· by Michael Blair, Nashville's Foundation Repair
Middle Tennessee sits on clay over limestone. Clay holds water, swells when it takes it on and shrinks as it dries, and a foundation sitting on it experiences that as constant upward and downward pressure that varies by season.
The mechanism
Wet clay expands and pushes up and inward. Dry clay shrinks and pulls away, leaving a foundation less supported than it was. Neither is uniform across a house.
The differential is what causes damage. A house whose north side stays damp under shade while the south side bakes moves unevenly, and unevenly is what cracks things.
Why drainage is the whole answer
Keeping soil moisture as consistent as possible around the perimeter is the single most effective thing a homeowner can do. That means moving roof water away and grading so runoff does not pond.
Downspouts discharging beside a foundation are the most common contributor here, delivering thousands of gallons a year to one spot.
- Downspouts extended six to ten feet, or piped away
- Grade falling six inches over the first ten feet
- Gutters kept clear so they do not overflow at the wall
- No beds that hold water against the foundation
- Consistent watering in drought, not none then a lot
Trees and drought
A large tree close to a foundation removes a great deal of moisture from the clay in summer, and that localized drying causes settlement on that side.
The reverse also happens: removing a mature tree lets clay rehydrate and heave. Both are real and both are gradual enough to be missed.
Watering a foundation in drought
In an extended dry spell, soaker hoses set back a foot or two from the foundation keep clay from shrinking away entirely. It sounds counterintuitive and it is genuinely effective here.
The goal is consistency rather than volume. Alternating between bone dry and saturated is worse than either steady state.