What karst is
Karst is landscape formed on soluble rock, mostly limestone and dolomite. Rainwater, made slightly acidic by carbon dioxide from the air and soil, slowly dissolves the rock along cracks and bedding planes, creating voids, caves and channels. Soil sits on top of that rock. Over time, soil can wash down into the openings, leaving a cavity that eventually collapses as a sinkhole, or settles slowly as a depression.
Karst covers large areas of the eastern U.S. Well-known sinkhole regions include the Lehigh Valley and Lancaster, Lebanon, Cumberland and York Counties in Pennsylvania, East Tennessee around Knoxville, central Kentucky, the Missouri Ozarks, central and west-central Florida, and the Shenandoah Valley in Virginia.
Why water matters
Most sinkholes near homes are not caused by the rock suddenly failing, but by soil being washed downward into openings in the rock. Water is the transport. Anything that concentrates water in one place speeds this up: broken pipes, clogged storm drains, ponds and, on a smaller scale, downspouts that dump a whole roof’s water in one spot. Pennsylvania American Water and local townships in Lancaster County both point to heavy rain and stormwater runoff as aggravating sinkholes. The Pennsylvania Department of Environmental Protection advises that stormwater from development should not be directed into sinkholes.
A 2,000-square-foot roof sheds more than 1,200 gallons in an inch of rain. Split among four downspouts, that is 300 gallons each, poured onto a few square feet. In karst, that concentrated flow can open a path into the rock over time.
How to route downspouts in karst areas
- Spread the water out. Discharge onto a wide grassy area, a gravel bed or a shallow rain garden rather than a single point. Splash blocks and extensions that fan water out help.
- Keep it away from the foundation. At least 5 feet, and more where the lot allows, onto ground that slopes away.
- Never drain into a depression or known sinkhole. It may seem convenient because water disappears there, but that is exactly how sinkholes grow.
- Be cautious with dry wells. Dry wells concentrate water underground. In karst areas they are generally discouraged unless designed by an engineer who has evaluated the site.
- Check buried lines. A cracked underground downspout line can leak into the soil for years. Test lines with a hose and repair or replace broken ones.
- Consider connecting to a storm sewer where local rules allow, so water leaves the property entirely.
- Keep gutters from overflowing, which creates a line of concentrated water along the drip edge. See gutters overflowing in heavy rain.
Rain gardens in karst
Rain gardens are often recommended for downspouts, but in karst they should be shallow, spread out and located well away from the house and from any known depressions. Their job here is to slow and spread water over a wide area of soil, not to pond it deeply. Check with your township or county conservation district; some publish rain garden guidance specific to carbonate areas.
Warning signs
- A new depression or soft spot in the lawn, especially near a downspout or under a gutter line
- Circular cracks in soil or pavement
- Water disappearing quickly into the ground at one spot
- Doors and windows that suddenly stick, or new cracks in foundation walls
- Leaning fence posts or trees
- A downspout outlet that keeps washing out a hole
If you see these, stop directing water to that area immediately, keep people and pets away from any opening, and contact your municipality or a geotechnical professional. Many townships in karst areas have guidance for residents.
Gutters and the house itself
Sinkholes are not the only karst concern. Carbonate soils in these regions are often clay-rich residual soils left behind as the rock dissolved, and they can shrink, swell and settle unevenly when moisture changes. Roof water dumped at one corner keeps that spot wet while the rest of the foundation dries. Even moisture around the house, with water carried away and spread out, reduces both settlement and sinkhole risk. See downspouts and your foundation.
Building or buying in karst
If you are buying a home in a karst area, ask about any history of sinkholes or depressions on the property and nearby, look for signs of settlement, and check where downspouts, sump pumps and septic systems discharge. Many counties and state geological surveys publish karst or sinkhole maps. For new construction, local ordinances often require a geotechnical evaluation and stormwater plans that avoid concentrating water. When new gutters are installed on a home in karst, plan the downspout locations and discharge areas before the gutters go up, not after.
Sump pumps and other water sources
Roof water is only one source. Sump pump discharge, air-conditioner condensate, pool drains and leaking water or sewer lines also concentrate water. Route sump discharge the same way as downspouts: away from the house, spread out, not into depressions. Fix leaking pipes promptly; a slow leak can feed a sinkhole for years before it appears.
Who is responsible?
Local governments in karst areas, including Knox County, Tennessee, and townships in Lancaster County, Pennsylvania, generally say that sinkholes on private property are the owner’s responsibility unless they threaten public infrastructure. Standard homeowners policies often exclude earth movement; some states require insurers to offer sinkhole or catastrophic ground cover collapse coverage, notably Florida. Check your policy.
Gutter choices in karst areas
- More downspouts, each carrying less water, spread the load. See downspout size and placement.
- Guards reduce overflow from clogged gutters. See are gutter guards worth it?
- Rain barrels hold some water back, with overflow routed safely. See rainwater harvesting.
- Rain gardens sized and located away from the house spread water over a wide area. Avoid deep infiltration features without advice.
A simple yearly check
- Walk the yard after a heavy rain and note where water collects or disappears.
- Look at each downspout outlet for holes, trenches or soft ground.
- Check that extensions are still in place and pointing where they should.
- Look along the gutter line for overflow marks on siding and a line of erosion below.
- Photograph any depression and watch whether it grows.
Regions we serve with karst
Lehigh Valley and Berks, Harrisburg, Lancaster and York, Knoxville, Nashville, Louisville, St. Louis, Tampa Bay and Orlando.