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WOOD STRUCTURE ASSESSMENT
Test methods for determining the soundness of wood structural members: This article surveys methods used to test & evaluate the structural integrity of wood-framed buildings where focus is on the condition of structural wood posts, beams and other framing members. We discuss the problems surrounding hidden rot or decay, the presence or absence of moisture or other instrument-detectable clues, and the problem of subjective decisions to replace or not-replace suspect wood structural members. Page top image of micro-drilling, USDA FPL .
This article compares using micro-drilling with other methods to screen both standing trees and wood structural elements for hidden damage. The original authors, Probett et als., propose a technique to provide objective pass-fail data on the condition of in-situ but suspect structural wood using micro drilling to examine wood density. This service uses timber resistance drill technology to profile variations in timber density associated with timber decay. The work is done on site and the results are instant. This technology is objective and evidential in nature and provides assessment as to whether wood is significantly decayed, suspect or suitable for retention.
Also see our insect damage & wood-destroying insect type identification articles beginning at INSECT INFESTATION / DAMAGE and STRUCTURAL DAMAGE PROBING and see TIMBER FRAMING, ROT where we describe the relationship between construction details and the occurrence of structural rot in timber frame buildings.
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Paul Probett, Clinton Craig, Blake Probett, Incodo Forensic Building Specialists 
This article is adapted & expanded from the author's "An Introduction to Micro-Drilling Technology for N. Z. Structural Timber Assessment" and is used with permission. We [DF] have added comments, some illustrations, and additional article citations. At the references section we include a link to the original article as well as contact information for the authors and Incodo Ltd., a Tauranga, New Zealand forensic engineering firm. The original authors' article without the expanded discussion added here can be seen at An Introduction to Micro-Drilling Technology for N. Z. Structural Timber Assessment. Additional comments, illustrations, and technical citations addressing wood structure testing technologies have been added. We and the original authors invite and will reply to reader questions and comments using the comments box found at the end of this article. Initial technical review completed 8/6/201
Introduction: A Brief History of Methods Used for Standing Trees & Wood Timber Assessment
A variety of tools and methods for assessing the condition of standing trees as well as the structural integrity of in-use wood timbers & beams has been in use for decades.
Several methods for obtaining drill samples or core samples from standing trees have long been in use by agencies such as the U.S. Forest Service and agencies in other parts of the world as part of evaluating the condition of standing timber and forests.  In addition a variety of other methods such as ultrasound, stress waves, electrical resistance testing, mechanical coring or boring, visual inspection, and even a simple plastic hammer have been used to evaluate possible decay or insect damage in both trees and other exposed, accessible wood structural elements.
Methods used initially to evaluate standing timbers were quite naturally expanded to permit assessment of the structural integrity and condition of key wood structural members that are completely exposed and thus readily accessible, such as timber-constructed bridges, bridge pilings, telephone poles, railroad ties, as well as engineered lumber such as block-laminated timbers.  and 
The presence of those tools and methods for testing exposed timbers suggested possible applications in the evaluation of buildings and other structures in which their structural wood members may be partly or even completely hidden by finish materials.
More recently, Bohumil Kasal & Thomas Tannert have led research on the design of microdrilling methods and on the reliability of such tests in predicting the actual bending or breaking strength of structural wood members.
It was Kasal who developed recent technology of taking micro specimens of wood that in turn might be used to obtain direct measurements of the modulus of elasticity and strength of wood (in the area tested).
A direct measurement of the tension properties of wood along the wood fibers permits bending strength evaluation - important because bending strength is a key property in evaluating the structural integrity of a wood beam. Kasal's significant observation was that
Here, in expansion of an original article by Probett et als , here we discuss the range of structural wood beam or timber assessment methods where wood beams, timbers, or other structural members are used in buildings and where there is known or suspected risk of structural damage from leaks, decay by wood rotting fungi, or damage by wood destroying insects.
Because Probett et als propose and discuss the applicability of micro-drilling test methods for assessing wood structural members in-situ with or without having to perform more extensive demolition to actually expose the members, special attention is given to this method of structural assessment of wood, including Probett et als' test protocol and progress in evaluating the efficacy of this method.
That 2005 study provides important independent experience and assessment of the micro-drilling approach to structural wood testing.
As you will read in our inserted opinions and warning remarks in the article below, wood testing equipment should not be used by itself to form conclusions about a structure. Excerpting from the Brashaw micro-drilling study conclusions:
Why perform in-situ timber or wood structural member assessment?
In situ Timber Assessment Compared with Other Current Bases for Wood Structural Member Assessment
These conditions result in two significant error risks:
In situ Timber Assessment Impact on Government Departments and Local Authorities
Current In-situ Wood Beam, Timber, or other Wood Framing Member Structural Condition Assessment Methods
Current choices for assessing the structural integrity of wood timbers, beams, or other framing members include the following:
Visual Inspection Methods for Wood Structural Member Assessment
Note: I [DF] agree that the actual visual inspection of building conditions, lacking "X-ray vision", is limited to external or surface observations. However a skilled and experienced observer can predict areas where hidden damage is most likely at a particular building by considering building and site history, construction materials, construction methods, and by familiarity with construction details that tend to lead to building leaks, moisture problems, rot, damage. Therefore I have added these comments. Also see ADVANCED INSPECTION METHODS "Developing your X-Ray Vision" - DF
More-expert inspectors make limited use of instruments such as moisture meters, infra-red scanners & thermography, and more significantly, by familiarity with construction methods and materials, building science, and building failures, an expert inspector can focus attention on areas where hidden damage is most likely to occur. But as we and the USDA FPL warned at the start of this article, an expert who approaches building assessment most effectively does not allow the use of an instrument to substitute for a thorough visual inspection and site history recording.
Microbiologist / Mycologist Methods for Wood Structural Member Assessment
Fractometer Methods for Wood Structural Member Assessment
Pilodyn Methods for Wood Structural Member Assessment
Micro-Drilling: Resistance drill Methods for Wood Structural Member Assessment
Additional Wood Decay or Damage Testing Instruments / Methods
Bob Monk, USDA Forest Service, discussed the use of various types of equipment capable of detecting decay in standing trees - the epitome of in-situ testing. Monk described and tested an extensive list of tools and methods for assessing the condition of trees, possibly also used for structural wood member evaluation. Monk's USDA article expands upon the types of options and test instrument applications discussed here. The following table of tree decay detection equipment is adapted from Mr. Monk's USDA article  - DF
Moisture Meters Useful in Spotting Areas of Extra Risk of Structural Rot Damage
This tool accepts a wide range of input sensors including 16+ variations on pin type electrodes for measuring moisture in wood. Quoting from the manufacturer:
Multifunctional measuring instruments with digital precision without the measured value drift disadvantage of analogue instruments. SDI input for serial sensors such as temperature, relative humidity, flow speed and destruction-free humidity measurement. In addition to the Trotec sensors you can not only connect electrodes from other manufacturers to the device with adapter cables but [the tool] will also be able to combine future sensor developments with the T2000. 
We find the “lollipop” dielectric head particularly useful and the microwave head is of some use.
The unit also comes with a temp/humidity probe that is only 4 mm diameter and we use it on cavities and to determine moisture in concrete.
Interestingly the Trotec Manual also includes research from Aachen Univ that allows resistance readings in concrete and other non wood materials to be converted to MC%. - P.D.
For a study testing the effectiveness of several types of moisture meters for finding wet materials or surfaces hidden in wall cavities of a building with known current, recent, and previous leaks see MOISTURE METER STUDY.
We tried pin type and electronic moisture encounter meters as well as more invasive methods such as test cuts in suspect areas, followed by a complete interior surface demolition to disclose actual leak areas, both recent and historic.
Our photo (above left) illustrates the chance involved in using even a long-pin Delmhorst moisture meter to detect an exterior wall water leak that sent water running inside the wall cavity but down the side of the stud.
None of the moisture meters we tested could find this leak until we cut open the wall.
In general, pin type moisture meters work well at producing moisture content profiles in materials that have been directly wet or dampened such as wood or drywall that are directly accessible. Probing deep inside wall cavities, as our photo above illustrates, is a hit-or miss proposition. Electronic moisture encounters work well at producing moisture content profiles on all surfaces and can detect moisture behind a ceramic tile wall. But these devices can be fooled by foil faced insulation, expanded metal lath, piping and wiring.
Also see MOISTURE CONTROL in BUILDINGS- DF.
Are Thermal Images Reliable for Spotting Areas of Extra Risk of Structural Rot Damage or Hidden Mold in Buildings?
OPINION: DF: Thermal images and IR have been widely used with some success at spotting areas of heat loss in buildings and these tools have a longer history of use in examining overheated electrical connections, motors, etc. But for finding hidden mold, thermography is a risky proposition.
Image at left, courtesy Paul Probett, Incodo 
Watch out: in the hands of the un-trained or unscrupulous these and other tools can wreak havoc or harm to consumers. The most egregious instrument snafu I've come across [DF] recently was a Hudson Valley New York "mold remediator" uses an IR camera to tell his clients where the hidden mold is located in their home - it was a modern version of the guy with the light meter who sold people replacement windows by showing clients where their heat loss was occurring - wherever there was light.
I did find areas of basement water entry and moldy insulation - in an area not addressed by the New York mold-thermographer. Details about using thermal imaging to look for hidden mold are at THERMAL IMAGING MOLD SCANS. Also see FIBERGLASS INSULATION MOLD
Paul Probett adds: We had major problems with people buying thermal imagers, using ex military units and making ridiculous claims. om 2008 I gave a power-point presentation  to a conference explaining how IR results can be fudged and I described the limitations of thermal imagers. (Our staff had been through the Infraspection Institute USA on-line course to level 2 the year before).
Mr. Probett's power point presentation on using thermography in building damage or mold surveys is adapted and expanded at THERMAL IMAGING, THERMOGRAPHY
For thermal building heat loss screening see HOUSE DOCTOR, how-to be, and for a more broad look at spotting air leaks and heat losses see AIR LEAK DETECTION TOOLS and HEAT LOSS INDICATORS along with HEAT LOSS DETECTION TOOLS. The photos below illustrate detection of overheating aluminum electrical wiring.
Visual Inspection Compared with Micro-drilling for Wood Structure Assessment
How does Micro Drilling Timber Assessment Work?
The drill is computer controlled with drill bit rotation rpms and linear penetration independent of each other and at pre-set levels applicable to the timber specie. This is a change from older generation microdrills that had rpm and penetration linked and power applied controlled by the operator.
Results are readable in real time via the onboard display as the graph is formed.
Typical testing for 100 mm p.rad framing is about 15 seconds.
Results are also processed according to developed timber specie parameters and appropriate standards (where applicable) so onboard software instantly gives a preliminary pass, fail or retest assessment of timber. Settings are adjustable to assist with the identifying of quality infractions, decay areas and splits or checks.
As highlighted in the 2010 RILEM State of the Art Report – In Situ Assessment of Structural Timber by Kasal and Tannert “ If strength values are require(d) the best drills available on the market are required as well as a high level of education and experience. This, proper education of the user is a critical point before applying resistance drilling” quote
Incodo endorse the above and suggest general usage, adjusting
programmable settings interpretation and moderation of results is critical.
Micro-drilling In Situ Timber assessment
Test Results: Summary
Rilem - State of the Art Report In Situ Assessment of Structural Timber, Bohumil Kasal & Thomas Tannert, Springer 2010. 
In addition reliance has been based on studies by others* involving micro-drilling in Europe, USA and Australia related to structural timber in buildings and bridges. The technology has been used for many years. The big difference is that the PD model is a new generation micro-drill with onboard firmware
*Ref www.printfu.org search “Resistograph” or “Sibtec”
“Wood density is the property which will be used as an example because of its’ close correlation with timber strength (Dadswell and Nicholls, 1959, Elliot 1970) …and because it is widely held by wood technologists to be the single most important wood variable (de Zeeuw 1965) According to Panshin et al (1963) the relationship between wood density and timber strength is expressed by
S = K (D)n
Where S is any of the strength properties, K is a constant, D is density and n is an exponent which depends on the strength property being considered”
- WOOD DENSITY OF RADIATA PINE: ITS VARIATION AND MANIPULATION – D J Cown, FRI
Determining consistency of results using sound timber and establishing preliminary variable settings for software to determine Pass/Warning/Fail, Quality Infraction levels and identification of decay type voids.
Build larger database of sound timber graphic levels to use as controls / base levels.
Phase 3: Outside consultant and further field testing (completed but ongoing consultation)
Consult ex FRI staff and engineers as to draft reports and use of terms.
Determine further testing requirements.
Phase 4: Effect of moisture on consistency and reliability of test results (Completed – report at draft stage as at 05/12)
Assessment of impact of varied moisture levels on microdrilling results.
Undertake in-house testing using resistance moisture meter, H3.1 framing (to ensure no internal decay to skew results), Contherm drying oven, etc. Limited to 4 samples dried from 88-100%+MC to 12-14% Results (currently being tabulated)
Preliminary indications are that moisture level effects are limited and do not impact on micro-drilling assessments sufficiently to be of concern. (likely reason is considered to be that drill readings are a resistance reading and this is a combination reading of lateral penetrative resistance as well as shear loads required by the bit tip to slice through timber fibres at various angles to fibre orientation eg shearing occurs parallel and at random angles to timber fibre direction )
Sample 2 graph at left, at 88.0% Moisture Content (MC) indicated
and next graph (below) at indicated 14.4%MC.
Readings taken from new SG8 pinus radiata using Protimeter resistance meter.
Readings taken from points at 20mm centres, along plane of timber grain to ensure minimal impact to test in regard to variance of earlywood and latewood.
Net result indicates minimal graphical difference values between high and EMC (equilibrium moisture content) approximated moisture levels.
Comparison between test results and timber testing properties in accord with NZS: 3622:2004.
Further testing involving taking samples from critical points in sample(s) identified by test graphs, where there is a transition from sound to decayed wood and determining the degree of correlation to timber strength when tested for compression, bending and tension using an IML Fractometer Mark
Programmed testing also involves the same methodology being applied to other timber species as samples become available and demand and demand for micro-drilling testing is required.
Specific testing involves has involved sourcing dendochronological level dry wood core drills capable of extracting parallel to grain cores to test bending stiffness.
Microbiological comparison of samples and phase 5 results by independent laboratory such as Biodet.
NOTE: The value of this testing is being considered and is not currently scheduled.
Arguably the decision to retain or replace building elements in leaky homes should be driven by timber meeting or failing the requirements of NZS:3602, :3603, :3604 and :3622. These are “strength” related criteria – not decay presence criteria.
As micro-drilling is considered to evaluate strength – it becomes a moot point whether decay impact is a Building Consent / Safe & Sanitary criteria that warrants consideration.
Argument in simple terms is - if the timber meets Building Code requirements, that is all that is required to meet for compliance.
This is the same for supplied timber for remediation which is tested for strength only.
Presence of incipient, surface or minor decay is irrelevant to decisions to retain or replace as effective remediation practice should ensure future moisture levels remain at or below 18% at which level any remaining timber decay fungi reportedly cannot thrive.
Current best practice remediation also includes in situ preservative treatment and removal of decayed timber which severely limits opportunities for framing to become structurally non-compliant at a later date. (If this was not the case complete exterior frame replacement should be the norm)
Assess building ready for extensive remediation and reclad using NDT moisture meter to help determine focus points from inside only and using micro-drill to test largely through external wall skirtings and or window liners etc
Note drill used in concert with metal detector/ stud finder for safety etc
Compare report(s) with actuals after cladding removed
Watch out: for moisture meter limitations: we [DF] have tested and demonstrated that while moisture meters are an invaluable tool in assessing building conditions, none of the moisture meter designs presently on the market can reliably detect all wet or leak in conditions in all sorts of building structures and cavities.
Note: Above information is copyright, commercially sensitive and is yet to be peer reviewed. It is not to be copied or circulated without the express and written permission of Incodo Limited. Adapted here by InspectaPedia.com, with permission from Incodo 8/5/2012, technical review is in process.
V4a 06/12 - ####
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