Most nights, Roy Scott Jr. sleeps in a wheelchair, with his left arm draped over a blue scaffolding pipe, his graying curls shielded from rain by the rusting planks overhead. This is not the reason that the tunnel-like wood-and-steel structures called sidewalk sheds shroud walkways across New York City. They are supposed to catch debris from repairs overhead.

But sheltering Mr. Scott and his fellow transients has become the raison d’être for the sidewalk shed at the corner of Lenox Avenue and 123rd Street in Harlem.

This month it is entering its second decade. The moldering structure’s first 10 years, neighbors say, passed without ever being tested by significant construction overhead.

With its murky corners and tiers of blue piping, the shed has become a jungle gym for strapping men and a hideaway for drug deals. Evenings feature camaraderie among street friends, occasional outdoor sex and the usual neighborhood drama, Mr. Scott said last week, adding that it can sometimes escalate into brawls. A few steps away from the curb that the group uses as a bathroom, young families try to dash into their brownstones without catching a whiff — an impossible task on stifling summer days.