In his answer to the question about whether tree roots can dry out the soil beneath a foundation enough to cause the concrete to crack (Q&A, 7/09), Bill Palmer refers to expansive soils. But in fact all nonexpansive clayey soils will also shrink as tree roots “suck” the moisture out of the soil — a process known as desiccation. When desiccation occurs with roots that extend beneath the bottom of the footing, the foundation will indeed settle, usually resulting in cracking.
When we investigate a settlement problem in an area known to have clayey soils, trees are the first things we look for. If a tree canopy extends over the roof or is even near the wall line in the area that is settling, then dessication is the most likely cause, since the tree’s root system has about the same diameter as the tree canopy. If the...
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