Measuring Moisture in Timber: Choosing the Right Method

Moisture content (MC) is the ratio of the mass of water in a piece of timber to its oven-dry mass, expressed as a percentage. It is defined in AS/NZS 1080.1 Timber - Methods of test - Moisture content as:

This simple relationship underpins almost every compliance and performance check in timber construction.

This measure underpins all compliance checks. For example:

• Structural framing is typically limited to ≤18% MC at installation.
• Appearance products such as linings and joinery should be in the 12-14% MC range.
• Flooring and cladding are usually acceptable at 10-18% MC depending on species.

Fibre Saturation Point (FSP)

The Fibre Saturation Point is the threshold between “free water” and “bound water” in wood. 

• Above the FSP of approximately 26-30% MC (depending on the species), cell walls are saturated and additional water fills cell cavities without affecting dimensions or strength. This is shown in Figure 2a.
• At the FSP, cell walls are exactly saturated, creating the upper bound of swelling. This is shown in Figure 2b. Note the geometric consistency between Figure 2a and 2b.
• Below the FSP, moisture loss occurs from the cell walls themselves, causing shrinkage and increasing strength and stiffness.

 Most moisture-related movement occurs as timber cycles between the FSP and its equilibrium with ambient conditions.

Figure 1: Fibre saturation point in wood cells

Equilibrium moisture content (EMC)

The EMC is the point where timber is neither gaining nor losing moisture relative to its environment. That is, the timber has reached a state of equilibrium with its environment and is no longer undergoing any consistent directional change in moisture content (small diurnal and seasonal variations are expected). EMC varies by climate and exposure: 

• Coastal and humid regions  13-16%
• Dry inland regions:  8-12%
• Climate-controlled interiors 8-10%

EMC is the benchmark against which field meter readings should be interpreted. It is also the target moisture level timber should reach before installation. Installing material significantly above or below local EMC leads to swelling, shrinkage, and distortion as the timber adjusts to its environment.

Practical Implications

• Compliance: AS 1684 and the NCC rely on moisture limits that assume timber is near EMC for its service environment.
• Durability: Timber stored or enclosed above EMC can remain above the decay threshold (> 20 % MC) long enough to enable fungal growth.
• Calibration: All field meters (pin or pinless) are ultimately calibrated against oven-dry MC values derived at known EMC conditions.

Accurate understanding of these baseline concepts allows the correct interpretation of readings from any measurement device.

Resistance meters measure electrical resistance between two insulated steel pins driven into the timber. Because dry wood is a poor conductor, resistance falls sharply and predictably as moisture increases. The meter converts this resistance into a percentage moisture content, usually calibrated against oven-dry test data from AS/NZS 1080.1.

Figure 2: Use and mechanism of pin-type moisture meters

Pros

• Provides quick, quantitative readings for most softwoods and many hardwoods.
• Can reach moisture below the surface when used with long insulated probes.
• Well suited for verifying structural framing, engineered timber, or concealed members before enclosure.
• Readings correlate strongly with oven-dry results when correctly calibrated.

Cons

• Species-dependent
  • Dense hardwoods and resinous species (e.g. Spotted Gum, Ironbark) require calibration corrections; otherwise readings may understate actual MC by up to 4-5 %.
• Temperature-sensitive
  • Cold timber gives artificially low readings unless adjusted.
• Invasive
  • Leaves small pinholes that may be undesirable in exposed or appearance-grade elements.
• Measures moisture only between the probe tips
  • Significant gradients may exist beyond the test zone.

Calibration and Corrections

Every resistance meter should be used with the manufacturer’s correction tables or digital settings for both species and temperature:

Meters should be verified at least annually by comparing readings on known samples against oven-dry test results (AS/NZS 1080.1). Periodic calibration maintains confidence in field data and prevents drift between instruments.

Best Practice for Reliable Readings

• Drive pins to at least one-third of the section depth (or as deep as possible for large members).
• Avoid knots, resin streaks, fasteners, and surface coatings.
• Take multiple readings along different boards or elements and average results.
• Record species, temperature, humidity, and location with each measurement for traceability.
• Store meters in dry, stable conditions and check battery health regularly; low voltage affects resistance measurement accuracy.

Field Tip

In practice, resistance meters provide the most reliable in-depth measurement available on site. Use them as your “confirmation tool” after scanning with a capacitance meter, or as your primary device for delivery and pre-enclosure checks.

Capacitance meters, often called pinless meters, determine timber moisture content by measuring changes in its dielectric constant. The device emits a low-frequency electric field from a flat sensor plate placed against the timber surface. As the timber’s moisture content changes, so does its ability to store an electric charge. The meter converts this change into a percentage moisture value, typically calibrated against oven-dry reference data from AS/NZS 1080.1.

Figure 3: Use and mechanism of capacitance-type moisture meters

Pros

• Non-destructive: leaves no pinholes or surface marks.
• Fast and simple: ideal for scanning large areas such as flooring, cladding, or mass-timber panels.
• Effective screening tool: quickly identifies high-moisture “hot spots” before targeted testing with a resistance meter.
• Useful on finished surfaces or when repeated testing is required during drying or maintenance inspections.

Cons

• Limited penetration depth
  • Typically 5 - 20 mm; internal moisture beyond this depth may go undetected.
• Surface condition sensitivity
  • Rough-sawn faces, coatings, or high surface resin content can distort readings.
• Species and density dependence
  • Dense hardwoods reduce field penetration and can under-report MC.
• Calibration drift
  • Readings may deviate over time without periodic verification.

Calibration and Corrections

• Species calibration
  • Select or input the correct density group (if the meter allows) or apply manufacturer correction charts.
• Surface preparation
  • Ensure firm, full-contact placement on clean, dry surfaces; avoid gaps or surface irregularities.
• Reference checks
  • Periodically compare readings against known-MC samples or oven-dry results to confirm accuracy.
• Depth awareness
  • Remember that capacitance meters primarily measure surface moisture; when readings exceed thresholds, confirm with a resistance meter to assess core moisture.

Best Practices

• Take several scans across the surface in both grain directions and average results.
• Use capacitance meters as a first-pass screening method, not as sole confirmation of compliance.
• Mark or log high-moisture areas and re-check with a resistance meter for verification.
• Maintain consistent contact pressure to avoid variation between readings.
• Keep the meter clean and free of resin or dust build-up that can affect sensor response.

Field Tip

Capacitance meters are best viewed as the non-invasive scout in the moisture-measurement toolkit. They excel in identifying zones of elevated moisture during inspections of flooring, cladding, and mass-timber panels, especially where surface finish or appearance precludes pin testing. However, always follow up anomalies with resistance or oven-dry verification.

The oven-dry method determines the true moisture content of timber by measuring weight loss as all water is removed under controlled conditions.

A small sample is weighed, dried in an oven at a prescribed temperature until its mass remains constant, and then re-weighed. The difference between the initial and final mass represents the water content.
The oven-dry method is defined in AS/NZS 1080.1 Timber - Methods of test - Moisture content, which forms the benchmark calibration reference for all resistance and capacitance meters.

Figure 4: Oven baking of E. Nitens samples. Sirswal et al.

Pros

• Most accurate method (±0.1%).
• Reference method used to calibrate all field meters.
• Recognised for QA documentation, certification, and dispute resolution.

Cons

• Destructive - samples cannot be reused.
• Slow - requires 24-48 hours of drying at 103 ± 2°C until mass stabilises.
• Requires laboratory oven and weighing equipment; impractical for on-site use.

Applications

• Used by laboratories, certifiers, and researchers when precise MC data is essential.
• Appropriate for testing sample offcuts or when compliance or warranty claims are disputed.
• Recommended for calibrating site meters where unusual readings occur (e.g. dense hardwoods, engineered products).

Ensuring Representative Results

• Use at least three samples per batch or moisture condition.
• Compare with field-meter readings to develop project-specific calibration factors.
• Document all test conditions (oven temperature, air circulation, initial MC estimate) for repeatability.
• For composite or engineered timbers, avoid adhesive lines or include multiple lamella samples to capture variation.

Field Tip

When in doubt, the oven-dry method provides the final word on moisture content. It should be used to resolve ambiguous or borderline site readings, or when establishing calibration baselines for new species, engineered timbers, or imported materials.
 

No single measurement method suits all situations. The choice depends on the purpose, level of accuracy required, and whether testing is being done on-site or in a laboratory.

• Framing timber at delivery or on-site - use a resistance (pin-type) meter, taking corrected readings across representative members.
• Mass timber panels, flooring, or cladding - start with a capacitance (pinless) meter for scanning, then confirm with pin-type tests at critical locations.
• Quality assurance, disputes, or research - confirming with the oven-dry method as the definitive reference standard (AS/NZS 1080.1).

Best practice is to use meters in combination. For example, capacitance meters can quickly locate high-moisture zones, which can then be tested with pin meters to confirm moisture depth and level. Where uncertainty remains, oven-dry testing should be undertaken.

Using Thermal Imaging as a Supplementary Tool

Thermal imaging can be a valuable non-contact diagnostic method for locating areas of potential moisture accumulation before using moisture meters. Moist areas in timber components generally appear cooler than surrounding dry areas due to evaporative cooling and higher thermal conductivity.

Applications:

• Identifying hidden leaks behind linings, in roof spaces, or under flooring before invasive testing.
• Scanning mass timber panels for localised cooling patterns that may indicate moisture ingress.
• Assessing differential drying across large CLT or framing assemblies during enclosure.

Practical guidance:

• Modern phone-mounted thermal cameras are effective for field diagnostics, though limited in temperature resolution. They are best used for pattern detection, not quantitative measurement.
• Always correlate thermal anomalies with follow-up readings from resistance or capacitance meters to confirm moisture presence.
• Avoid direct sunlight or strong heat sources during imaging, as these can mask or exaggerate thermal contrasts.
• For accurate interpretation, ensure surfaces are thermally stabilised, ideally early morning or late evening when gradients are modest.

Method Comparison

Field Comparison

Even with the right tools, moisture measurement errors are common. The reliability of results depends on understanding the equipment’s limitations, applying proper corrections, and documenting readings in context. 

Common Mistakes

• Ignoring species calibration
  • Resistance and capacitance meters read differently depending on timber density; failing to apply correction factors can shift results by ±3-5%.
• Not correcting for temperature
  • Cold timber may appear drier than it really is.
• Measuring only at the surface
  • Moisture gradients often exist; surface readings can understate internal moisture levels.
• Using non-calibrated meters
  • Meters drift over time and need periodic recalibration.
• Relying on a single reading
  • Isolated results may not represent the whole load or assembly.
• Interpreting thermal images as proof of moisture
  • Thermal variations may result from shading, airflow, or materials, along with moisture. Thermal imaging should be used as a screening tool, verified with a moisture meter.

Best Practices

• Use a two-method approach. Combine capacitance scanning and pin-type testing for both surface and depth information.
• Check calibration frequently. Compare readings from different meters on the same sample; large deviations signal the need for recalibration.
• Log all readings. Maintain a simple spreadsheet or digital app with date, RH, temperature, and meter model, helpful for handover records or warranty documentation.
• Monitor over time. Where elevated readings are found, repeat measurements after 24-48 hours of ventilation or heating to assess drying trends.
• Verify near threshold values. If readings are close to NCC or AS 1684 limits, confirm by oven-dry testing.
• Protect meters. Store instruments in stable humidity and temperature conditions; replace probe pins when corroded or bent.
• Use thermal imaging strategically. Perform scans early morning or evening when thermal gradients are moderate. Mark anomalies and re-test with a meter.
• Report findings in context. Include reference to the site’s expected EMC range and service class when communicating results to clients or certifiers.

Calibration Drift and Quality Control

Over time, moisture meters (especially capacitance models) can drift due to changes in internal circuitry, wear of sensor coatings, or environmental exposure. Even minor drift (1-2 % MC) can lead to non-compliance if left unchecked.

• Keep a reference sample of known MC (verified by oven-dry testing) to check calibration monthly.
• Record deviations and, where necessary, apply an offset correction.
• For larger organisations or recurring projects, maintain a calibration logbook noting test dates, reference conditions, and corrections applied.

Field Tip

Always interpret readings relative to species, climate, and intended service condition. For example, a reading of 14 % MC might be ideal for interior framing in Adelaide but too high for a conditioned space in Sydney. Use local Equilibrium Moisture Content (EMC) values as the benchmark, and verify results against NCC 2022 and AS 1684 requirements before proceeding.

Moisture readings have little meaning without context. Correct interpretation requires considering timber species, product type, and climate conditions at the time of measurement. Timber is continually seeking its equilibrium moisture content (EMC) with surrounding air, and readings should be assessed against what is normal for that environment.

Species Considerations

Different species have unique cell structures and densities, which affect how moisture is stored and how meters respond.

Engineered Timber and Composite Products

For laminated or adhesive-bonded members, moisture distribution can be non-uniform:

• Adhesive lines act as partial vapour barriers, trapping moisture in the core during drying.
• Capacitance meters are limited to surface readings (5-20 mm depth), which may not represent the internal state.
• Resistance meters may bridge across glue lines, creating erratic readings. In such cases, interpret values conservatively and verify with oven-dry testing of core offcuts if discrepancies arise.

Climate Considerations

Timber’s moisture content trends toward the Equilibrium Moisture Content (EMC) dictated by local temperature and humidity. This relationship is illustrated below.

For example, flooring installed at 16% MC in Sydney may later dry back to 10-12% indoors, causing shrinkage if expansion joints are not detailed. Conversely, timber brought from a dry inland mill at 10% MC may swell when installed in coastal locations.

Interpreting Field Readings

When evaluating site readings:

• Compare with EMC expectations
  • If readings are within ±2 % of local EMC, the timber is in equilibrium.
• Check the service class
  • Dry-service applications (Class 1–2) expect ≤ 18 % MC; exterior sheltered elements (Class 3) can tolerate slightly higher values.
• Assess gradients
  • Surface and core differences greater than 3 % indicate incomplete conditioning or residual moisture from wet exposure.
• Investigate anomalies
  • If isolated members read higher than others, assess for leaks, trapped water, or poor ventilation.
• Consider time of year
  • Readings taken in summer (high humidity) or winter (low humidity) may deviate seasonally by up to 3 %.

Field Tip

A reliable rule of thumb:

Timber should be installed within ±2 % of the expected EMC for its service environment.
Deviation beyond this range warrants investigation or acclimatisation prior to enclosure.

Accurate measurement of timber moisture content underpins every aspect of durability, serviceability, and code compliance. When readings are taken carefully, recorded systematically, and interpreted in context, they provide confidence that the structure will perform as designed for its full service life.

Moisture control is a construction and maintenance responsibility. Consistent, calibrated measurement protects projects from long-term decay, deformation, and costly remediation.

Essential Practices

• Use more than one method. Combine scanning and depth measurement to verify results.
• Calibrate regularly. Reference to AS/NZS 1080.1 ensures consistency across projects.
• Record context. Note temperature, relative humidity, species, and test location with every reading.
• Interpretation against EMC. Compare results to the expected equilibrium range for the local climate.
• Verify readings near thresholds. Where readings approach NCC or AS 1684 limits, confirm by additional readings and methods, or perhaps by oven-dry testing.
• Act promptly. Elevated moisture should trigger drying, ventilation, or enclosure delay until levels stabilise.