How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What is my chance of buying a Florida home over a sinkhole?
Friday, October 19, 2018
While sinkholes are not the big problem in Alachua County that they are in Florida counties farther south—such as in Pinellas, Hernando, and Hillsborough—they do happen occasionally throughout most of Florida. The limestone underlayment of much of this area, called “karst,” is the root of the problem. Deterioration of the underground karst due to percolation of acidic rainwater through the soil causes voids to form, which collapse slowly, or sometimes abruptly, like in the dramatic newspaper photos of homes swallowed up overnight by a sinkhole.
The “Devil’s Millhopper” in northwest Gainesville is the largest sinkhole in Florida. It’s a state park, and the stairs to the bottom, shown below, are an enduring tourist attraction. Plus, many of the lakes in the area were formed by sinkholes.
There are three major types of sinkholes: 1) solution sinkholes, 2) cover subsidence sinkholes, and 3) cover collapse sinkholes. The first two types sink slowly over time. It’s the cover collapse sinkholes, which usually fall down abruptly and dramatically, that are on the evening TV news. To learn more about the different categories of sinkholes, see our blog post What causes sinkholes?
But the more prevalent underground defect in some North and Central Florida areas is clay soil. The elasticity of clay causes it to shrink during dry spells, then swell during seasons with heavy rains. The swell/shrink cycle causes the ground under a home built over a layer of clay soil to heave up and down in seasons that are extra wet or dry. A hilly band of clay soil that geologists have labeled the “Hawthorne Formation,” for example, runs through the middle of Alachua County, roughly north-south along I-75 and east-west along Newberry Road. The cross-section below is from a USDA soil survey of Alachua County from 1985.
Determining whether structural cracks in a home are due to sinkhole activity, clay soil, or erosion, is complicated. And engineering experts sometimes reach different conclusions about the cause of a structural problem when examining the same home. Go to our blog post How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil? for more on this.
The determination of what’s causing a home to start cracking apart can a make a big difference as far as getting your homeowner’s insurance company to pay for repairs. Clay soil is considered an existing condition and is not covered, while sinkholes are. Or, at least, sinkholes can be covered, if you pay additional for it when the policy is written. They are typically excluded in standard policies nowadays. To get a better understanding of sinkhole insurance coverage, see our blog post How can homebuyers protect themselves against buying a house over a sinkhole?
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