At the Cambridge, Mass., project, contractor Tom Silva frames the library roof; plumbing-heating expert Richard Trethewey explains the software system he used to design the climate-control system; and landscaper Roger Cook and homeowner George Mabry select landscaping materials at a stone yard. Meanwhile, host Kevin O'Connor visits the Cambridge Common, a park near Harvard Square. On “Ask This Old House,” projects include a slow-draining bathtub and a raised garden bed.