Issue: March 9, 2002

Tree roots and basement wall

Question:

I have a Honey Locust tree about 30 feet from my house. About 15 years ago I had broken a section of cement out of our basement to put in a bathroom and dug out smaller-sized roots that came from that tree. We've developed some cracks in the foundation across from where the tree is located. Recently the cracks have gotten bigger. We have a brick and stucco home. Could the tree roots be the cause? How far and deep from the tree trunk do the roots grow? The tree is about 50 years old. James P.

Answer:

If there is sufficient water around the foundation of the house to encourage roots to grow there, it is possible that the roots growing next to the foundation are enlarging and stressing the foundation. However, here in New Mexico with our very dry climate, such a situation is usually caused by our watering a flower bed next to the foundation. Since you indicate that the tree's roots were present next to the foundation 15 years ago, it is likely that they are still there and have enlarged over time. As roots enlarge, they can slowly exert great pressure. However, I wouldn't expect a lot of damage from the roots unless the concrete wall of the basement was not reinforced or unless it was quite thin. Regarding the location of roots, roots can grow three or more times the height of the tree from the tree, so they are certainly able to reach your foundation. However, the thick roots tend to be near the trunk, tapering rapidly with distance. The distance of the tree from your home (30 feet) also suggests that the roots are not major culprits. At that distance the roots should have tapered to a smaller size which would not be as damaging. It is often reported that 95 percent of tree roots are in the top 3 feet of soil, and 90 percent in the top foot. There are deeper roots, but relatively few roots below the depths described above. This also provides evidence acquitting the roots as likely culprits. I prefer to recommend that flower beds and other plantings be kept a couple of feet from the foundation and that water be diverted from soil near the foundation. In our dry climate, that will discourage growth of roots at the foundation and may have some beneficial effect in discouraging termites, as termites, like roots, thrive moist soil.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.