Wet Years, Dry Years: The Clay Soil Under San Antonio Homes

In the Bexar County soil survey, the NRCS rating written for Texas homes on a concrete slab marks about 92% of rated acreage as limited, 36% of it very limited, and cites high shrink-swell on about 83% of the major soil acreage (USDA NRCS). Clay like this swells when wet and shrinks when dry, lifting and dropping a slab unevenly.

What the soil survey says about Bexar County clay soil

The USDA soil survey maps about 804,600 acres in Bexar County. Its most extensive soils are Eckrant, a stony clay often shallow over limestone, and Houston Black, a deep clay. About a quarter of the mapped acreage belongs to the Vertisols, the soil order defined by clay that shrinks and swells. For homes on a concrete slab, the NRCS rates the county's soils this way:

• Very limited: about 36% of rated acreage
• Somewhat limited: about 56%
• Not limited: about 8%

High shrink-swell is the reason it cites most, ahead of shallow bedrock, slope, large stones and flooding (USDA NRCS, Bexar County soil survey).

Cracks you can put your hand in

The official description of Houston Black clay says that when it dries, cracks 0.5 to 4 inches wide open from the surface to a depth of a foot or more, and that water moves through the soil very slowly (USDA NRCS). Picture that against a slab edge in August. The first heavy storm pours straight down those cracks and soaks the soil along the edge long before the soil under the middle of the house gets wet. The edge swells first and the center later, which is one reason slabs here crack and tilt rather than simply sinking.

A rain record full of extremes

• Driest years: 1917 with 10.11 inches, then 2022 with 11.51 (NWS Austin/San Antonio).
• Wettest year: 1973, with 52.28 inches.
• Wettest day: 11.26 inches at the airport on October 17, 1998, in a month that totaled 18.07 inches, still the wettest on record.
• Second wettest day: 9.87 inches on May 25, 2013.
• Most recent federal declaration: FEMA DR-4935, declared September 1, 2026, for severe storms and flooding from July 12 to August 4, 2026, with Bexar County among the counties covered.

In an average year the airport still sees about 122 days at 90°F or hotter (NOAA 1991–2020 normals), so the soil dries hard between rains.

Keeping the moisture steady

• Make sure gutters drain and downspouts empty several feet from the slab.
• Keep soil sloping away from the house, and don't let raised flower beds trap water against the foundation.
• In long dry spells, a soaker hose set back from the slab keeps the edge from drying out. SAWS groups soaker hoses with sprinklers in its watering rules, so check the current drought stage first.
• Large trees close to the house pull a lot of water from the soil. Ask a specialist whether a root barrier makes sense before removing a tree.
• Fix plumbing leaks quickly: water from a broken line under the slab can swell the clay beneath it.

When the house has already moved

Steady moisture slows future movement but won't lift a slab that has already dropped. Read about leveling a settled slab with piers or repairing a pier and beam floor, and check typical prices in the 2026 San Antonio cost guide.