Tree Root Damage to Garden Walls

Cause

• Roots do not usually cause a garden wall to crack by forcing their way into the structure, as is commonly thought. While a root may grow into an existing crack and widen it over time, the majority of structural damage is from heaving caused by the gradual growth in circumference of a root. This problem is exacerbated in areas with shrinking and expanding soil types. As roots draw moisture from soil next to a wall during dry periods, the shrinking soil may cause the wall to subside.

Prevention

• The best prevention for root damage is not to plant trees near a garden wall or build a garden wall near trees. Failing that, incorporating expansion joints in the wall will at least limit the section of wall that will need to be replaced. Installing root barriers inside the wall forces new root growth downward under the wall, limiting damage from heaving. Layering foam board underneath the wall also helps, since the roots will crush the foam before damaging the wall.

Repair

• Unfortunately, once tree roots damage a garden wall, the options for repair are limited. The roots may be pruned, the wall may be moved or it may be rebuilt in the same place with the expectation that the damage will reoccur in the future. In either of the first two options, it is beneficial to employ some of the ideas used for prevention as well, such as root barriers or foam board. Incorporating porous gravel around the rebuilt wall may also discourage root regrowth and future damage.

Tree Health

• In situations where neither the tree nor the wall can be moved, consider the impact of root pruning on tree health before deciding whether to prune the roots or remove the tree altogether. Young trees tolerate root pruning better than mature trees, and trees that are sheltered from high winds do better than exposed trees. Trees suffering from disease or defect are unlikely to thrive after root pruning and should be removed instead.