It’s become a fall tradition in Salem — and no, not that fall tradition. I’m talking about the ritual roof installation on the more than 200-year-old Yin Yu Tang Chinese house, the crown jewel of the venerable Peabody Essex Museum. If you haven’t been inside, Yin Yu Tang, which housed generations of the Huang family in a rural village in southeastern China, offers a fascinating window into Chinese domestic life, with its compact rooms, central courtyard, weathered heirlooms, even an old red loudspeaker that once broadcast the Communist Party line. The house was dismantled and then reassembled in Salem over several years, first welcoming visitors in 2003. From May to November, the house is open to the sky, as it was in China. Each November, the museum brings in a crane crew to lower four glass-and-steel roof panels on top, to protect the structure — and the koi swimming in two ponds — from winter’s reach. On a recent morning, onlookers watched from downtown sidewalks as a giant, whining yellow crane hoisted the panels into place. Workers lying on the roof tightened the bolts with screw guns, giving off a staccato rat-a-tat. Lit by the sun, the koi ponds bubbled quietly underneath.