What happened one night under the same roof as the 28th Ward alderman’s office did not stay in the 28th Ward.

So Ald. Jason Ervin has joined the ranks of politicians who’ve learned the hard way that it’s easy for camera phones and social media to take private moments and make them very public.

Toronto Mayor Rob Ford has become the world’s most ridiculed municipal elected official thanks to cellphone recordings, including images of him smoking crack with criminals.

Depending on your views of exotic dancing, you could argue that Ervin has done nothing remotely as revolting as Ford’s best secretly recorded behavior, much less anything illegal.

Still, Ervin had to spend much of Monday trying to explain away two minutes and 23 seconds that were captured at his November 2012 bachelor party and surfaced on Facebook over the weekend.

The revelry — at least two strippers, gyrating in the most suggestive manner possible — took place in the same West Side building where Ervin handles constituent city-service requests. Near the end of the video, a woman tells the man who’s recording the scene to stop.

“Baby, this ain’t nothin’ but silhouette,” the man replies.

“Don’t do that because they will stop their show, baby,” the woman insists, adding that the video could turn up “on the Internet.”

Sure enough, the clip popped up — and disappeared quickly — over the weekend on the Facebook page of a self-styled “grass-roots” political activist named Mark Carter. Carter’s social-media headline was “Women for Ervin in 2015,” when he’s expected to stand for re-election.

Monday at City Hall, the alderman declined to tell Chicago Sun-Times reporter Fran Spielman where the video was recorded because “it was a private event at a private place . . . It has nothing to do with my official capacity.”

The party that began at 9 p.m. on Nov. 10, 2012, took place on the 3rd floor of 2602 W. 16th St., according to the text-message invitations Ervin sent from his own phone. That’s two stories up from Ervin’s ground-level ward office and the headquarters of his 28th Ward Democratic organization.

Told that, Ervin soon sent an e-mail statement saying the event involved friends and family and happened on “another floor of the building, where neither city nor political funds are spent.”

Ervin’s office did not reply to a request for a copy of his lease. The building owner could not be reached Monday, so it wasn’t immediately clear who, if anybody, was the rent-paying 3rd-floor tenant at the time of Ervin’s party.

The building is in the heart of the sprawling Cinescape film studio complex. What happened at the party was better suited to late-night programming on Cinemax rather than prime-time shows like “Chicago Fire.”

Billed in the invitation as “Jason’s Last Stand Part II,” the event was to rival the bachelor party before his first wedding in 2000, Ervin promised.

The room was darkly lit, but the alderman clearly ambles into view at about the 75-second mark of the video. He takes a seat and leans back with his hands behind his head. It gives him a perfect perspective on a pair of curvaceous women as they bend down in front of other men and rhythmically shake their behinds to loud music.

I met the man who says he shot the video at a soul food restaurant on Monday. He acknowledged “sour grapes” toward Ervin for not giving him a city job but told me he had not wanted the video to become public and didn’t want to be identified.

Ervin said he believed someone he would not name had betrayed him. But he added, “I apologize to the residents of the 28th Ward for the attention that this has brought.”

And he could not help but conclude in true Rob Ford fashion — invoking the spouse he wed a few days after the party: “My wife and I are committed to continued service of our community, and we shall move forward.”

Even if the party had been held in Vegas, there’s no guarantee what happened would have stayed there, rather than winding its way back to the West Side via Facebook. In any case, and in, well, hindsight, Ervin must certainly wish he would have kept the strippers much further from his 28th Ward office, at least as far as away as Harvey or the Wisconsin Dells.