One World Trade Centre will surpass Chicago’s Willis Tower (above) as the tallest building in the US when it opens next year. Photo: Reuters

The skyscraper at New York’s World Trade Centre has been designated the tallest building in the US by virtue of its spire, surpassing Chicago’s Willis Tower, an international body said.

The Chicago-based Council on Tall Buildings and Urban Habitat said its Height Committee ruled the mast atop the building, built on the site of the attacks of September 11, 2001 and formerly called the Freedom Tower, was a spire and therefore a permanent feature.

If the mast were considered an antenna instead, it would be considered functional equipment and subject to change.

With the spire, One World Trade Centre reaches 541 metres compared with 442 metres for the Willis Tower, formerly known as the Sears Tower. Counting its antenna, the Willis Tower reaches 527 metres.

The Port Authority of New York and New Jersey, the public entity that owns the building, changed the name in 2009, saying it would make the office tower more marketable for tenants.

The council’s Height Committee convened a group of 25 architects, engineers and façade consultants from around the world on November 8 to consider the issue, and ultimately decided the mast is a spire, the council said.

Because One World Trade Centre is still incomplete, its designation will become official once the building is occupied in 2014

Because One World Trade Centre is still incomplete, its designation will become official once the building is occupied in 2014, the council said. It is one of four skyscrapers planned at the site alongside a memorial, a museum, a transit centre and a performing arts centre.

It would then become the third-highest building in the world after Dubai’s Burj Khalifa at 828 metres and the Makkah Royal Clock Tower in Mecca, Saudi Arabia, at 601 metres.

However, four other buildings under construction in China and one in South Korea would be taller than One World Trade Centre, according to Emporis, a database for building information.