With five months to completion, U.S. Bank Stadium has sprung a leak.

M.A. Mortenson Co. executive John Wood announced Friday that the gutter on the $1.1 billion building was leaky and needed about $4 million in repairs.

Last fall, workers noticed dampness on the parapet wall and some pooling of water in the gutter, but the water had yet to seep inside, Wood said.

"We're happy they found it now," Wood said. "Stuff happens on projects."

The moisture was on the inner wall of the snow gutter atop the building. The leak didn't affect the building's signature clear plastic roof, which has held up through the winter.

The replacement work will affect about 30 percent of the facade, most noticeably on the large eastern face where the black panels will need to be taken down. In doing so, workers also will temporarily remove the white U.S. Bank Stadium logo, Wood said.

As many homeowners know, finding the source and path of leaks is tricky. Wood said it took months to diagnose the problem and determine a suitable fix.

A faulty vapor barrier was found to be the problem, Wood said. In coming weeks, the barriers will be removed and replaced with a different material. The barriers had yet to be installed on the other three sides of the building — so they don't require replacement. The new material will be used as they are installed.

The additional cost doesn't affect the Vikings, the taxpayers' share or the Minnesota Sports Facilities Authority (MSFA); Wood said Mortenson and the design team would pay for the repairs.

The building is still expected to open in August in time for a Luke Bryan concert and the 2016 NFL season.

Wood made the announcement at a regularly scheduled MSFA meeting, although it was clear insiders knew about the issue already.

The mishap carried echoes of the Metrodome roof collapse in 2010, when a heavy snow punctured the Dome's roof and poured onto the field, an incident captured on video and widely viewed.

The project partners have said no such thing will happen at U.S. Bank Stadium because unlike the air-supported Metrodome roof, the new stadium has a steel superstructure and multiple compartments of plastic.

Rochelle Olson • 612-673-1747

Twitter: @rochelleolson