A shivering knot of college students stands outside Motor City Java House as John George unlocks the front door. It’s 15 degrees in Detroit on a February morning, and fresh snow covers the Old Redford business district. Cold weather doesn’t stanch the flow of volunteers coming to the city’s northwest corner: They show up every Saturday, arriving in shifts, ready to swing sledgehammers and twist crowbars. George’s wife, Alicia Marion, has learned to expect this traffic since she opened the coffeehouse in 2010. She puts on an apron and starts brewing.