Work begins on the Concord, Mass., barn-conversion project and host Kevin O'Connor joins in, building temporary walls to take the weight off a bowed exterior wall that needs bracing. O'Connor also reviews plans with architect Holly Cratsley and visits an already-converted carriage house in Winchester, Mass. On “Ask This Old House”: “Housecall” projects include a slate walkway in a state of disrepair and a leaky fiberglass shower stall. Also: O'Connor, landscaper Roger Cook, plumbing-and-heating expert Richard Trethewey and contractor Tom Silva guess the uses of unfamiliar objects.