Why do the edges of a house sink in dry Oklahoma weather?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: Perimeter drying settlement, or edge drop, happens when expansive clay around the outside of a house dries out and shrinks while the soil under the middle stays moist. The edges of the foundation lose support and sink, leaving the center relatively high. It is seasonal, worst in dry periods and drought, and partly recovers when rain returns.

How does edge drop happen?

Expansive clay shrinks as it loses moisture and swells as it gains it. Engineers describe seasonal moisture loss letting the perimeter of a slab settle, and seasonal moisture gain pushing it back up ¹. Ground-supported slabs are flexible and tend to follow the shape of the soil beneath them ¹, so when the soil at the edges shrinks, the edges come down with it.

There is frequently a moisture difference between the soil beneath a house and soil more exposed to the weather ². A frequent source of damage is differential swelling where pockets of moist soil sit next to dry soil ², which is the situation at a drying perimeter.

Step by step

1. Heatwaves, drought and the water demand of vegetation pull moisture from the clay around the house ⁵.
2. The clay shrinks, leaving voids and cracks in the soil ².
3. The perimeter beam loses support and drops with the soil beneath it ¹.
4. The center, still near its original moisture, is left relatively high compared with the edges ¹.
5. The cycle repeats with the seasons, since shrink-swell cycles occur annually in climates with pronounced wet and dry seasons ².

How deep does the drying go?

The zone of soil whose moisture changes with the weather is called the active zone.

• In Oklahoma, it has been estimated at roughly 0.5 to 4 meters (about 1.5 to 13 feet), depending on soil properties and climate ³.
• A PreventionWeb hazard profile puts the zone of seasonal moisture change usually in the upper 1.5 to 2 meters, reaching about 5 meters in some cases ⁵.
• Engineering literature reports seasonal fluctuation from about 3 to 40 feet deep in expansive soils, with cracks reaching deeper than often assumed and multi-year droughts drying soil deeper than previously believed ².

See shrink-swell and the active zone.

Why do Oklahoma soils and climate cause edge drop?

Nearly all of Oklahoma has medium or high soil swelling potential, based on USDA soil survey data ³. OSU testing of benchmark soils found the fine clay in Bethany, Kirkland-like and Norge-like subsoils dominantly montmorillonite (smectite), a strongly swelling clay mineral, with coefficients of linear extensibility up to about 0.095 in Bethany and 0.090 in Kirkland ⁴. Oklahoma's climate ranges from humid subtropical in the east to semi-arid in the west ³.

How do trees and drought affect edge drop?

Below the depth reached by surface drying, the water demand of vegetation, particularly trees on clay soils, drives the moisture changes that cause shrinking and swelling ⁵. Soil drying from trees and other large vegetation, especially during drought, is a common cause of foundation subsidence ⁶. Texas engineering guidelines note that removing trees may stop the shrinkage or partly restore lowered areas, but can also cause upheaval, and that pruning may reduce future settlement without restoring elevation ⁷. See tree-root desiccation settlement.

What are the signs of edge drop?

• stair-step brick cracks wider at the top
• soil pulled away from the foundation
• cracks that open and close seasonally
• brick separating from window or door frames, frieze board or trim separation
• doors that bind, diagonal drywall cracks and sloping floors toward exterior walls

How is edge drop different from center heave?

Both can leave a floor high in the middle. In edge drop, the edges went down; in center heave, the center came up. Engineers associate center lift with long-term moisture gain, typically in older foundations, and edge movement with seasonal moisture changes ¹. A single elevation survey shows the foundation's shape at one moment, while repeated surveys over months or years show how it is changing ⁷. Without a baseline survey, movement cannot be determined accurately ⁶. Compare center heave.

How is edge drop typically fixed?

• Underpinning below the active zone. A standard risk-reduction measure for shrink-swell ground is extending foundations below the zone of moisture fluctuation, adjusted for vegetation ⁵. In Oklahoma steel push pier installation records, typical pier depths were about 8 to 16 feet, with a median of about 12 feet in Tulsa; piers were installed at about 7-foot spacing, with installation pressures around 9,000 psi and a 6,000 psi minimum ⁸. See steel push piers.
• Moisture control. Non-structural measures and monitoring may be prudent before or instead of structural repairs ⁷. See root barriers and moisture control.
• Monitoring through a cycle. Repeated elevation surveys show whether movement is seasonal and how large it is ⁷.

Related

• Perimeter Wetting Heave (Edge Lift)
• Tree-Root Desiccation Settlement
• Seasonal Movement Calendar
• Heave vs Settlement
• Helical Piers
• OKC Metro