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Wood Deterioration

By Keith M. Bouchard, P.E.

Timber is perhaps the oldest and most widely used building material available.  It grows naturally and is  renewable.  It can have great strength-to-weight characteristics and it is easy to work with.  Unfortunately, like

 any naturally grown organic material, it decays naturally too.  There are an abundance of organisms that feed on wood as part of the natural life cycle of the forest.  Wood that have been harvested and used for buildings will fall prey to these organisms if not properly protected.  On the other hand, wood that has been properly protected can remain sound for centuries.The primary factor separating sound wood from a pile of compost is moisture.  If your building is properly detailed to keep water out then there is very little chance that wood-deteriorating organisms will affect the structure.  However, any prolonged leak, condensation, or other source of moisture will be an invitation to a host of fungi and insects looking for a meal.  The most common of these organisms in the Northeast are the following:

Fungi

  • Brown Rot: This type of deterioration is caused by fungi that attack the cellulose in wood fiber, giving the damaged wood a brown hue.  This is the most common form of wood deterioration from fungus and includes “dry rot.”  Dry rot is a bit of a misnomer as it does require the presence of some moisture.  The

     fungus has the ability to transport water though strands in the affected timber allowing it to attack seemingly dry wood.

  • White Rot: The fungus causing this type of rot attacks both the cellulose and lignin in the wood fiber,leaving the affected wood with a blanched appearance.  White rot is more common in hardwoods, which are typically used for flooring or furniture.  Soft woods are typically used in structural applications and these are more prone to dry rot.

Insects

  • Termites: While not as big of a problem in the Northeast as in warmer climates, termites can still have a major impact on wood buildings with moisture issues.  The termites create colonies within the wood, destroying it from the inside.  They create long dropping-filled feeding galleries within the wood that are generally oriented parallel to the wood grain.  Active colonies are sometimes recognized by mud trails leading from the affected timber.
  • Carpenter Ants: Carpenter ants create nests in wood, though they do not feed on it.  Ants

    create tunnels that are generally across the grain of the wood and are free of frass (droppings).  Some forms of carpenter ants may nest in dry woods, however, this is rare in the Northeast.

  • Powder-Post Beetles: These beetles bore holes in moist wood to feed and lay their eggs, creating an ongoing infestation.  These infestations are apparent by small round bore holes present at the exterior of the wood with a trail of sawdust leading from them.  There are various species of this beetle, some of which will attack hardwoods while others attack softwoods.

Though not a comprehensive list, these are the main culprits leading to wood deterioration in the Northeast.  The best way to avoid damage to your building is by maintaining a tight building envelope that screens and sheds water away from vulnerable elements.  However, if rot or an infestation are detected early, there are some chemicals that may neutralize the organisms to prevent further damage.  Boric acid treatments, either in the form of borate rods or Bora-Care surface treatment, can be effective preservatives for damaged woods.  If the damage is beyond repair, replacement of the entire timber element is likely required.  Don’t be hesitant to replace the damaged material with new wood, as long as the repair is properly detailed to stay dry.  After all, the wood does, in fact, grow on trees!  (Sorry for that…)