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Most reported damage is secondary in nature. The problem is confined largely to soil types that shrink considerably on drying. In the UK these are mainly certain types of clay and some peaty, fen soils. Under conditions of drought when tree roots remove water from shrinkable soils, they accelerate the drying and shrinking process. This can lead to loss of support to inadequate foundations and subsequent subsidence. Continued drought, even in the absence of trees, can lead to subsidence in such circumstances.

Tree root spread data have most relevance to relatively ‘safe’ planting distances when relating to trees growing on shrinkable clay soils. Planting distances on other soil types that do not contract on drying have to be determined more by consideration of the respective scale of the buildings and trees, the ultimate size of the trees, and the desirable or undesirable qualities of the trees concerned. In all cases it is wise to consult a qualified expert.