Q. Can the roots of a tree actually dry out the soil beneath a foundation enough to cause the concrete to crack?
A. Bill Palmer, president of Complete Construction Consultants in Lyons, Colo., responds: Trees consume enormous amounts of moisture, so it’s conceivable that a root system growing too close to a home could suck enough moisture out of the soil to crack the foundation. This is particularly true in dry areas with very expansive soils, which shrink or expand with changes in moisture content.
In nonexpansive soils, damage from tree roots is often caused by a different mechanism. Root structures seeking water can grow to the point where the pressure they exert on a wall causes foundation walls to bulge and slabs to crack and move. There are even cases where tree roots have grown into or through concrete walls, cracking them and causing further structural damage.
According to HUD guidelines, trees and plants should be planted no closer to a building foundation than the anticipated height of the particular plant. Neither the Concrete Foundation Association nor the American Concrete Institute offer formal rules on how far trees should be from a structure. One CFA member I spoke with told me that in states with expansive soils, where builders have to disclose expansive conditions and take proper precautions for drainage and moisture content, the recommended clearance between a tree and a foundation is one and a half times the height of the tree. At that distance, the amount of moisture that the tree would draw from the ground would not be enough to cause significant settling.
