Tree Hazards: Roots and Foundations

Trees are generally a desirable feature of home landscaping, but in some circumstances they can pose a threat to buildings. Overgrown, damaged, and carelessly planted trees can all have a negative impact not just on your home’s value. Tree hazards also present health and safety risks to your property.

Contrary to popular belief, InterNACHI has found that tree roots cannot normally pierce through a building’s foundation. They can, however, damage a foundation in the following ways:

• Roots can sometimes penetrate a building’s foundation through pre-existing cracks. The cold climate we have up here in Michigan, combined with the high number of older homes, means that foundation cracks are quite common.
• Large root systems that extend beneath a house can cause foundation uplift.
• Roots can leech water from the soil beneath foundations, causing the structures to settle and sink unevenly.

Tree Hazards: Other Dangers

• Trees that are too close to buildings may be fire hazards. Additionally, trees near power lines present a fire hazard and should be trimmed appropriately.
• Leaves and broken branches can clog gutters, potentially causing ice dams or water penetration into the building.
• Old, damaged or otherwise weak trees may fall and endanger lives and property. Large, weak branches, too, are a hazard, especially if weighed down by ice. 
• Tree roots can potentially penetrate underground drainage pipes, especially when they leak. Water that leaks from a drainage or sanitary pipe can encourage root growth in the direction of the leak, where the roots may eventually enter the pipe and obstruct its flow.
• Trees may be used by insects and rodents to gain access to the building. 
• Falling trees and branches can topple power lines and communication lines.

Structural Defects in Trees

Trees with structural defects likely to cause failure to all or part of a tree can damage nearby buildings. The following are indications that a tree has a structural defect:

• Dead twigs, dead branches, or small, off-color leaves
• Species-specific defects. Some species of maple, ash and pear often form weak branch unions, while some other fast-growing species are weak-wooded and prone to breakage at a relatively young age.
• Cankers, which are localized areas on branches or stems of a tree where the bark is sunken or missing. Cankers are caused by wounding or disease. The presence of a canker increases the chance that the stem will break near the canker. A tree with a canker that encompasses more than half of the tree’s circumference may be hazardous even if the exposed wood appears healthy.
• Hollowed trunks
• Advanced decay (wood that is soft, punky or crumbly, or a cavity where the wood is missing) can create serious tree hazards. Evidence of fungal activity is an indication of advanced decay. Trees with sound outer wood shells may be relatively safe, but this depends on the ratio of sound-to-decayed wood, and other defects that might be present.
• Cracks, which are deep splits through the bark, extending into the wood of the tree. Cracks are very dangerous because they indicate that the tree is presently failing.
• V-shaped forks. Elm, oak, maple, yellow poplar and willow are especially prone to breakage at weak forks.
• The tree leans at more than 15 degrees from vertical. Generally, trees bent to this degree should be removed if they pose a danger. Trees that have grown in a leaning orientation are not as hazardous as trees that were originally straight but subsequently developed a lean due to wind or root damage. Large trees that have tipped in intense winds seldom recover.

Tips for Tree Safety:

• Binoculars are helpful for examining the higher portions of tall trees for damage. 
• When planting trees, they should be kept far from the house. An experienced landscaping professional should be consulted if you have any doubt.
• Do not damage roots. In addition to providing nutrition for the tree, roots anchor the tree to the ground. Trees with damaged roots are more likely to lean and topple than trees with healthy roots. Vehicles can damage a tree’s root system.
• Dead trees within the range of a house should be removed. If they are not removed, the small twigs will fall first, followed by the larger branches, and eventually the trunk. This process can take several years.
• Inspect your trees periodically for tree hazards, especially in large, old trees. Every tree likely to have a problem should be inspected from bottom to top. Look for signs of decay and continue up the trunk toward the crown, noting anything that might indicate potential tree hazards.

While most home inspectors do have basic knowledge of tree health and safety, our primary expertise is in the building itself. Of course, if there are tree hazards, that will be noted on the inspection report. However, for questions about tree health, or advice on where to plant new trees, an experienced landscaping professional is usually your best contact.

The original version of this article can be found on the InterNACHI website here.