KeyArena will begin reattaching its roof by late February. Then we might know a completion date.

KeyArena’s 44-million-pound roof now is completely suspended by dozens of temporary posts and should start being reconnected to the foundation by late February of what’s to be a completely rebuilt venue.

That was the word from the media tour Tuesday of the construction site, where digging continues beneath the demolished structure on what will eventually be a $930 million rebuild into a modern NHL and NBA-caliber arena. Ken Johnsen, construction executive for the project developer, the Oak View Group (OVG), said concrete footings were to begin being poured Tuesday ahead of reattaching the foundation and roof and that he should have a more accurate completion date target by late April or early May.

“I think it’s safe to say this is really unique,’’ Johnsen said of the project, due for completion by summer 2021 and ahead of an NHL Seattle expansion team launch. “This is a one-of-a-kind of this size of a roof, this weight of a roof, being held in place while building what we’re building underneath it.

“I think that means for everybody working on it, it’s interesting, it’s challenging and it’s something that they’re going to remember for a long time.’’

The roof, designed by Northwest architect Paul Thiry ahead of the arena being built for the 1962 World’s Fair, was designated for historical preservation. That’s resulted in months of work to keep it suspended intact above the construction zone, where the arena has been demolished while workers dig down an extra 15 feet — reaching more than 50 feet below ground level — to expand the venue’s new walls farther outward.

By the time that’s done, the new arena beneath the old roof will be roughly 800,000 square feet compared to its prior 368,000.

First, though, crews must finish excavating some 600,000 cubic yards of soil to create the new venue’s foundation. Johnsen said they’ve “gotten lucky with the weather’’ starting in November and have averaged about 7,000 cubic yards of removal per day instead of the 6,000 previously envisioned.

“We’ll be done with that by the end of February,’’ he said.

But digging already has been completed in the northwest corner of the site, where new foundation began being drilled Jan. 5.

Greg Huber, project executive for lead contractor Mortenson, said permanent steel posts will then be built back up from that foundation piece by piece and eventually reattached to the roof.

“It takes about a month,’’ Huber said. “But we’ll have several of them being worked on at the same time.’’

The privately funded project, originally pegged at $600 million, has seen costs soar due to its complexity as well as upgrades ordered by OVG and NHL Seattle. Timing for completion remains an issue, with the groups hoping to hold  the NHL draft and expansion draft there in late June 2021 ahead of a debut season four months later.

The Seattle Storm also needs to be in the arena by June 2021 for the start of its season. Johnsen said it will take about a year to build up the arena — barring unforeseen circumstances — once the roof is reconnected by spring.

“On a project like this, everyday there’s challenge,’’ Johnsen said. “The building was built well and the roof was built well. And I think that’s helping us out with how we’re saving it. It wasn’t like it was overly fragile. It was pretty robust. And that was a pleasant — not surprise, but confirmation.’’