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Tree Dangers
By Nick Gromicko and Rob
London 
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, 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. |
Other Dangers:
| 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. |
| Falling fruit, particularly heavy fruit such as walnuts, can damage
vehicles and other property. Fruit and leaf litter can make a mess of
driveways, patios, decks, etc. |
| 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; |
| hollowed trunks; |
| 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; |
| 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. 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
| 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 and other types of damage.
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