The Resistograph, or resistance drill, is a very effective nondestructive tool for assessing the condition of wooden timbers and trees. I have been using one for over 15 years and recently upgraded to the PD Series, which comes equipped with computer software, excellent graphical imaging, as well as increased efficiency in drilling time. Therefore, as a timber inspector I can gather a lot of field data for my clients in a relatively short period of time. Common structures I inspect are old mill buildings, covered bridges, historical structures, utility poles, marine pilings and foundation pilings, as well as the inspection of wooden members in parks and challenge courses, and even trees.
The drill itself is equipped with a 12" long needle that has a 1/8" diameter tip. There is virtually no damage made to a timber that is sampled. As the drill advances the needle into a timber the resistance that the needle encounters is plotted on a graphical chart. Regions with little to no resistance are generally indicative of hidden wood decay or insect attack. Consistent relative density levels are generally indicative of sound wood. A knowledge of wood and its structure is recommended for interpretation of results. In buildings, several locations can be drilled in a relatively short period of time. The same applies to testing utility poles and pilings.
The drill is very effective for detecting internal voids and advanced decay, but the drill is not as sensitive to detecting early wood decay, or incipient decay, which can only be observed using a light microscope. At present, I am unaware of any non-destructive test equipment that can detect early wood decay. The biggest problem associated with early wood decay is the difficultly detecting its presence in a wooden timber unless you are a qualified inspector. Even at wood weight losses of 2%, strength loss can be as much as 50% where the decay is located. Therefore, I have always recommended to my clients that in addition to collecting Resistograph charts, microscopic analyses should also be periodically performed to verify the presence or absence of early wood decay. To perform a microbiological analysis I simply extract an increment core from a timber in question. The 1/4" diameter hole is then filled with a treated wood plug and can even be further covered with wood filler if needed. I then cut very thin sections of wood along the length of the increment core and examine them with a light microscope. By analyzing the core along the entire length I can document if any early wood decay is present. Generally, my goal in these circumstances is to collect a series of consistent profiles and to be certain that the wood is sound where I have drilled. I recommend a microbiological analysis to be certain.
I can use the drill to map out regions of wood decay in timbers, poles and pilings. Alternatively I have even used the Resistograph to collect data and document good, or sound, conditions in timbers for piece of mind. However, if advanced decay is detected in any particular member then microbiological analyses will likely need to be performed. Especially if the client, or engineer of record, needs to have the ability to make accurate decisions regarding structural repair and remediation.