Q. One of the trees we planted close to our house 20 years ago has gotten fairly large, and we're concerned about the roots damaging our foundation. Will the roots put pressure against our basement wall? Someone else told us that it's not so much the large roots as the change in soil moisture next to a foundation full of plantings that will damage a wall, any truth to that?
A. When you see the damage that a tree’s surface roots can do to a sidewalk, it may seem reasonable to infer they could damage a building’s foundation in a similar fashion. However, most tree roots are found in the top 6 – 18 inches of soil where there is a favorable balance of moisture, oxygen and nutrients. While roots can damage a relatively thin layer of pavement overlying the soil as they grow, most foundations in western Pennsylvania are much deeper. The surrounding soil at such depths stays too wet for too long a time to support much root growth. And many homes have eaves that keep the soil right near the foundation dry year round, another situation that is not conducive to root growth.
It is true that the change in soil moisture near a foundation is the most damaging, especially in times of severe drought such as we experienced in 1988. Fortunately, it is rare that we experience such severe drought conditions in western Pennsylvania. And as much as gardeners complain about our clay soil, it's much more stable for building foundations than sand is.
Trees and shrubs planted near a foundation can compound the situation because they will take up any available moisture during such major drought events. When the soil around a foundation dries out completely, it shrinks away from it and makes the foundation less stable.
When planting trees near your house, it best to plant them away from the house at a distance slightly greater than the expected crown radius at maturity. This not only keeps the majority of roots away from the foundation, but also keeps branches from damaging the siding or roof as they move in the wind.
Of course, you will need to do a little research on that cute little treasure you just bought at the nursery if you are unsure of its mature size. Better yet, do your research BEFORE you visit your local nursery so you do not wind up falling in love with a tree that you do not have room to accommodate when it is mature.