A bulky couch. A narrow stairwell. A sprinkler head.

Put them together and what do you get?

Millions of dollars in property damage. And an insurance company that refuses to pay because the incident — couch vs. sprinkler head — happened inside a luxury downtown condominium, and not on the street.

This sorry, soggy tale is playing out at the Textile Building — at 66 Leonard St. in Tribeca — according to a current lawsuit.

The case hinges on the definition of “loading and unloading” — and whether the moving truck was being loaded while the errant couch was inside, still upstairs.

The stately, 14-story edifice — an office building converted to condos in 2001 — holds apartments that sell in the $3 million to $5 million range.

Dating from around 1900, the building was home to two recently deceased celebrities: Sylvia Weinstock, the cake designer, and Toni Morrison, the novelist. A current $4.95 million listing has nearly 3,000 square feet, and monthly common charges and taxes of more than $10,000.

The headache began a year and a half ago, when someone on the sixth floor was selling a couch through Furnishare, also known as Kaiyo — an online marketplace for high-quality used furniture.

The company promotes its convenient and environmentally friendly process, which keeps furniture out of landfills. Furnishare picks up the furniture, cleans it, stores it in its warehouse, sells it on consignment and delivers it to its new home.

The service eliminates the pain point of buying and selling furniture — the hassle of transporting big, bulky items.

“We do all the heavy lifting for you so that good furniture doesn’t go to waste,” the website says.

That day, Furnishare’s movers were removing a couch from the sixth-floor unit — a 2,300-square-foot $4 million duplex with three bedrooms and four bathrooms — to take it downstairs to its truck, which was “waiting off-premises, adjacent to the city sidewalk,” the lawsuit says.

The couch never made it.

The movers “began to descend the staircase with the couch,” according to court papers filed by Furnishare’s lawyer, Elizabeth Morris of Latham & Watkins, who declined to talk to The Post. “At that point, the couch accidentally struck an exposed sprinkler head, resulting in a substantial release of water.”

The stairs were flooded. Millions of dollars’ worth of damage ensued: ceilings split, floors buckled, mold spread.

Three months later, emergency mold remediation began in the hallways and stairwells. The estimated repair cost was $400,000. The work took eight months, according to permits from the Department of Buildings.

Several apartments on lower floors also sustained severe water damage — more than $500,000 each.

Furnishare had adequate insurance — or so they thought. Instead, their insurance company, Travelers, is refusing to pay, saying the incident occurred during “loading and unloading” the truck — and that the policy excludes such loading and unloading activities.

The insurance policy defines “loading and unloading” in ambiguous and hard-to-interpret terms. It means “the handling of property . . . after it is moved from the place where it is accepted for movement into or onto” a vehicle.

“An accident in a sixth-floor stairwell does not arise in the course of ‘loading or unloading’ a truck parked outside the building, across the sidewalk on the curb of a public way,” say the court papers filed on behalf of Furnishare. “This is common sense; and just as important, it is the law of New York.”

Nor does loading extend to “acts in preparation for loading,” the court papers say.

Furthermore, “there was no motor vehicle parked in the sixth-floor stairwell.”

Furnishare had a separate policy with State Farm to cover its trucks. But State Farm won’t pay, either, declaring that no motor vehicle was involved in the incident.

“The overarching question presented by this case is whether . . . an accident in an internal stairwell occurs during the loading of a vehicle,” writes Furnishare’s lawyer.

She cites a precedent from 20 years ago, involving a wheelchair patient being transported down stairs to a waiting ambulette. The wheelchair’s brakes failed in the stairwell; the patient fell and was injured. The court decided the situation did not fit the category of “loading or unloading.”

Both incidents were “removed in space and time from the sidewalk where any loading onto a vehicle would later occur,” the court papers say.

“Furnishare paid substantial premiums” for insurance “to protect against the risks of running a furniture moving business, including the risk of causing third-party damage during moves,” says the lawsuit.

Travelers’ lawyer, Amy S. Gross, did not respond to messages, nor did representatives of Founder Shield, the company that sold the Travelers policy, or Baldwin Risk Partners, owner of Founder Shield.

As for the couch? It was “irreparably damaged.”