Non-destructive testing (NDT) on glued laminated timber structures and X-Lam panels allows assessment of the conservation state of structural timber elements without extracting samples or compromising the integrity of the cross-section. Teknoprogetti Engineering Srl carries out instrumental testing campaigns combining pin penetration tests, electrical moisture measurement and Resistograph penetrometric inspections on load-bearing timber elements in new construction, existing buildings or conservation restoration projects, in the Milan-Brianza area and throughout Italy.
1. Why X-Lam timber structures require non-destructive testing
Cross-Laminated Timber (X-Lam) has become in recent years one of the most widely used structural materials in school, residential and public construction. Its light weight, fast assembly time and good seismic performance make it advantageous over reinforced concrete, but its organic nature exposes it to risks that RC does not face: moisture variations, water infiltration, biological degradation from fungi and wood-boring insects.
Unlike concrete, in timber the most serious damage — density reduction, internal rot, layer delamination — is not detectable by obvious surface signs. An X-Lam panel with internal moisture content above 20% may appear perfectly intact on the outside. This makes instrumental non-destructive testing not an optional check, but the technical tool necessary to determine the actual conservation state of the element.
NDT investigations on timber are explicitly covered by certain UNI standards “Cultural heritage — Wooden artefacts — Load-bearing structures of buildings — In-situ inspection for the diagnosis of elements in use” and form an integral part of the static testing and construction management process for timber structures subject to an investigation plan.
2. When NDT investigations on timber structures are required
There are various contexts in which investigations are technically necessary or strongly recommended:
| Context | Technical justification | Reference |
|---|---|---|
| Building under construction — moisture detection from infiltration during construction | Assess whether degradation is superficial or has reached the core of the element before it is covered by finishes | Static testing / Site supervision |
| Static testing of timber structures | Provide the inspector with objective instrumental data on the conservation state of load-bearing elements | NTC 2018 |
| Conservation restoration of historic buildings with timber beams | Identify elements to be consolidated or replaced without destructive sampling | Current UNI standard |
| Post-infiltration or post-fire inspections | Quantify the extent of degradation to define the intervention plan | Site supervision / Insurance report |
| DTechnical due diligence before purchase | Assess the actual condition of existing timber structures before a real estate transaction | Technical repor |
3. The three methodologies: principle, instrumentation and standards
Preliminary note: Visual Analysis
Visual inspection consists of observing the structure as a whole and each individual timber element that forms part of it, with the aim of gathering all the information necessary for the investigation. To make this inspection feasible, the timber structure must be accessible and the timber surfaces must be visible.
In addition to an accurate description of the structural type, the following information will be recorded for each timber element: timber species, moisture content, geometry, category, biological degradation (wood-boring insects and decay fungi) and mechanical degradation (fractures, cracks, etc.), connection efficiency.
3.1 Pin penetration tests on timber — Wood Pecker
The timber sclerometer is a penetrometric instrument that drives a needle into the timber tissue with a preset number of blows. The penetration depth of the needle — measured in millimetres with a resolution of 0.1 mm — can be correlated to the mechanical properties of the timber and its conservation state.
The principle is analogous to the Schmidt hammer used on concrete, but adapted to the fibrous structure of timber. The measurement is non-destructive: the hole left by the needle is negligible in size and does not compromise the integrity of the cross-section.
Instrumentation: Wood Pecker mechanical sclerometer.
What is measured: the needle penetration depth (in mm). A low penetration value indicates dense, intact timber. A high value signals density reduction — possible degradation. The heterogeneity coefficient Cs classifies the homogeneity of the response (< < 10% / Good 10–15% / Fair 15–20% / > > 20%).
Warning: the pin penetration test on timber is an indirect test. Penetration values do not directly provide the mechanical resistance of the element. They must be combined with other tests and interpreted by a qualified engineer. Used as the sole tool to determine mechanical properties, it can lead to serious errors.
3.2 Electrical moisture measurement (according to UNI standards)
The electrical method measures the resistance between two electrodes placed in contact with the timber surface. Since the electrical conductivity of timber is strongly influenced by its water content, the resistance is correlated to the percentage moisture content by weight.
Instrumentation: TESTO 606-2 electrical resistance hygrometer, with two electrodes at a preset distance. Resolution 0.1%.
Moisture measurement at the base of an X-Lam wall with TESTO electrical hygrometer: infiltration anomaly
| Service condition | Equilibrium moisture content |
|---|---|
| Enclosed building on all sides with heating | 9 ± 3% |
| Enclosed building on all sides without heating | 12 ± 3% (max limit: 15%) |
| Building with roof covering, without walls | 15 ± 3% |
| Open-air structures exposed to weather on all sides | 18 ± 6% |
The electrical method does not provide the absolute value of internal moisture but that of the surface layer at the electrodes. Significant variations between adjacent measurements on the same element indicate localised anomalies to be investigated further with the Resistograph.
3.3 Resistograph penetrometric inspections (according to UNI standards)
The Resistograph is the most sophisticated diagnostic tool of the three. A 3 mm diameter bit drills into the timber element to a depth of up to 40 cm, continuously measuring the resistance to drilling. Results are displayed as a graph (resistogram) in which the X axis represents the penetration depth and the Y axis a dimensionless resistance index.
Instrumentation: IML-RESI PD400 (s/n PD400-0377), calibrated to DIN EN ISO 9001 — the same instrument used for the investigation campaign documented in this article.
How to read the resistogram: a flat, consistent profile along the entire cross-section indicates homogeneous, intact timber. Sharp drops in the curve — especially at the core of the section — signal zones of density reduction: possible rot, voids, biological attack, or layer separation in the case of X-Lam.
Difference from the sclerometer: while the sclerometer measures an integrated surface response, the Resistograph provides the density profile across the entire cross-section. It is the only NDT instrument capable of diagnosing internal degradation that cannot be reached from the surface.
4. Real case: investigations on an X-Lam school building under construction (Frassinoro, MO)
In March 2025, Teknoprogetti Engineering Srl was commissioned to carry out an instrumental testing campaign on structural timber elements of a school building under construction in the Municipality of Frassinoro (MO), client: Municipal Administration (DET. No. 45 of 05/03/2025). The commission was requested by the Static Inspector and the Site Supervisor following visual detection of moisture from infiltration on certain load-bearing elements.
4.1 Building description
The building consists of a partially underground floor and one above-ground floor. The foundations and the central staircase/lift shaft are built in reinforced concrete. The entire above-ground structure is built with prefabricated X-Lam timber panels. The construction site was still open at the time of the investigations.
4.2 Sampling plan
The campaign covered 20 sampled elements, selected based on their structural position (columns, walls, ridge beams, portal beams, ridge purlins, floor decks) and the presence of visible moisture. On each element, tests were carried out at one or more points, for a total of 31 Resistograph measurements, an equal number of moisture measurements and systematic pin penetration tests.
Types of elements investigated: columns, X-Lam walls, sole plates, floor decking and joists, portal beams, ridge purlins.
4.3 Pin penetration test results
The average penetration value recorded across the sample as a whole was 18.75 mm, with overall limited dispersion. Two elements showed anomalous values:
- Column: one side of the column that remained exposed to rain and sun shows a higher penetration value, with degradation estimated at 10–15 mm surface depth.
- Window jamb: the area below the sill, where infiltration moisture was concentrated, shows significantly higher penetration values with a Cs (heterogeneity) of 83%, indicating strong local inhomogeneity.
Interpretation: the sclerometric degradation is in both cases localised and superficial in nature. It does not indicate a failure of the overall load-bearing capacity of the element, but signals a reduction in surface resistance near the exposed face.
4.4 Moisture measurement results
Almost all the elements analysed showed moisture values below 15% by weight — the limit adopted for the service condition “enclosed building on all sides without heating”. Two significant exceptions:
- Floor decking and joists: moisture detected between 28% and 38%. The element is in contact with straw infill soaked with water from infiltration through the not-yet-completed roof covering.
- Window jamb, area below sill: moisture of 39.6%, confirming the anomaly already identified by the pin penetration test. The area above the sill of the same element records 10.3% — a normal value.
These data confirm that the problem is strictly localised at specific points and not widespread across the entire structure.
4.5 Resistograph inspection results
All 31 Resistograph measurements returned consistent resistographic profiles across the entire cross-section, with no curve drops indicating zones of density decay.
This finding is particularly significant for the elements that showed visible moisture: even in the zones affected by infiltration, the profile was homogeneous from the very first millimetres of drilling (within 5 mm of the surface).
The data demonstrate that the infiltration produced a localised increase in moisture without biological degradation (fungi, rot) having had time to affect the internal structure of the element. The diagnosis made it possible to rule out replacement interventions and to limit the recommendations to controlled drying measures and monitoring.
Frequent errors in the interpretation of NDT investigations on wood
In professional practice, some recurring errors are encountered that can lead to incorrect assessments:
- Using the sclerometric test as the sole diagnostic tool: the penetration value is a comparative index, not an absolute measure of mechanical strength. On its own, it is not sufficient for making structural decisions.
- Confusing surface moisture with structural deterioration: a moisture content of 35% in an element indicates a problem to be addressed, but it does not imply that the load-bearing structure is compromised. Only the Resistograph can rule out internal deterioration.
- Do not consider the season or the building’s climatic history: humidity values must be interpreted in relation to the context (building closed/open, heated/unheated, construction phase/in operation).
- Insufficient sampling: the standard requires a representative sampling plan. Investigating only elements with visible damage does not allow for a reliable assessment of the overall condition.
- Do not fail to distinguish between X-Lam panels and traditional solid timber: in X-Lam panels, the individual layers are oriented orthogonally. A Resistograph test performed parallel to the grain of one layer passes through layers with different orientations, producing resistance variations that indicate the panel’s structure rather than deterioration. The technician’s experience in interpreting the results is essential.
6. Comparison of NDT methodologies on timber
| Method | Depth of investigation | What it detects | Limitations | Relative cost |
|---|---|---|---|---|
| Sclerometry (Wood Pecker) | Superficial (a few mm) | Surface hardness, external deterioration | Does not diagnose the core of the section | Low |
| Moisture measurement (electrical method) | Superficial (electrodes in contact) | Moisture content in the outer layer | Does not measure internal moisture. Relative value, not absolute. | Low |
| Resistograph (drilling resistance measurement) | Entire cross-section (up to 40 cm) | Density variations, internal decay, cavities, separations | Leaves a 3 mm hole (negligible). Requires an experienced technician | Medium |
| Ultrasonic testing on timber | Entire section (indirect) | Elastic modulus, homogeneity | Difficult on irregular sections; highly sensitive to geometry | Medium-high |
7. Implicazioni progettuali: come le indagini NDT influenzano le scelte di intervento
The data provided by instrumental investigations are not an end in themselves, but a tool to guide the technical decisions of the Construction Manager, the Inspector, and the Structural Designer. In the case of Frassinoro, the results made it possible to:
- Exclude the replacement of structural elements: the uniform Resistograph profile demonstrated that the deterioration was exclusively superficial and that moisture had not compromised the load-bearing core of the X-Lam panels.
- Prescribe targeted drying measures: the site management was able to specify appropriate procedures (forced ventilation, periodic hygrometric monitoring) instead of invasive interventions.
- Accurately locate the anomalous areas: the combination of sclerometry and Resistograph testing made it possible to identify the issue only on the area below the window sill, excluding the rest of the element.
- Provide objective documentation for the inspection: the technical report with resistograms and moisture tables forms part of the commissioning documentation, with legal and insurance value.
An NDT investigation campaign on timber does not generate value only when it detects problems: a negative result—an intact structure—is also a highly valuable technical outcome, allowing subsequent design or commissioning phases to proceed with confidence.
8. Operational checklist: NDT investigations on timber structures
Pre-investigation checklist
Before starting an investigation campaign, during testing, and in the reporting phase, there are precise technical steps to follow. The main ones are:
A — Preliminary phase
- Define the purpose of the investigations (commissioning / site supervision / technical appraisal / due diligence).
- Verify accessibility to elements and instrument availability
- + 4 additional checks for the complete checklist
B — During testing
- Measure moisture before pin penetration testing
- Identify and photograph each measurement point with a unique code.
- + 5 additional checks for the complete checklist
C — Processing and reporting
- Correlate sclerometric anomalies with moisture data and resistograph results.
- Produce the technical report with tables, resistograms, and photographic documentation.
- + 5 additional checks for the complete checklist
📥 Download the complete checklist in PDF — 19 operational control points, copyable and adaptable to your investigation plan.
9. Frequently asked questions (FAQ)
Are NDT investigations on timber structures mandatory by law?
There is no general legal requirement comparable to that for reinforced concrete under the 2018 Technical Standards (NTC 2018). However, in the case of the static acceptance testing of timber structures, the inspector may request instrumental investigations to support their assessment. The NTC 2018 require that knowledge of the structure be documented, and NDT tests are the main means of achieving knowledge levels LC2 and LC3 without carrying out destructive testing.
What is the difference between Resistograph and ultrasonic testing on timber?
Both investigate the entire cross-section, but based on different principles. The Resistograph physically penetrates the wood and measures mechanical resistance to drilling, providing a direct density profile. Ultrasound measures the propagation speed of a wave through the material, providing information on elastic modulus and homogeneity. The Resistograph is more suitable for detecting localized decay and cavities; ultrasound is preferable for assessing elastic modulus in regular sections. In many cases, the two methods are complementary.
How many Resistograph measurements are needed for a reliable investigation plan?
There is no standardized minimum number: it depends on the structural typology, the area to be investigated, and the required level of knowledge. As an operational reference, an investigation plan for a medium-sized X-Lam structure typically includes at least 2 drillings per sample element (on opposite sides of the section), with a minimum sampling of 20–30% of the main structural elements. For historic buildings or restoration projects, the plan may require inspections of all visible load-bearing elements.
What is the moisture threshold beyond which a structural timber element is compromised?
Some DIN standards indicate equilibrium moisture content values depending on the usage context. For an unheated enclosed building, the reference limit is 15% by weight. Values consistently above 20% promote the development of staining fungi, while values above 28–30% for prolonged periods can lead to decay. However, a single moisture measurement is not sufficient: it must be interpreted together with the climatic history of the element, ventilation conditions, and Resistograph results, which is the only tool capable of detecting internal deterioration that has already occurred.
Can NDT investigations on timber replace destructive testing?
In many cases yes, and this is precisely their main advantage. UNI standards explicitly recognize non-destructive testing as an in-situ diagnostic method for timber structures. However, as the same standards specify, penetration tests (sclerometry and Resistograph) should ideally be calibrated with destructive tests on sample elements taken on site, when possible. In high-value historic buildings or new X-Lam structures, the exclusive use of NDT is standard practice and is technically accepted.