Phillipsburg and Belvidere both have a median house from 1938, and in each town about half the homes went up before 1940. A roof that old is rarely the first one on the house, and a second layer of shingles nailed over the first is a common find. Under them there is often plank decking from the original build.
Greenwich and White sit at the other end of the scale. Greenwich's median house dates to 1994 and White's to 1991, and close to a third of White's homes were built this century. A 1990s house may still carry its original roof, and shingles from then are often near the end of their life.
The whole county sits in the State Climatologist's Northern Zone, colder than the rest of New Jersey, with 40 to 50 inches of snow in an average winter. Snow on a roof melts from underneath when the attic is warm, and the water can freeze again at the cold edge.
A 1938 roof and a 1994 roof can look alike from the road. We climb up when it is safe, count the layers and check the attic before a price goes on paper.