Why are retaining walls, fences and trees leaning on my slope?

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

In short: When retaining walls, fence posts and trees on a slope all lean the same way, the upper soil may be slowly moving downhill. This slow movement, called creep, can also pull at foundations near the slope, so the leaning features are checked alongside the house itself.

What does this sign look like?

• Retaining walls tipping outward or bulging at mid-height.
• Fence posts leaning downhill in a row, or fence lines that have bowed.
• Trees tilted downhill.

Any house on or near a slope can show these, whether it sits on a slab, crawl space or basement.

Why does it happen?

Creep of clay soils

Creep is very slow downhill movement of the upper soil, often too slow to see except through its effects on things anchored in it. Inspection guidance notes that clay soils on slopes can slowly drift downhill, causing settlement ¹. Expansive clay swells as it gains moisture and shrinks as it loses moisture ⁵. The mechanism is covered in slope creep and lateral movement.

Retaining walls

Masonry walls that lean almost always lean outward, and leaning walls can sometimes be tied back to the structure ¹. Poor surface drainage can cause soil swelling and erosion around footings ¹, and when soil particles fill the drainage path, water pressure builds up against the wall ¹. See lateral earth and hydrostatic pressure on walls.

Landslides in Oklahoma

The Oklahoma Geological Survey monitors and maps landslides across the state, with research focused on the causes of shallow landslides in eastern Oklahoma, and it has produced a preliminary landslide susceptibility map of Sugarloaf Mountain using high-resolution LiDAR elevation data ². Creeks and steep banks are covered in erosion, creeks and slopes.

How is it documented?

A common inspection rule says a wall is usually considered unsafe if a plumb line through its center of gravity does not fall within the middle third of its base, and that a structural engineer should be consulted in that case ¹. Floor readings are covered in elevation surveys.

How is drainage handled on sloped lots?

Building guidance calls for grassy swales (shallow drainage channels) to slope at least about 1 percent and no more than about 15 percent unless lined with riprap, meaning rock armor ³. Foundation design guidance calls for at least a 5 percent slope away from the house in the first 10 feet and keeps drainage swales at least 4 feet from the foundation ⁴. Approaches for unstable slopes are described in slope and erosion stabilization.

Related

• Slope Creep & Lateral Movement
• Erosion Creeks and Slopes
• Slope and Erosion Stabilization
• Surface Drainage and Grading
• Sloping Floors