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Enabling better global research outcomes in soil, plant & environmental monitoring.

Practical application of Micro-Resistance drilling for Timber Inspection

Resistance drilling has been used to inspect trees and timber since 1987. However, several application areas provide specific requirements regarding precision, resolution, and technical application properties of the machine. Meanwhile, nearly ten different resistance drills are available. Because they differ significantly in many properties, it is very important to know and understand the major practical aspects of the applications as described in this paper to be able to select the appropriate drill version for a specific task. The user has to understand that not every resistance drill is a real Resistograph®, because this internationally registered trademark is exclusively allowed to be used for labeling high-resolution precision drills providing a high linear correlation to wood density – the sole base for reliable profile interpretation.


Since 1987, recorded resistance drilling using thin needles has been used by experts to determine the condition of structural timber and joints by measuring density profiles of wood. Experienced users are able to identify decay, insect damage, and cracks in the profile. In addition, the method allows the expert to find invisible beams below floor and inspect hidden timber (Rinn, 1989a). The information obtained on the internal condition of beams and joints helps engineers evaluate historic and modern timber structures (Görlacher and Hättich, 1990) and is the basis for repair planning.

Meanwhile, nearly ten types and generations of resistance drilling machines are available, differing significantly in many ways. Some types are applicable for timber inspection. Unfortunately, many applications of this method do not lead to satisfactory results, because the selected machines are not appropriate for the given task and because of missing knowledge about how to apply and how to interpret the results. Thus, a basic understanding of the method and knowledge about technical properties of the machines is mandatory for being able to properly select the appropriate machine type and generation for any kind of application.

Machine Type Selection

Technical properties of the drilling machine and needle strongly influence the quality and reliability of the resulting profiles. In addition, the applicability on site is determined by size, design, handling and weight of the machine. The economic efficiency of the application is not only determined by the price of the equipment but furthermore by costs per drilling (for paper and needle), speed of drilling, time needed for paper and needle replacement, data management, and maintenance. The available drilling machine types differ to such an extent, that it is very important that before purchasing, renting or using, you clearly analyze which tasks have to be fulfilled by the machine in order to be able to select the appropriate one.

For detecting voids in trees or hardwood timber, a simple machine with low resolution may be enough. If conifers (as full- size or laminated beams) or other softwood species shall be inspected for decay, high resolution and electronically measured profiles with a linear scaled ordinate (y-axis) are mandatory in order to enable the user to distinguish decay from intact soft wood reliably (the same applies for quantitative wood density and tree-ring and analysis). This is because in soft, intact wood the resistance can be lower than in decayed parts of originally higher density. Only the change in the variations between earlywood-latewood-zones can be interpreted in terms of potential presence of decay. If the absolute level of the profile seems to be normal, only the shape of the tree-ring variations may help distinguishing between soft but intact and decayed parts. This requires a high resolution linearly scaled profile. Although most resistance drilling machines currently on the market do not fulfill this condition, the users commonly are not aware of this limitation and are thus very likely to misinterpret the results.

Wilcox (1978) showed that brown rot fungi, for example, can lead to an 80% loss of bending strength in early stages while density is only lowered by about 10-20%. This means that the profiles have to provide high resolution and a linear scaled curve, calibratable to wood density. Otherwise the profiles cannot be used to identify decay of early but dangerous stages. Reliable identification of decay requires knowledge about the typical shape of profiles and intra-annual fluctuations.
However, aside from resolution and precision of the profile, there are many other aspects to be regarded before deciding what machine to take and how to apply. In historic timber structures, for example, it is critical that the drill is as thin as possible in order to be able to reach narrow areas. For this, in addition, the handle and main switches have to be at the end rather than at the front of the machine, otherwise it cannot be operated.

Machine length and drilling depth

In narrow historic roof structures, a thin and short machine is required in order to access many points of interest, such as the joint of rafters and ceiling beams. Most historic beams are less than 30cm in diameter. Thus, a drilling depth of 30cm is sufficient. The same applies to utility pole inspection: mostly, 30cm drilling depth is enough. But, in some cases, 40cm or even 45cm is required because the pole is of a larger diameter or because the drilling has to reach deeper down below ground level. It is also far more convenient if the machine is longer because drilling at the base of the pole can be done while standing. For this application, in addition, it is important that the handling switches are at the end of the drill and not at the front because this makes drilling much more convenient.

Figure 1: This conifer ceiling beam (Pinus sylvestris) was inspected visually and with an endoscope. No decay was found. Weeks later the beam broke – two carpenters fell down one floor, they were not badly injured. A detailed analysis of the beam showed early stages of brown rot in the center of the beam.

For inspecting invisible ceiling beams below floor, the penetration depth mostly has to be around 40cm, if not more, in order to enable the machine to penetrate floor, support beams and then the ceiling beams completely. Inspecting glue-laminated beams often requires drilling much deeper than 30cm because the dimensions of the beams can reach up to 1m, or even more. Many modern structures are built in such a way that inspection of the beams, especially in joints, is quite difficult. Because the beams cannot be accessed from all sides, a resistance drill with long drilling depth is required. But, especially due to the mostly unpredictable and changing orientation of the tree-ring borders within a glue-laminated beam, the needles often do not penetrate straight as soon as drilling depth exceeds 40cm with the standard needles. This is a consequence of the stiffness of the steel commonly used for the 1.5 mm drill bits. If the tree-ring border is not penetrated perpendicular, the needle is deflected from the straight path due to the abrupt density change from earlywood to latewood. The same can happen when the needle hits a knot in an angle different from 90°. In full size beams, the drilling orientation can be adapted to the expected tree-ring geometry in order to ensure a straight drilling. In laminated timber, the tree-ring orientation mostly changes from one lamella to the next. Consequently, deeper drilling more often penetrates tree-ring borders at a non-perpendicular angle. As soon the needle starts deflecting, this trend can even increase and lead to a needle coming out of the wood perpendicular to the original drilling direction.

Figure 2: a more detailed close up of the beam from figure 1, showing the spotted areas of brown rot decay. In this stage only earlywood is effected, as can be seen by the location and size of the decayed areas. Resistance drills thus have to provide high resolution and linearly scaled calibratable profiles to allow the user to identify decay.

This will not normally destroy the needle, but the profile will not contain reasonable information about wood condition because shaft friction dominates the results. However, this can be prevented by observing the sound of the drilling: any severe bending of the needle can be identified by the corresponding noise of increased shaft friction. When this occurs it is better to pull back and start another drilling at another angle or at another spot.
Summarizing experiences from thousands of timber structure inspections, a drilling depth of 40-45cm has been shown to be a good compromise as long as the machine length is less than 70cm. With such a device, most tasks can be fulfilled in terms of machine size and drilling depth. If, occasionally, a much deeper drilling has to be done, machines with more than one meter drilling depths can be rented.
For achieving a straight drilling, especially for pole and gluelam inspection and for drilling at an angle, needles with bigger shaft diameter can be used if the machine offers this feature. The disadvantage is that the damage to the sample is bigger, because the tip of the needle should always be at least twice the diameter of the shaft in order to minimize the influence of shaft friction.
Taking into account all aspects of machine properties and the typical tasks in structural timber inspection, a few drill resistance machines types available on the market are suitable for timber inspection, others may be better for trees.


Two reasons clearly speak for a plastic casing of the resistance drill instead of metal: temperature and electric isolation. The needle can always touch an electric power cable. This may happen in trees as well as in structures, especially if hidden timber is drilled. There is always the chance that an electric cable is attached to the back side of a beam that cannot be seen and checked before drilling. A plastic casing allows for reliable isolation of the potentially electrified needle from the user. In addition, metal cases tend to become too cold in winter and too hot in summer to be held by hand.
Machine handling
In some machines, the main switch to start the drill is at the back end, some have a gun-like handle, others an ordinary drill attached at the front. For drilling utility poles at the base and ceiling beams from below or top, switches at the back of the machine are preferable. Then the operator can still stand while starting, controlling, and stopping the machine.
For many applications, it is critical to be able to operate the machine with one hand while drilling because the other hand is needed to hold oneself on a ladder or scaffold. This requires the switches close to the handle and reachable with the hand that holds the machine.
The force the machine pushes back itself from the wood mainly depends on the ratio between needle revolutions per minute and needle feed rate. With some machines it equals around 10 N, with others it can reach up to more than 100 N. This pressure has to be carried in addition to the machine’s own weight (and vibration) when drilling overhead into ceiling beams, for example. If no tripod is available, the operator has to be strong enough to hold the machine still for the duration of drilling (typically in the order of one minute).

Figure 4: Only short drilling machines can be used in such narrow conditions at the base point of historic roofing, they need to vertically drill though the head of the ceiling beam beside the finger joint connection with the rafter. The same applies to horizontal drilling between ceiling beams at this point to detect the finger within its hole in the ceiling beam.

In many cases while inspecting timber, drilling has to be done in an angle of, for example, 45° to the surface. For these applications, the drill needs to provide a guiding adapter at the front. It is important that this adapter is long enough to guide the needle’s tip at an angle into wood. Consequently, the front-adapter has to be longer than half of the longest side of the front cross section of the drilling machine.
During drilling, the noise of the machine is an important source of information. The variations of the resistance can be tracked by observing the motors noise. However, this is not possible with some machines: being loud by construction and producing vibration during drilling. It is convenient to be able to directly see the profile while drilling, however, in many circumstances, the operator has no way to watch it because he has to hold the device at a height or angle, which inhibits a direct observation of the profile shown in the machine. Because of that, some devices are offered with an external printer that can be put into a position to be observed during drilling. If the printer is connected wirelessly, it is important that the connection is established automatically by the machine and that the machine is able to operate without the printer. Although the machine should not be moved during drilling in order to prevent distortion of the reading, a tripod is usually not required if the operator can hold the machine still.

Power supply

Some machines have an integrated battery and thus are heavier in weight. Other machines are lighter but require a cable connection to a battery pack. The external battery pack usually provides more power and its weight is not of concern because it does not have to be carried while drilling.
When using drills with integrated rechargeable batteries, it is important to check that they are shock-proof and not dangerous if shaken.
While climbing up a tree or scaffolding for inspection, as well as in many other instances, it [battery shock/shaking] may happen even bumping against a structure. In such cases, modern high-capacity lithium-ion batteries can be damaged, and even burn or explode. Because this has happened already several times, strict requirements for transporting such rechargeable batteries have been established (www.iata.org). Thus, only battery types that cannot explode should be used for mobile resistance drills. It has to be taken into account that only some special kinds of modern high-capacity lithium batteries, for example, are shock proof and will not burn or explode when “mistreated” mechanically.


Printout and storage
Amongst other reasons, clients may be present while drilling beams. For analysis and evaluation it was shown to be best to have a profile printout in hand on the spot immediately after drilling, providing a 1:1 scaled curve. At 1:1 scale, the expert can easily and directly evaluate the meaning of the curve – the major measurement result. This can easily be explained and shown to others using a 1:1 scale curve. In addition, a paper printout is always a useful backup in case of any electronic storage problem.
If the machine has an integrated printer, it may be a requirement to change the paper after every drilling. This takes time and may require opening the machine which can result in dust, dirt or rain entering the casing. If the integrated printer is built as a pin scratching on pressure-sensitive paper, it has to be taken into account that the paper has to be handled with care because touching or scratching can “erase” the profile. Thus, the pressure sensitive paper has to be marked and stored carefully in order to protect the profile information (Do not touch!).
In other systems, a thermal printer stores the information on long paper rolls, allowing the user to carry out dozens of consecutive drillings without the need for a change of paper or paper storage. Such a machine may allow for far more than 100 drillings a day because much less time is spent on paper handling. However, in many inspection projects, a majority of time is consumed taking notes and moving from one spot to the next. Since the machines drill much faster [than taking notes], the drilling itself is only a small part of the complete job in terms of time consumption.

Some drilling machine types store each profile in internal memory and/or transmit them via Bluetooth or cable to a portable computer, probably even at the same time.
In timber structures, drilling often has to be done while standing on scaffolds or on a ladder or while crawling and squeezing oneself into narrow edges to reach joints. Consequently, paper handling possibilities are very limited. Interrupting the inspection after every drilling is time consuming and inefficient. The same applies for projects, where many drillings have to be carried out in a short time.

Efficiency and reliability of the application as well as the reporting depends on how quickly the machines can be operated and how they display and store the results. For reporting, digitally stored profiles are preferable because the software can allow printing hundreds of profiles with one click, or they can be saved as a PDF and attached to the report as an appendix.
Because reliable documentation of the obtained results is prerequisite for reliable reports and required not only by law but the engineers insurance, profiles that are only stored on paper should immediately be copied or scanned after the inspection for safe long-term storage.
Experience from thousands of drillings since 1987 has clearly shown that printed profiles, preferably in 1:1-scale, are very helpful and required on the spot for interpretation and evaluation. Such profiles may even be given to clients or other interested people. Digitally stored profiles are best for efficient and reliable reporting.

Figure 7: In both historic and modern structures, it is often a requirement to drill in at an angle. Because the needle is made of a thin, flexible steel, it will not penetrate the wood at such an angle without being guided. The front adapter has to be long enough to hit the corner of the wood while the front edges of the device touch the beams on one or both sides.

Resolution, precision, and linearity
There are basically two groups of resistance drilling machines on the market: electric or electronic recording, and mechanical recording. The first prototype by the inventors [of Resistance Drilling], Kamm & Voss recorded the resistance mechanically with a scratch pin on wax paper contained within the drill. Due to resonance and damping effects, these profiles were not linearly correlated to wood density and therefore could not be interpreted reliably. Because of that, Kamm & Voss developed an electrically recording drill that was then further developed and equipped with electronic regulation and recording.
With these machines it was possible to prove the method’s suitability for tree and timber inspection. These electronically recorded profiles can be linearly correlated to wood density. Within this group of machines, the provided resolution in both drilling depth and measured signal differs strongly, leading to a correspondingly different precision as measured by the coefficient of determination of the correlation to density. Some machines record one value per mm penetration depth, other up to 100. The resistance itself in some machines is digitized in eight bit (0-128), others provide twelve bit (0-4096). Machines with high resolution can reveal tree-ring density variations and incipient decay in contrast to soft intact wood and only such machines are allowed to be labeled with the internationally registered trademark “Resistograph”.