There’s a hidden concern lurking beneath some of Boston’s priciest properties. Thousands of homes and buildings face structural risk as underground timber pilings deteriorate due to falling groundwater levels.
Approximately 8,000 buildings built on filled land between the mid-1800s and early 1900s in Boston are possibly affected, Christian Simonelli, executive director of The Boston Groundwater Trust, tells Realtor.com®.
Established by the Boston City Council in 1986, The Boston Groundwater Trust tracks groundwater levels throughout the city’s filled-in neighborhoods to help ensure the wooden pilings beneath buildings stay underwater to prevent rotting.
The problems date back centuries ago when the first European colonists reached the Shawmut Peninsula—large swaths of what is now Boston lay beneath the waters of Massachusetts Bay.
Over roughly a century starting in the late 1700s, Boston undertook massive land-reclamation projects, dumping earth and gravel into the bay to create new buildable ground.
Today, roughly 5,250 acres of the city sit on man-made land, according to Simonelli.
Construction crews hammered long wooden logs down through the loose fill, stopping only when they struck something firm enough to bear weight. Stone foundations were then stacked on top of this timber framework.
“The first time I heard about this, I thought this doesn’t make any sense that houses are built on tree trunks,” says Simonelli. “But it originated in Europe. The whole city of Venice is built on wood piles.”
This approach was standard practice in Boston for well over a century before modern concrete and steel techniques made it obsolete in the early 20th century.
Around 6,000 buildings in Boston are supported on wood pilings, according to Simonelli.
Kept permanently wet beneath the water table, untreated piles could survive for generations without any sign of deterioration.
But urban development—including subway tunnels, sewer lines, and deep basements—gradually drained groundwater from the surrounding soil, leaving piling tops exposed to open air. Once oxygen reaches the untreated wood, decay sets in quickly, and the foundations above begin to drop.
Many of Boston’s best-known landmarks have undergone foundation repairs over the years to address these concerns, including the Boston Public Library, Trinity Church, and Old South Church.
Identifying the damage
It’s hard, if not impossible, to spot rotting wood and foundation damage with the naked eye, according to Simonelli.
The Boston Groundwater Trust maintains a database on its website logging known piling damage and any repairs that have been made.
“People think these buildings have historically been in lower Beacon Hill or Back Bay, but they’re in areas all over the city,” says Simonelli.
Would-be homeowners can also call The Boston Groundwater Trust directly to find out if a home is located on solid ground or fill.
“People do that all the time, whether they are buying or selling,” says Simonelli. “We’ll walk you through our online database of what buildings are on wood and what buildings are on concrete.”
Foundation issues are definitely on buyers’ radars, according to real estate professionals.
“Foundation condition has become part of the standard due-diligence conversation, particularly as media coverage has increased awareness,” George Sarkis, co-founder of The Sarkis Team at Douglas Elliman, tells Realtor.com.
A professional inspection can uncover potential foundation issues before a purchase is finalized.
“Experienced inspectors can identify warning signs such as settlement, uneven floors, cracking in masonry, or moisture issues, but pilings themselves aren’t always directly visible without specialized evaluation,” says Sarkis. “In many cases, structural engineers are brought in to conduct more detailed assessments.”
In addition, if a seller is aware of foundation movement, water intrusion, or structural compromise, that information cannot be withheld. “Failure to disclose known issues can create serious liability after closing,” says Sarkis.
A state law that took effect recently makes it illegal for buyers to skip home inspections in an effort to outbid competitors.
“This change is particularly important in a city like Boston, where structural and foundation considerations are part of many properties’ realities,” says Sarkis.
Foundation repair is a long process
Repairing a failing foundation is a massive undertaking.
Simonelli says that, depending on the size and scope of the project, it can take anywhere from a year to 18 months to complete.
“If you’re living in the building when it’s under construction, it can take even longer,” he adds.
According to Simonelli, foundation work can run in the ballpark of $200,000 to $300,000.
And these are costs homeowners will almost certainly have to shoulder themselves, since he says most insurers won’t pay for this type of repair.
“I’ve seen buyers walk away when inspection findings reveal significant structural work that materially changes the financial equation,” says Sarkis. “Foundation repairs can be substantial, and not every buyer has the appetite for that risk.”
Mitigating future risks
City regulations now call for new developments to include systems that replenish underground water supplies, along with surface materials that let rain soak through rather than run off.
Meanwhile, The Boston Groundwater Trust soon expects to begin installing next-generation monitoring equipment, giving officials a more detailed, real-time picture of conditions underground.
“Hopefully that will be rolling out this summer,” says Simonelli.