been featured in Popular Mechanics. It also is on the forefront of a new trend; the Atlanta Falcons' new stadium will have ETFE on the roof, and HKS, the architect that designed U.S. Bank Stadium, also included an ETFE canopy in plans for the Los Angeles Rams' new stadium.

"You really do feel like you're sitting outside," Minnesota Sports Facilities Authority chairwoman Michele Kelm-Helgen said. "And yet, the roof has fritting on it, so it kind of diffuses the light. It's not like a stark blaze of light; it's really kind of soft. You see clouds, you see the sun. You get that indoor-outdoor feel."

The stadium's glass doors, standing 95 feet tall at their highest point, will also open to downtown Minneapolis on the west side of the stadium, allowing fresh air in from the outside on seasonable days and adding to the open-air feel of the facility. Even on an overcast Tuesday morning, with few indoor lights turned on inside the facility, daylight illuminated the stadium during a media tour.

"If you go to another closed, covered stadium, it's dark on days like today," Bagley said. "But here, it's very bright and natural."

As the Vikings parsed HKS' options for the facility before the Minnesota State Legislature approved it in spring 2012, they realized a retractable roof, in Minnesota's fickle climate, didn't make sense. The ETFE roof became the team's preferred option after some initial research, and as they unveiled the design to the public in May 2013, they did so with a catchphrase: Clear is the new retractable.

"We looked at Indianapolis; they have a retractable-roof stadium, and they only open it a couple times a year," Vikings executive VP of public affairs Lester Bagley said. "We liked the concept of that indoor-outdoor feel. We'll keep those doors open as long as possible during the Vikings season, have our fans feel the elements -- and the visiting team feel the elements -- and really have that stadium for all seasons. That's kind of the vision."

The vision will be exposed to the world over the next several years, when U.S. Bank Stadium hosts Super Bowl LII in 2018 and the NCAA men's Final Four in 2019. By that point, the Falcons' new stadium will be open, and the Rams' new facility will be in its final phase of construction. The latest trend in NFL stadiums, though, will have started with a cutting-edge solution in a cold climate.

"At noon on a Sunday in the fall, we should have optimal sunlight," Bagley said. "No matter the weather outside, you're in perfect comfort inside, and you get the feel of an outdoor experience."