Thousands of historic Boston buildings in the city's core are ticking financial time bombs due to a hidden risk. Be wary of what lies beneath them

Christian Simonelli, executive director of The Boston Groundwater Trust, told the news channel that 6,000 buildings are at risk.

In an interview with Realtor.com, he added that he was surprised to learn about the heritage building’s wood foundations (2).

“The first time I heard about this, I thought this doesn't make any sense that houses are built on tree trunks," he said. "But it originated in Europe. The whole city of Venice is built on wood piles."

Here’s why wood foundations designed to stand the test of time in Boston are under pressure.

Rising risk from plummeting groundwater

The challenge dates back more than two centuries to the late 1700s, when developers launched a massive land reclamation project, filling mud flats in the area with sand and gravel.

That reclaimed land accounts for 5,250 acres of modern Boston, home to many of the city’s most historic and expensive properties.

Over the next 100 years, well into the late 1800s, builders used tree trunks for foundations, burying the wooden pilings up to 25 feet underground, and then stacking stone on top of that

The buildings and foundations were meant to last for centuries — as long as the wood pilings remained submerged in groundwater.

But today, the city’s groundwater levels are steadily dropping, according to The Boston Groundwater Trust.

That has exposed the pilings to air and, consequently, microbes. That means rotting foundations, leaving buildings at risk of collapse.

The historic buildings can be saved.

The foundations of some of the city’s most noteworthy landmarks — including Boston Public Library, Trinity Church, and Old South Church — have already been repaired to address this issue (2).

But those repairs come at a cost, which could be prohibitive for many homeowners: $300,000 and up for a standard rowhouse, according to Boston Groundwater Trust (3).

The work is also time-consuming, taking up to 18 months.

Simply put, many ordinary Bostonians are sitting on financial time bombs.

What can homeowners and buyers do?

For those who live in and around Boston’s downtown core, it’s probably a good idea to check if your home is at risk.

The Boston Groundwater Trust maintains a free online database at bostongroundwater.org that can help you figure out if your home is located in vulnerable areas of the city.

The database also includes information about groundwater levels across the area, which should give you some clue about how exposed your property is.

If you believe your property sits on a wooden foundation in mud flats, consider hiring a structural engineer to inspect the foundation for any signs of rot (4).

Also, check your homeowners insurance policy to see if any clauses mention these types of risks.

For potential homebuyers, checking the foundation should be a part of the due diligence process.