The roof is made up of 60% Ethylene tetrafluoroethylene (ETFE), a fluorine-based clear plastic, and is the largest in North America, spanning supplied and installed by the firm Vector Foiltec. ETFE's low R-factor and the roof's slanted design, inspired by Nordic vernacular architecture, allows the stadium to endure heavy snow loads. Snow accumulates in areas that are more safely and easily accessible, and also moves down the slanted roof into a heated gutter, the water from which drains to the nearby Mississippi River. The translucent roof and large wall panels also give fans a view of downtown Minneapolis. The glass operable wall panels will allow the stadium to experience some of the outdoor elements while providing protection from the snow, rain, and the cold winter weather. The stadium is aligned northwest and the elevation at street level is approximately above sea level. Years before construction began on the stadium, local and national conservation groups - including the Minnesota Department of Natural Resources and the Audubon Society - requested a "bird-friendly" design of the stadium's exterior using slightly less transparent bird-safe glass. Designers ignored the advice and instead used highly reflective glass for aesthetic reasons. The reflective glass, combined with the stadium lying along the Mississippi Flyway migration route, has resulted in a large number of bird deaths, double any other building in Minneapolis. A "bird fatality study" being financed by the Vikings and the Minnesota Sports Facilities Authority is expected to be completed in 2019. If changes are made, it will now cost about $10 million to replace the existing glass with bird-safe glass rather than the $1 million it would have added to the original construction. The design for U.S. Bank Stadium has been compared to the Christ Cathedral in Garden Grove, California, which was created by architect Philip Johnson. Opened in 1980, the then-named Crystal Cathedral was previously considered America's largest glass-dominated building. The stadium, which likewise sports transparent roofs, walls, and giant rotating doors, has the world's five largest pivoting doors. The seating capacity is 66,860 for most games, slightly more than the Metrodome, and can be expanded to 73,000 for soccer, concerts, and special events, such as the Super Bowl. The Vikings' lease with the Metropolitan Sports Facilities Commission (MSFC), as signed by both parties in August 1979, kept them in the Metrodome until 2011. The lease was considered one of the least lucrative among NFL teams; it included provisions where the commission owned the stadium, and the Vikings were locked into paying rent until the end of the 2011 season.