Winston, my co-worker, arrived at the job site a few minutes before me, and called because he thought he was in the wrong place.  “I thought we were looking at a rotten porch.  The place I’m looking at is new construction.” The triple-decker condo building in Jamaica Plain was completed in late 2005, but one of the owners called me recently to look at problems with the rear decks.     The first sign of problems was the brown staining on the white ceiling — visible if you look closely at the second floor ceiling above.  Then the client showed me some of the failing deck boards.   and up close:   In one location, they had replaced some of the decking, but the…