Correction: An earlier version of this story misstated the weight of the dome’s cast iron. It is 9 million pounds, not tons. The story has been corrected.

When the U.S. Capitol dome was built in the 1860s, cast iron was the high-tech building material of its day, lighter and easier to erect than stone, more fireproof than wood. But if the people’s business never stops below, neither do the wind and rain above — and now, after 150 years of duty as Washington’s all-weather symbol of democracy, the dome is getting an overdue metal makeover.

Preparations for the long-planned $60 million restoration project got underway this week as officials suspended tours of the upper structure and prepared to string protective netting in the vaulted — and vaunted — space above the Rotunda. Capitol architects took reporters for a final climb to show where leaks coming through the battered dome are beginning to threaten the historic interior. Some massive metal parts have already been removed to keep them from plunging to the always-crowded floor below.

“It’s like having a bridge as the roof of your building,” Kevin Hildebrand, of the Architect of the Capitol’s office, said Thursday as he climbed through the web of girders and braces lacing the dome’s upper reaches. The space is like a concave Eiffel Tower sandwiched between two sloping sheets of metal. On some of the softball-size bolts, the name “M.C. Meigs” can still be seen beneath multiple coats of paint, a bit of stamped graffiti from the legendary Army captain who supervised the building of the dome.

“Today, this would never be made of cast iron,” Hildebrand said. “It would probably be made of steel and glass. It’s an archaic material. But it is the symbol of our country. It’s an icon that has to preserved.”

Access to the Rotunda floor will continue mostly uninterrupted (for a few weeks in February, a covered walkway will be installed during some dicey aerial work above).

In the spring, a towering web of scaffolding will encase the entire top of the 288-foot edifice, transforming the skyline of the Mall for at least two years. LED lights attached to the scaffolding will provide some nighttime visual interest, however. And the recent repairs of the Washington Monument, which was damaged in the 2011 earthquake, showed that tourists can take these national “excuse-our-dust” periods in stride. Some residents even petitioned the White House to make the monument’s temporary lights permanent.

“I’m just glad they are taking care of [the dome] — it’s so beautiful,” said Janice Bradley of Pennsylvania, who was touring the Capitol during a holiday visit. “I just wish they would fix our roads.”

Moviemakers may be more put out, according to John Latenser, a local location scout who has set up shots of the Capitol for “The West Wing,” “Veep” and other productions.

“It’s one of the shots that tell you you’re in D.C.,” he said. “It will change some scripts, change some angles. Some people may try to take out the scaffolding” in post-production.

Workers will deploy a range of newly developed techniques and materials to fix more than 1,000 cracks in the 9 million pounds of cast iron, almost every piece of which was cast in New York and shipped to Capitol Hill during the Civil War.

The work famously continued during the war, which struck an inevitably symbolic chord as the republic struggled to endure during the conflict. It might have also been a rare triumph of government procurement: According to architectural historian William Allen, the contractors couldn’t afford to let the 1.3 million pounds of iron already on the site go to waste.

“It was under contract, so the work was able to continue,” Hildebrand said.

In preparation for the restoration, Architect of the Capitol workers traveled to see recent repairs done on the aged statehouses of New Jersey and Ohio. For painting tips, they inspected the Chesapeake Bay Bridge.

Tests have shown that the old iron of the Capitol dome is too soft for welding in most places and too brittle for standard bolts. So technicians will “stitch” the cracks closed with pins specially designed by a fabricator in California. Other sections will be replaced altogether.

The great contoured sheets of iron that form the famous exterior will be sandblasted to bare metal, with tons of 1950s-era lead paint carefully contained and removed. That process will be a race against time — and rust.

“They will have only about eight hours to set the primer,” said Hildebrand. He spoke on a narrow outside catwalk just below the cupola and the Statue of Freedom that caps the dome. The view of Washington below, on a clear sunny afternoon, was spectacular.

It was up here three years ago that a worker, rappelling down the dome’s face during repairs of the cupola, noticed that one of the decorative iron acorns on the exterior had deteriorated nearly to the point of falling off. And that was only the latest warning sign: After a rainstorm in 1990, inspectors were tracing the origins of a major puddle on the Rotunda floor when they noticed that birds had nested throughout the upper gutter systems. They’ve been patching cracks ever since.

“It became clear there were major problems,” Hildebrand said, pointing out ominous dark stains spreading along the colonnade above the Rotunda, just yards from the base of the huge fresco that covers the ceiling. “We don’t want it to get to this level of damage again.”