One day in 2004, workmen arrived at the corner of West 123rd Street and Lenox Avenue in Harlem to erect a sidewalk shed, one of those unsightly steel-and-wooden structures that pop up any time a building is under construction. Twelve years later, it's still there.

"It's completely demoralizing," said Laurent Delly, a real estate broker with a master's degree in civil engineering who lives down the street from the shed. Sidewalk sheds and the scaffolding that usually sits atop them are intended to protect pedestrians from falling bricks and debris. But the chief purpose of this particular shed, on a handsome block in the Mount Morris Park Historic District, seems to be to collect garbage and provide shelter to the loiterers who lurk underneath. Repairs at the building it surrounds move at a glacial pace.

Delly has complained over and over to the city, elected officials and building inspectors, who have fined the owner for numerous violations. He contacted The New York Times, which wrote a feature on it two years ago. Still, the shed remains.

"This is a beautiful block, and people here care about our streets," Delly said. "Do you think this would be allowed to happen if we lived on the corner of Park Avenue and 72nd Street?"

Plenty of New Yorkers can understand his frustration. Across the city, sidewalk sheds and scaffolding spread like kudzu. They devour precious sidewalk space, cut off sunlight, create safety hazards and hurt businesses. There are now nearly 9,000 sheds entombing city streets, according to the Department of Buildings, up from about 3,500 in 2003. That's 190 miles worth of sheds, or 1 million linear feet—equal to the distance between Gansevoort Street in the West Village and the hamlet of Gansevoort in upstate Saratoga County.

"New York is insatiable right now when it comes to sheds," said George Mihalko, a shed-equipment supplier who said his biggest challenge is finding inventory to fill the five 25-foot-long truckfuls of steel pipes and beams that he sends out daily from his North Bergen, N.J., warehouse. "I've never seen anything like it in 30 years."

The unprecedented demand is driven in part by the new wave of construction fueled by the city's robust economy. But there's another, more important reason: Thirty-six years ago, the city passed a law requiring regular inspections of older buildings to ensure concrete and bricks don't fall on pedestrians. And since then, the City Council has strengthened the law while adding new ones, giving rise to an industry that generates $1 billion a year—$200 million of that is for the street-level sheds, and the rest pays for the scaffolding and the workers who repair the façades.

One of the shed boom's biggest winners is William Laffey, a 36-year-old native of Bellerose, Queens, who, after graduating from Albany's Siena College in 2001, started out managing the tool inventory at a Home Depot. He is now president of Spring Scaffolding, a Long Island City-based firm that builds more sheds than anyone else, with 583 standing around the city, according to Buildings Department records. "I wouldn't say this is recession-proof, but someone always needs scaffolding," Laffey said.

Of course, what some might call boom times for sheds would be described by others as an epidemic.

"Sheds are awful and everywhere," lamented Dan Biederman, president of the 34th Street Partnership, a midtown business district where sheds cover about 20% of the sidewalk space.

"So many things about the city have improved, yet sheds are the same gloomy thing they've been for more than 40 years," groaned Stephen Varone, president of Rand Engineering & Architecture.