“Raising Sand,” the 2007 duet album by Robert Plant and Alison Krauss, started as an experiment, a modest side project for two longtime bandleaders to revisit old and recent songs. It was a hushed, long-breathed album with a haunted twang, yet it turned into a blockbuster — selling more than a million copies and winning five Grammy Awards including album of the year.

A follow-up would have seemed like an obvious next step. Yet it has taken 14 years for the arrival of that sequel: “Raise the Roof,” due Nov. 19.

“Raise the Roof” almost magically reclaims the spectral tone of “Raising Sand,” then finds ways to expand on it, delving further into both quiet subtleties and wailing intensity. “It’s a little bit more smoky, a little bit more lustrous than the first record,” Plant, 73, said by phone from his home in western England.

“It’s definitely different, even though it might be coming out of the same sort of crevasse, the same fork in the landscape of our musical lives. It has a mood to it, which is laced with time and with the actual age and maturity of the songs themselves.”

But the musicians needed a decade of reflection between albums. “If we had thought we knew what we were doing in the first place, we could probably have repeated it,” T Bone Burnett, 73, the producer and linchpin of both albums, said by telephone from Nashville. “But we didn’t. At the time, we were just kind of goofing off, having fun. And that’s what we were up against. We’ve been waiting for it to get to that point where we could just have fun doing it again.”

Plant and Krauss were an unlikely pairing from the start. “We were from two radically different worlds,” Plant said. He was the world-conquering, musically restless rock singer who had fronted Led Zeppelin. Krauss was already a luminary in the more close-knit world of bluegrass and Americana, leading the string band Union Station.

They were also strikingly disparate singers, with contrasting musical instincts. Krauss, 50, grew up harmonizing in bluegrass groups, figuring out and delivering restrained, precise, locked-in ensemble parts. “I’m a regimented-type singer,” she said. “Bluegrass people sing things very consistently, because there’s three parts going on most times. And if someone pulls around and goes and does something different, now the other two want to run you over with their car.”

Plant was used to a lead singer’s free rein; he would improvise with every take. “I try to sing across the beat quite a bit,” he said. “If it’s a straightforward groove, I like to bounce across the left and right of the groove. I did it in Zeppelin. I kind of scuttle it, accelerate it, slow it down.” He chuckled. “It drives them mad.”

Krauss grew to appreciate their differences. “It makes you feel like you’re hanging off the edge of a cliff,” she said by telephone from her home in Nashville. “It is so exciting and so magnificent.” Plant and Krauss first sang together as part of a 2004 tribute to Lead Belly, and Plant proposed that they try recording together when their schedules aligned; that took more than a year. Plant initially suggested trying just three days in the studio to see if anything worked out.

They enlisted Burnett, who had recently reimagined old-timey Appalachian music for the soundtrack to “O Brother, Where Art Thou?,” which featured Union Station. For “Raising Sand,” the three gathered songs — mostly about tragic lost loves — and transfigured them with close harmonies and an aura of suspended time. Burnett’s studio band let tempos hover and undulate; Plant and Krauss discovered how uncannily their voices could fit together.

“A funny thing happens with them,” Burnett said. “When the two of them sing, it creates a third voice, a third part in their harmonies when there are only two parts. You know, one plus one equals two unless you’re counting, say, drops of rain. Then one plus one could equal one, or one plus one could equal a fine mist. Their voices are in that relative space where they sing together and it creates a fine mist.”

“Raising Sand” was an otherworldly alternative to virtually all of its pop contemporaries (its competition at the Grammys included Lil Wayne’s “Tha Carter III” and Ne-Yo’s “Year of the Gentleman”), and although it was released on the folky independent label Rounder, eager listeners sought it out. It reached No. 2 on the Billboard 200 album chart.

Plant already had another project underway in 2007: the arena-sized last hurrah of Led Zeppelin that December. But the Led Zeppelin performance was an endpoint, while “Raising Sand” was a new beginning. Plant and Krauss toured for much of the next year, with concert sets that included some revamped Led Zeppelin songs. They plan to tour again in 2022.

“We’ve got a kind of a personality which we could pursue as two singers, a neat place that we made for ourselves,” Plant said. “I just liked the idea of actually singing together throughout an entire show, more or less with somebody. Concentrating, listening, being free-form at times. Letting it rip, then being pretty controlled and organized and following instructions from her. And then, sometimes, letting go so she can’t catch me.”

Yet having a hit album also brought self-consciousness and pressure. Plant and Krauss tried recording new duets with their touring band just after their Grammy sweep in 2009, but scrapped those sessions. “Nothing happened that was really horrible,” Krauss said. “We just felt like it was too much at once.”

They then returned to their own bands and projects: Krauss with Union Station; Plant leading his Americana-rooted Band of Joy and then, for much of the 2010s, the psychedelia-, trip-hop- and world-music-infused Sensational Space Shifters. “We really enjoyed the fact that we have no idea about our corresponding alternative lives,” Plant said.

Still, the “Raising Sand” collaborators stayed in touch. “We’ve been sending songs back and forth for almost 14 years, trying to figure out how to continue,” Burnett said.

Finally, in 2019, they regrouped. A decade of other work had made the sequel less fraught although “there was a little bit of trepidation on my part,” Plant said. “I wasn’t sure whether we could reinvoke what we had. But it was very short-lived, that question of whether or not it was real. It was like, I bow to her, and she curtsies to me, and we see what we can do.”

They went back to the venerable Nashville studio, Sound Emporium, where they had recorded “Raising Sand,” and where Burnett and Krauss have frequently recorded since. (Plant returned there this year, he said, for sessions with the 1950s guitar titans Duane Eddy and James Burton.)

The core rhythm section from “Raising Sand,” Jay Bellerose on drums and Dennis Crouch on bass, had continued to work with Burnett and returned for the new album. They were joined by an expanded assortment of guitarists including Marc Ribot; Bill Frisell; David Hidalgo from Los Lobos; and Buddy Miller, a Nashville stalwart who was in Plant’s Band of Joy. A few songs added collectors’ item string instruments like a Marxophone and a dolceola, both zithers played with keyboards: tinkling, evocative, echoey, unexpected timbres. Plant and Krauss finished recording in February 2020, just before the pandemic lockdowns.

“Raise the Roof” opens with a song from the Arizona band Calexico, “Quattro (World Drifts In),” which is filled with images of desolation, escape and war, perhaps conjuring Afghanistan: “No choice but to run to the mountains where no poppies grow/You have to hit the ground running.”

While most of the other songs on “Raise the Roof” ponder love, separation and longing, the album has a discreet through line. “As we were going through the material,” Burnett said, “it was clear that a story was being told concerning a man, a woman and war. And it became clear which songs fit and the sequence they went in.”

The collaborators returned to some of the songwriters from “Raising Sand,” picking up the Everly Brothers’ “The Price of Love” and the Allen Toussaint song “Trouble With My Lover,” which was recorded by Betty Harris. And as on “Raising Sand,” they remade tracks that started as blues, old-timey, soul, country, gospel and rock.

Their versions are far removed from the originals, often close to inside-out. Most often, Plant said, “We have a kind of languid, sometimes pensive sound, with the pathos of the original song taken into another place.”

They stripped songs down to just lyrics and melodies, and rebuilt them intuitively in the studio, often around sparse, subtle beats from Bellerose. They shifted “Trouble With My Lover” from a major to a minor key, and Krauss trades Harris’s New Orleans soul resilience for a neo-Appalachian plaint, lingering over the song’s loneliness and hints of betrayal.

Krauss chose “Going Where the Lonely Go,” a doleful ballad that Merle Haggard released in the 1980s. Plant seized the chance to record a soul song he had been singing since his teens: “Searching for My Love,” by Bobby Moore & the Rhythm Aces. He also brought material from Britain’s 1960s folk revival: Bert Jansch’s stoically intransigent “It Don’t Bother Me,” which brings out Krauss’s defiant streak; and Anne Briggs’s “Go Your Way,” a wife’s troubled farewell song to a soldier she may never see again.

At one of the album’s extremes, Plant unleashes his Led Zeppelin wail and echoes of “Kashmir” in “High and Lonesome,” a song that grew out of a studio jam session. Burnett and the rhythm section were toying with a Bo Diddley beat. Plant happened to have his book of potential lyrics with him. The title is a tongue-in-cheek country cliché; the song is not. It is equally biblical and bluesy, wondering, “If I should lose my soul, would you still care for me?”

At the other end of the dynamic scale is “The Price of Love.” The Everly Brothers’ own version is an exuberant two-minute, harmonica-topped stomp, though they’re singing about a cheater’s bitter regrets. Plant, Krauss and Burnett took the song down to half-speed and removed any distractions. The track opens with half a minute of near-ambience as instruments quietly drop in: a bowed bass drone, shakers, a distant fiddle, eventually a few guitar notes before the beat and chords solidify and Krauss arrives like an accusatory wraith: “You won’t forget her,” she warns. By taking their time, they concentrate the essence of the song. And as they did with “Raising Sand,” they calmly defy the impatience of 21st-century pop.

The song “kind of forms before your ears,” Plant said. “When people stick stuff on the radio now, I think you’re allowed like 16 seconds or even less before you’re actually hitting a chorus. But then again, we’re fishing in a different pool. In fact, we’re not even fishing. We’re just trying to swim.”