Rain doesn't stop preliminary exterior work on the 1950s-modern house in Cambridge, Mass., as trees are cleared and excavation for the new foundation has been completed, but rot and termite damage have been found. Also: Homeowner George Mabry inspects exterior materials and host Kevin O'Connor visits a Lincoln, Mass., house designed by Walter Gropius in 1938. On “Ask This Old House”: planting a drought-resistant tree; installing an under-sink water filter. Filtration expert Bill McTighe guests.