The errant antiaircraft round gouged the building in 1942. The gold Mylar confetti fluttered to the roof during the year 2000 festivities. The cracks in the huge marble blocks formed when the 2011 earthquake shook the massive structure.

For almost a century the timeless Lincoln Memorial has stood exposed to the sun, wind and rain as Washington changed around it and marked it with evidence of the passage of time.

Now it is getting a $25 million top-to-bottom makeover, and Thursday the National Park Service provided a glimpse of how the project is going and showed some of the work that has to be done.

Park Service experts trooped up a narrow internal stairway and then scaled several levels of construction scaffolding to reach the windy roof of the memorial and the “penthouse,” a smaller structure that sits atop the memorial.

They showed where the Aug. 23, 2011, magnitude-5.8 earthquake had nudged some of the giant blocks of Colorado Yule marble out of place, caused long, jagged cracks and broke off pieces of the stone.

Much of the work underway now is aimed at replacing the failing, almost 30-year-old roof. When the old roof slates were removed, workers discovered numerous bits of gold confetti from the 2000 celebration marking the millennium.

“There were just hundreds and hundreds of these,” said Park Service spokesman Mike Litterst, noting that they were fired from some sort of confetti cannon.

Extensive work also is planned on the earthquake-damaged stone and deteriorated mortar between the blocks. Litterst also pointed out a baseball-size gouge in the marble of the penthouse’s ornate outer wall that was made by an errant antiaircraft bullet in 1942.

It was in the midst of World War II. The gun was set up near a local bridge to defend the city against air attack. And the gunner accidentally pulled the trigger, hitting the memorial’s east side.

“You don’t have to stretch it too much to say the Lincoln Memorial’s one of the few buildings in Washington, D.C., that came under attack in World War II,” he said.

From “friendly fire” no less, he said.

The hole had been patched before, but the fix fell out. There are no plans to patch again.

The project is probably the biggest overhaul of the building since the structure was dedicated 96 years ago, officials have said. It is being funded largely through an $18 million donation from billionaire philanthropist David Rubenstein.

Much of the rest of the work will take place inside, beneath the massive chamber that holds the statue.

The current exhibit space will be greatly expanded, along with the memorial’s tiny bookstore and antiquated restrooms. New exhibits will be added, and visitors will be able to see the massive pilings and foundation that support the memorial in its undercroft.

The project began in 2016, and the Park Service would like it finished for the memorial’s 2022 centennial.

The memorial, which attracts 7 million visitors a year, will remain open during the work, although parts of it may be closed off from time to time.

The marble-columned edifice, which houses the 120-ton statue of a seated and contemplative Abraham Lincoln, is one of the most elegant and hallowed memorials in the ­country.

Construction began in 1914. It was dedicated May 30, 1922, in the presence of Lincoln’s 78-year-old son, Robert Lincoln.

The 38,000-ton building sits on a man-made eminence vulnerable to freeze-thaw cycles and open to the elements, including wind off the adjacent Potomac River.

It was built on fill dredged from the Potomac River. It is supported by huge concrete pilings sunk 44 to 65 feet through the fill down to bedrock.

“It’s a pretty punishing environment,” said Justine Bello, a Park Service architectural conservator. “It’s the highest thing around here. It’s not getting screened by trees . . . or other buildings. For better or worse, it’s a very prominent spot.”

An initial task was fixing the old roof, which had been installed in 1990 and was failing, said Audrey T. Tepper, a historical architect with the Park Service.

“It should have lasted longer than it did,” she said. “And the key to a healthy building is having a tight roof. Once the roof fails, everything else fails.”

“And this isn’t just any building,” she said. “This is a building that is very, very important to Washington, to the country, to the world.”

Much of the old roof has now been removed, and the new one is about to be installed. The problems, which led to poor drainage, have been discovered and will be addressed in the new roof, she said.

As for the marble blocks around the roof, all of the mortar will be replaced, Bello said.

“That is every joint between every stone that you see here,” she said as she stood on the roof Thursday.

The cracks and missing pieces of stone from the earthquake are mostly at the corners, where the structure is most exposed to the weather. They will all be repaired.

Pins will be installed to stabilize the stone.

“We don’t want to allow any further movement,” Bello said. “We can accept that the movement happened, but we don’t want it to move anymore. . . . Holes will be drilled, and stainless-steel rods will be inserted with an epoxy adhesive.”

The quake also moved several of the blocks out of alignment.

Some of the discoloration in the stone stems from natural elements in the marble that have emerged over time, and little can be done about that, Bello said.

And after all these years, Litterst said, “the warranty’s up. . . . We signed off on it.”