But they never look up, above the eyeline, to the silhouetted turrets and rooftops that frame their own city. Business is business, even for the beggars, prostitutes, or elderly women selling just-born kittens, and business takes place at ground zero.
If they did lift their gaze at the right time of night, as the light fades to a cobalt blue for an hour or two, they might glimpse a small group of foreigners, backpacks and cameras over their shoulders, earnestly picking a path among the chimneys as silently as possible. The rooftop tourism season is in full flood in St Petersburg, unknown to its four million residents: except, perhaps, those who hear the footsteps overhead.
Its chief advocate is Peter Kozyrev, a 26-year-old Russian freelance journalist and - a rare breed this, in a country whose population until recently could not usually go overseas - an avid and widely travelled backpacker. He never advertises his services as a roof guide, apart from distributing fliers in English at the international youth hostel. He never gives interviews to the Russian media. The essence of successfully roaming the roofs is to be as discreet, and low-key, as possible.
Being discreet, however, is not easy when you are clambering up a steel ladder in darkness but for the beam of Peter's small torch. Moments earlier, we had ducked (guiltily, I thought) into an archway, and walked up a dark six-storey staircase inside a damp-blistered, pee-reeking apartment block. Our destination was one of his favourite spots, a small, crumbling brick turret on a roof just off Nevski Prospect, the city's Oxford Street. It commands a view that sweeps from the port on the Gulf of Finland to the church that marks the spot where Tsar Alexander II was assassinated by a bomber, and beyond. "Shhhhh," Peter whispered, as I tripped over a pipe. "We don't want to piss off the neighbours." He patted a wall as we were passing through a dust-caked attic. "There are people living right behind here."
When we got there, the view was marvellous. "Once you are up here, above everything, it's all worth it," he said, eyes shining. "It is a feeling that you cannot get anywhere else in St Petersburg. You see, there are no skyscrapers here, and no hills." He calls his tours an "urban version of mountaineering".
St Petersburg has plenty of official guides, bustling women who used to go from museum to museum spouting history of behalf of the KGB-controlled Intourist agency, and have carried on ever since, though under new management. (It is not hard to imagine how their former bosses would have reacted to the idea of Western tourists creeping about the roofs). Peter Kozyrev is not one of these. He is, as he points out in his perfect English, a guide for the Alternative Tourist. He does not have a guide's licence - which is one reason for the need for discretion; nor could he ever get one for his rooftop work. His clients are few in number and, he says, "like-minded people", mostly other youth hostelers.
Putting together his tour was a sizeable undertaking. For weeks he wandered the streets, padding up and down apartment stairways and trying the doors used by the city workers who go on to the roofs to clear the snow and icicles in winter. Even as we walked along Nevski Prospect later, he seemed to be constantly looking upwards, wistfully.
"It takes a long time to find a good one," he said. "You walk along the street, see a building with a nice railing, and say to yourself, `It must be great up there.' You try several staircases. If you can find one that's open, you try to find a safe way in. Sometimes the neighbours give you a hard time. If not, you can start taking people in."
The residents are his biggest bane, even though he says most of the roofs he tramps upon are municipal property. He used to have access to the roof of the tenement in which Dostoyevksy housed Raskolnikov in Crime and Punishment, but the inhabitants got fed up with the disturbance and bolted the door. It can be far worse: on one occasion he and some friends were locked up on a roof by outraged tenants. On another, he was confronted by an angry man wielding a rifle.
He has persevered, not only because his work earns him a bob or two (though not much more), but also because he loves showing off his patch. With only 1.9 million tourists last year, St Petersburg has failed to make the most of its beauty, because of its crime - an Australian was murdered last month, and contract killings are a blight; overcharging - the top hotels are ludicrously expensive; and Soviet practices - you can still be fined for not registering your visa.
He admits to all these problems, but points to the city's fabulous art and architecture, the legacy of nearly 300 years. "People say Petersburg is the Venice of the north but that's bullshit. St Petersburg is St Petersburg. If anything, Venice is the St Petersburg of the south."
For now, he and his fellow back-packers are enjoying these riches from above, moving in the shadows above the melee. And, for now, Peter Kozyrev sees no reason why he shouldn't carry on: "We are invisible. No one looks up. Have you ever seen anyone around looking up?"