Tree roots and shrubs can be the source of home foundation problems. Trees and shrubs withdraw large amounts of water from the surrounding soil and in areas of expansive soils that could be problematic.
The southern part of the United States is home to large areas of expansive soils, which expand in volume when wet and shrink in volume when dry. This process of expansion and shrinkage is repeated many times and is called horizontal movement, vertical movement, uplift, upheaval and/or soil movement. Unfortunately for home foundations this soil movement is strong enough to crack and damage them. Usually the moisture content of the soil under the center part of the foundation remains stable. Usually it is the soil around the perimeter of the foundation that is alternately exposed to large volumes of water (rain) and long periods of drought. The repeated swelling and shrinkage of the soil will cause portions of the foundation to rise and to fall.
Tree roots and shrubs can cause foundation problems when they absorb large amounts of water from areas near and under a specific portion of the foundation. Most people do not realize that tree roots usually extend outward to the extent of the tree canopy and up to three times the height of the tree. A medium size tree about one foot in diameter and twenty-five to thirty feet in height can absorb 150 gallons of water a day. If this tree is close to the home foundation then the tree absorption plus evaporation during the summer can leave the soil in that area much dryer than the soil under other areas of the foundation.
What happens when a large tree or dense shrubbery removes a disproportionate amount of water from the soil on one side of a home? Since the loss of water is not uniform around the entire perimeter of the home foundation, the soil near the tree or shrubbery will shrink to a disproportionate degree. The soil will then lose contact with the foundation and there is nothing but air between the foundation and the soil. The foundation will then “sag,” “sink,” or settle on that side of the home and the both the foundation and the home will suffer damage.
Large trees such as Live Oak, Red Oak, Arizona Ash and Chinese Tallow have extensive tap root systems that extract large amounts of water from the soil. If trees are too close to the home then their roots can also extract moisture and water from the soil under the center of the foundation. If too much moisture is lost from this area then interior portions of the foundation may “settle” or sag and collapse.
In most cases the solution for tree roots and the risk of foundation problems is to remove the tree and / or install tree barricades. A good rule of thumb is that a mature true should be located no closer than one-half of its canopy width to the home foundation. Another general guide is that a large tree should be no closer than ten to twenty feet to the home foundation. Trees that are closer than these guidelines should be considered for removal. However, sometimes there are exceptions to these guidelines for tree removal. When a large tree is older than the home then its removal may cause a large void under the foundation as the tree roots gradually die and decompose. In these situations an arborist should be consulted. Similarly shrubs and plants should be located no closer to the foundation than their mature width. Therefore if a mature shrub has a width of five feet it should be no closer than five feet to the foundation.
Tree barricades are good solutions for deflecting tree roots and the home foundation problems they cause. Generally a trench is dug approximately five feet from the exterior foundation. A series of overlapping sheets of Plexiglas (plastic) is placed in the trench to a depth of 30 inches. This will prevent the tree roots from growing toward the foundation and withdrawing a disproportionate amount of water.
Landscaping is another potential source of home foundation problems. The elevation of a landscape bed should be three to four inches below the top of the home foundation. And landscape beds should never cover weep holes and they should be angled so that rain water drains away from the home. Unfortunately landscape companies frequently commit these errors as well as placing shrubs and plants too close to the foundation.
Martin Dawson is the co-founder of Dawson Foundation Repair headquartered in Houston, Texas. He is a leading authority on repairing failed commercial and home foundations using the time tested and thoroughly researched drilled Bell Bottom Pier method. His company has serviced Texas and other southern states since 1984.
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