Transpiration of Trees and Shrubs

This is a very important factor which is not generally considered. The drying  effects of the transpiration of trees and shrubs are superimposed on the seasonal conditions mentioned above whenever the root system approaches the shallowly founded structures. The root systems of isolated trees generally spread to a radius greater than the height of the tree and have been observed to cause significant drying of fat clay soil to a depth of 3 in. The fast growing trees are especially dangerous and within 5 or 6 years the roots extend to a distance of 15 or  18 m and dry out the clay abnormally below the foundations of the nearest part of the house.  Sometimes a permanent depression of the ground is produced during the early period of the rapid growth when the tree demands more water than is available on the ground. Abnormal spells of dry weather have much more serious effects. 

It has been found that the differential shrinkage below the foundations can be sufficient to produce racking in the brickwork.  Such cracking can be most common at the western and southern sides of the house since these parts receive more radiation from the sun than the northern or eastern sides.  The drying which progress from the outside towards the inside edge of the footing causes outward tilting of the walls and corners; a movement which becomes magnified at the top of wall to 25 to 50 mm in the worse cases. The outward movement of the wall may drag the upper floor and roof with it and cause cracking to spread throughout the interior of the house.Trees growing close to a Building on shrinkable soil may cause cracks in the wall as shown in Figure. 

  Trees Growing Close to a Building on Shrinkable Soil may Cause Cracks in the Walls 

For overcoming the damaging effects of shrinkage the foundations should be taken sufficiently deep. A depth of 90 cm is necessary to avoid racking in normal brick houses away from fast growing trees. It has also been suggested that fast growing and water seeking trees should not be planted within 18 m (7.6 m min.) of buildings. Trees planted alongside roads have caused marked depressions along the edges of the roads and underneath cement concrete paths.