The high-tech plastic roof on the new Minnesota Vikings stadium looks like something from a household kitchen — a wobbly, paper-thin surface that could be used as a cutting board.
It’s called an ETFE pneumatic roof, or ETFE for short. That stands for ethylene tetrafluoroethylene, a polymer similar to Teflon with the same nonstick qualities.
It’s extremely durable and flexible, but still lightweight and translucent — qualities that make it ideal for what the Vikings want at U.S. Bank Stadium.
Vikings executive vice president Lester Bagley called ETFE “the best solution for this market and this climate. We wanted to do something innovative, but also something functional and practical.”
Last week, Mortenson Construction nailed its November deadline for completing the roof. Unlike at this time last year, site crews now will be working indoors and the place looks more like a stadium than a concrete bowl.
As of last week, roughly a quarter of the stadium’s purple seats had been installed. The video board on the east end was being lifted into place, panel by panel, by a crane on the dirt floor. Most of the glass windows were installed, and the building was so toasty that a jacket was optional.
A comfortable indoor climate is among the many key features of the $1.1 billion stadium, on track to open for the 2016 NFL season. State taxpayers are paying $348 million and Minneapolis is covering $150 million of the largest public-private effort in state history; the Vikings are covering the rest.
Because of Minnesota’s harsh winters, a retractable roof was considered for the team’s new home as an appealing alternative to the cavelike Metrodome, where the controlled indoor climate and diffused light made it feel claustrophobic even on sunny days.
But when the retractable option was set aside for cost reasons, the state and the Vikings sought a way to avoid the dark feel of closed stadiums. The solution from Dallas-based architect HKS was ETFE, a fritted film that can handle both heavy snow and scorching sunlight.
Bagley said that the translucent ETFE, the extensive glass throughout the building and the five pivoting glass doors on the west side of the building were designed to bathe the 65,400-seat stadium in light and provide a connection to outside.
One thing the new roof won’t do, according to everyone: collapse.
The Metrodome’s fiberglass roof infamously caved under the heft of deep snow in December 2010. That roof was held up mostly by air pressure.
Air supports the ETFE pillows in the new roof, but those pillows also are girded by a steel roof structure.
Edward Peck, vice president at Thornton Tomasetti, the facade engineers, said the air in the panels at the new stadium is used only to stabilize the ETFE. The underlying steel structure is self-supporting, so a loss of air pressure won’t cause the roof to sink like the Dome’s did, he said.
ETFE can be punctured by sharp objects but it’s easily repaired with a patch, Peck said. And nothing sticks to it, so it won’t get dirty or collect snow.
The plastic has been used on sports arenas and buildings throughout the world, so it’s been tested in all sorts of weather and has proved extremely durable, Peck said.
The hard roof on the stadium’s north side is steeply pitched and will easily shed snow into a diverter and gutter system encircling the roof. The diverter captures the snow as it comes off the roof and channels it into the gutter, which has a radiant heating system to ensure melting.
Brendan Moore, Mortenson’s senior project manager in charge of the enclosure and roof, said that silver fritting — a dotted pattern on the ETFE — will serve as a sun shield in the summer to cut glare and reduce the heat coming through the roof. While not entirely transparent, it’s translucent enough to make clouds visible from the seats.
In at least one way, the new roof will be similar to the Dome’s: It will make the place noisy.
Bagley said the roof on the new stadium is “acoustically reflective,” so “it’s going to be hard for other teams to play in there.”