A large tree stands close to a house with a shallow foundation on clay soil. The mechanism to describe is:

Clay soils are expansive: they swell as they take up water and shrink as they dry, and the volume change is large enough to move a building. A mature tree is a powerful and highly localized dewatering machine, drawing hundreds of gallons a day through its root zone during the growing season. Next to a shallow foundation on clay, that withdrawal dries the soil under one part of the footing while soil further away stays wetter, so the clay shrinks unevenly and support is lost on one side. The result is differential settlement, which is what cracks foundations and racks door and window openings — uniform settlement is far less damaging. The effect is seasonal and worsens in drought, which is why the damage often appears years after the tree was planted.

Roots withdraw moisture from the clay beneath and beside the foundation, and clay shrinks as it dries. Because the withdrawal is concentrated near the tree, the shrinkage is uneven and produces differential settlement of a shallow foundation.

Expansive clay soils change volume with moisture content. Shallow foundations bear within the zone affected by those changes, and vegetation that withdraws moisture unevenly — particularly large trees in drought conditions — is a recognized cause of differential settlement.

Trees Drink, Clay Shrinks, Footings Sink. The damage comes from what the roots take out of the soil, not from what the trunk pushes against.

Look for corroborating evidence on the same elevation as the tree: stair-step cracking in masonry, doors binding, and cracks that open and close with the seasons.

• •Attributing movement to the trunk pushing on the wall
• •Overlooking that the damage is differential rather than uniform settlement
• •Missing the seasonal pattern that distinguishes clay shrinkage from other causes