Although trees are generally a desirable feature of home landscaping, they can pose a threat to buildings in a number of different ways. Inspectors may want to educate themselves about tree dangers so that they can inform their clients about potentially dangerous situations.
Tree Roots and Foundations
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.
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.
Trees that are too close to buildings may be fire hazards. Soffit vents provide easy access for flames to enter a house.
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 of maple, aspen, ailanthus and willow 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;
Advanced decay (wood that is soft, punky or crumbly, or a cavity where the wood is missing) can create a serious hazard. Evidence of fungal activity, such as mushrooms, conks and brackets growing on root flares, stems or branches are indications of advanced decay. A tree usually decays from the inside out, eventually forming a cavity, but sound wood is also added to the outside of the tree as it grows. 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;
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. The general growth-form of the tree and any uplifted soil on the side of the tree opposite the lean provide clues as to when the lean developed.
Tips that inspectors can pass on to their clients:
Binoculars are helpful for examining the higher portions of tall trees for damage.
When planting trees, they should be kept far from the house. It is impossible for the homeowner to reliably predict how far the roots will spread, and trees that are too close to a building may be a fire hazard.
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 are capable of damaging 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 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 a potential hazard.
In summary, trees that are too close to buildings can potentially cause structural damage.