If you really want to know how old a historical house is, look to the tree rings

Michael Worthington’s job requires a drill, a microscope and a vast amount of accumulated knowledge. It requires something else, too: tact.

“There’s a lot of diplomacy in my job,” Worthington told Answer Man.

Worthington’s job is telling people how old their house is. How old it really is, not how old everyone thinks it is. Or hopes it is. Some people bristle when Worthington has finished his work and lops a few decades off the age of their building.

“With those people, you have to be a little kid-gloved,” he said. “You have to cajole them away from the idea they had and give them a new story. Once they have a new story they can hold onto, they are much more forgiving that their house isn’t the oldest building anymore.”

By what sorcery does Worthington discern when exactly a house was constructed? By looking at tree rings.

Changes in weather affect the width of each year’s tree ring. A ring grows wider in wet years, narrower in times of drought.

“Over a long period of time — 50-plus years — that becomes a unique sequence,” Worthington explained. “Any two trees growing at the same time will collect a similar sequence of wide and narrow rings.”

Worthington runs the Oxford Tree-Ring Laboratory in Baltimore and is a research associate at the University of Maryland. When he’s hired to date a building, he starts by finding original wood, such as in floor joists and wall posts. Using a drill, he takes six to 10 pencil-sized samples. He’s looking for pieces with more than 50 rings and a bark edge. (The bark edge shows when the tree was felled.)

Worthington has an archive of hundreds of samples of oak, yellow pine and tulip poplar for which he knows the date range of the rings.