Assessing Historic Structures
For historic buildings and structures, the engineer often relies on current standards and design values to determine the adequacy of wood members to remain in service, but current standards are generally based on lower-quality material than was typically used in historic buildings. Historic structures built before building codes or design values for wood products were established (or structures which lack grade stamps on individual wood members), present a quandary when determining what design values are appropriate. Frequently an assumed species and grade are assigned, only to show that the wood members are structurally deficient, despite the fact that the structure has stood for decades or centuries without failure. The results of assuming a species and/or grade are often an overly conservative estimate of the design values and unnecessary replacement, repair and retrofit decisions along with associated unnecessary project costs and the loss of historic fabric.
Determining the species and grade, however, are not sufficient for assessing the serviceability of wood elements within a historic structure. In addition to determining the species and grade of historic timbers, it is essential that the condition of the timber being graded be known. Deterioration due to decay or insect attack, member failure, mechanical damage or alterations and other conditions can adversely affect the performance of structural timber even though it may appear to meet the requirements for a particular grade. For this, a detailed wood inspection is essential. There are three primary reasons to conduct a wood inspection: (1) concerns about moisture and its effects, (2) the concerns about deterioration (both physical and biological) and (3) a need to know material properties. Of these reasons, determining the grade addresses only the need to know material properties. Wood behavior is highly variable and it is that variability relative to the use of wood in historic structures that it is important to understand. There are numerous references that provide information on the pertinent aspects of a wood condition assessment. The following paragraphs contain a summary explanation of wood condition assessment tools and procedures and are excerpted from an APT Bulletin Practice Points article by Anthony (2007).
All wood is subject to a variety of deterioration mechanisms, the most prominent of which is wood-decay fungi, which can ultimately lead to the inability of structural members to perform their function. Large timbers, depending on wood species, frequently will rot on the interior where absorbed moisture is retained, with no externally visible sign of the deterioration. Moisture absorption though end grain, checks or holes provides a highly favorable environment for decay fungi to attack the heartwood at the center of a large timber. The heartwood (the inner growth rings of the tree) typically has more decay resistance than the sapwood (the outer growth rings of the tree). However, even the heartwood of a decay-resistant species, such as chestnut, when exposed to enough moisture, will decay. Deterioration is a particular concern where the wood is in contact with the ground or other materials, such as porous stone, that may provide for moisture absorption into the wood.
There are three “tools” for a basic wood inspection – visual inspection, a probe (such as a blunt awl) and a moisture meter. Nondestructive evaluation equipment is available that can give much more information about wood condition, but use of such tools should be reserved for situations where a basic inspection cannot sufficiently answer the questions about the wood that need to be addressed by the architect, engineer or owner.
A visual inspection is just that – looking for things that do not appear to be right. Visual examination of the wood allows for identifying components that are missing, broken or in an advanced state of deterioration. Missing components are those which have been removed or have fallen away, frequently due to extensive deterioration. If missing components were intended to provide structural support or protection from the elements (e.g. prevent moisture intrusion), their replacement may be essential to prevent long-term damage to the structure. Missing roof shingles or sheathing that cause roof leaks are common examples of a missing elements that are essential to the long-term preservation of a historic structure.
Visual inspection also allows for the detection of past or current moisture problems, as evidenced by moisture stains on the exposed surface of the wood. Further, visual inspection enables detection of external wood decay fungi or insect activity as determined by the presence of decay fruiting bodies, fungal growth, insect bore holes, mud tubes, or wood substance removed by wood- destroying insects. Visual inspection provides a rapid means of identifying areas that may need further investigation.
Probing the wood with a blunt awl enables rapid detection of voids in the wood that may not be visible on the surface. Internal decay is often masked by the lack of evidence on the exposed surface of the wood. For advanced decay, where large internal voids are present near the surface, probing allows for detection of potentially serious deterioration. Even for the early stage of decay, termed incipient decay, probing is beneficial. Probing can often reveal areas of incipient decay in timber, which has experienced sufficient deterioration due to decay fungi to allow for easy entry of a probe although no void is yet present. Wood without incipient decay tends to offer more resistance to probing due to the higher density and more intact internal wood structure. For internal voids in large timbers, more advanced inspection methods are generally required to detect the void.
The true moisture content of wood can be determined only by oven drying a sample removed from a structure. Fortunately, portable moisture meters are available that allow us to take a reading of the approximate moisture content of wood without removing a sample from the structure. There are two primary types of portable moisture meters. A capacitance-type meter measures the electrical field within a small area of wood and does not require penetration of probes into the wood. It will generally provide the average moisture content throughout a certain depth, typically less than an inch. A conductance-type meter is based on the principal of electric current being conducted through wood between two probes, which come in different lengths, and can be inserted into the wood to various depths. This allows for determining the moisture content at a specific depth. This is particularly useful to determine whether wood is drying or taking up moisture.
Knowing where to inspect and what tools to use depends on the goal of the inspection. The condition of wood components is the most common reason for conducting an inspection. That being the case, it makes sense to look for problems where they are most likely to occur in a structure. A visual inspection will often locate areas that warrant further investigation. Missing or failed components, moisture stains, the presence of fungal fruiting bodies, decayed wood, insect bore holes, mud tubes or frass are indicators of areas that need closer investigation. An inspection should focus on areas where problems are known to be common, such as:
- Wood in ground contact
- Wood that exhibits moisture stains
- Wood with visible decay
- Material interfaces (e.g. wood and masonry), such as beam pockets
- Floor joists and girders
- Roof framing
- Sill beams and plates, particularly when in contact with masonry
- Top plates
- Structural lumber or timbers near openings (doors and windows)
- Crawl spaces and basements
- Areas of the structure that have been modified
- Exterior wood work, including cladding, shingles and soffits