Ken Burns is used to telling stories that have been told before. That comes with the territory when you work on documentaries covering momentous points in history, including the Civil War, World War II and the Prohibition era. But for his new seven-part, 14-hour documentary "The Roosevelts: An Intimate History," Burns and his longtime collaborator Geoffrey C. Ward have managed to find new material on Theodore, Franklin and Eleanor Roosevelt that many people have never seen.

"We had finished our film, it was long done and mastered, and we discovered four seconds of film through the University of Pennsylvania. At great expense, we cut that back into the film," Burns told the press at a discussion on Tuesday, PBS' day at the TCA summer press tour.

The footage provides a very rare glimpse of the polio-stricken FDR near the end of his life, struggling to disembark from a train in Bismarck, N.D. Though FDR's condition was not a secret, news crews at the time were regularly asked to turn off their cameras so as to avoid showing the president in his weakened state. However, bits and pieces, usually edited from newsreels, still survive, and it's those unseen bits that Burns was able to put into the film.

"We gathered extras that slipped through the net," Burns said, who included the footage along with many personal letters written by FDR that provide more insight into the personality of someone the filmmakers calls a "deviously opaque human being."

The film attempts for the first time to intertwine the lives of all three Roosevelts into one family story. Although each has received much individual attention from biographers over the years, Burns and Ward, who once corresponded with Eleanor when he was 11 years old, look at the family dynamics of this upper-crust family who had the ability to identify with the common man.

"They talked about each other all the time," Ward said. "They sometimes resented each other."

But though the film is subtitled "An Intimate History," Burns and Ward debated constantly about how much of their very private personal lives should be included.

"We debated [using the word] intimate," Burns said. "Because it suggested a movement over to the tabloid."

For instance, though there is much conjecture in the field of scholars studying the life of Eleanor Roosevelt as to whether or not she had bisexual relationships with people in her social circle, Burns and Ward chose not to speculate and present the facts of her life as presented through her personal correspondence.

But one thing they didn't debate was how much to show just how physically debilitated FDR was while president.

"Anyone who is handicapped ought to know how hard that man worked," Ward said.

Unfortunately, Burns pointed out that though the media of the time managed to turn a blind eye to the health shortcomings of both Teddy and Franklin Roosevelt, the modern media would not be as forgiving.

"In this culture where we're constantly tweeting and observing and watching ... Theodore and Franklin and Abraham Lincoln and Meriwether Lewis sort of couldn't get elected dog catcher today," Burns said.

The film premieres on PBS on Sept. 14.

Follow me on Twitter: @patrickkevinday