Basics of Wood Inspection: Considerations for Historic Preservation

Wood performs well in structures, often for hundreds of years or more, when it is kept dry and protected from the deleterious effects of moisture and biological deterioration. The open construction typical of historic structures, which provides air infiltration into the structure, makes it possible for the wood to dry quickly if it gets wet. Often this type of construction is criticized for permitting drafts through walls and around openings. On the other hand, this characteristic means that older buildings can breathe in such a way that if wood gets wet, it has a chance to dry. Such openness, of course, is not very energy efficient and has been largely eliminated in modern construction. However, sometimes moisture does get trapped, or the strength requirements of members change. Alterations may have compromised the ability of the wood to dry quickly or may have overloaded the structural members. Architects, engineers, and owners need to know when deterioration has occurred and whether the current strength of the structural members, even when they are oversized and provided more strength than required for the original use, is adequate for implementing a preservation plan for the structure. Wood inspection provides the means to acquire that information. There are three primary reasons to conduct a wood inspection: concerns about moisture and its effects, deterioration (both physical and biological), and a need to determine material properties. Wood behavior is highly variable, due to different wood species, rate of tree growth (typically measured in growth rings per inch), age of the tree, how the lumber was cut from the log, the presence of defects (such as knots), and end-use conditions (interior or exterior use). It is that variability relative to the use of wood that those working on such structures must understand. This article provides the reader with a brief description of how to conduct a basic wood inspection in a historic structure and why such an inspection may need to be done. A list of references with additional information on each topic is provided. Practice Points NUMBER 03