What Makes a Building Lean
From the Leaning Tower of Pisa to Amsterdam’s famous “Dancing Houses,” a surprising number of structures around the world are tilted without collapsing. The reason is usually not mystery, but engineering: soil conditions, foundation design and long-term changes underground can all nudge a building off balance.
Geotechnical expert Mandy Korff of Delft University of Technology explains that in places such as central Amsterdam, many buildings stand on wooden piles driven deep into soft ground made up of clay, peat or sand. When those piles remain in good condition, the buildings can remain stable for decades. But if the wood deteriorates, or if weight is not evenly distributed, cracks and gradual leaning can follow.
Some buildings also lean by design. In Amsterdam, many merchant houses were built to tilt slightly forward so goods could be moved more easily through upper openings and along the canals. A forward lean can be intentional; a sideways lean usually is not.
The Pisa Lesson
The Leaning Tower of Pisa is the best-known example of a structure that began shifting because of the ground beneath it. Professor Nunziante Squeglia of the University of Pisa, who helps monitor the tower, says the building started leaning almost immediately during construction because the soil was extremely soft. The tower sank several meters as it rose, setting the stage for centuries of concern.
That concern became urgent in the 20th century as measurements showed the tilt increasing. The crisis sharpened after the collapse of the civic tower in Pavia in 1989, which helped convince authorities to close Pisa’s tower the following year and move quickly to stabilize it.
The chosen fix was unusual: engineers removed 37 cubic meters of soil from the north side of the foundations without touching the tower itself. The work took 11 years and ended in 2001, reducing the lean by more than 40 centimeters. Engineers now believe the tower should remain safe for at least the next 200 years.
Why Some Tilts Are Dangerous
A leaning building is not automatically unsafe. Korff says a structure has to lean quite far before it becomes structurally unstable. The key issue is whether the movement is ongoing and whether the foundation is still supporting the load properly.
That is why some tilts are monitored rather than immediately corrected. In Amsterdam, for example, old wooden piles can still perform well if they stay dry and intact. But if they rot or if groundwater levels change, a building may begin to settle unevenly and shift further out of alignment.
Other causes include human changes to the landscape. Delft’s Oude Kerk leans partly because a canal changed the ground on one side, leaving one portion softer than the other. When soil is excavated or groundwater drops, the support beneath a building can weaken and create a slow but steady tilt.
Climate Pressure on Old Foundations
The problem is not limited to landmark towers. Korff’s research suggests that in the Netherlands alone, around 75,000 houses built on wooden piles are at risk of damage, with nearly three times as many threatened by shallow foundations. The issue could become more common as climate change alters groundwater levels.
If water tables fall, wooden piles can be exposed to air, which speeds up deterioration. Shifts in groundwater can also affect layers of soil, creating a ripple effect for different types of foundations. Even when the process is slow, the long-term costs can be high, especially for older cities with dense historic neighborhoods.
The lesson from Pisa and Amsterdam is clear: a building can lean for a long time and still stand, but only if engineers understand why it is moving and intervene before a tilt becomes a collapse risk.
What Readers Should Watch
For cities with historic buildings, the growing concern is not just architecture but infrastructure management. Foundation monitoring, groundwater control and careful repair work are becoming more important as older structures face new stress from climate change and aging materials. The story of the leaning tower is no longer just about one Italian landmark; it is a warning for heritage cities everywhere.