Non-destructive testing on X-Lam timber structures

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 variation, water infiltration, and biological degradation by fungi and wood-boring insects.

Unlike concrete, in timber the most serious damage — density reduction, internal rot, layer delamination — is not detectable from visible 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 essential technical tool for knowing the actual conservation state of the element.

NDT investigations on timber are explicitly required by applicable UNI standards “Cultural heritage — Wooden artefacts — Load-bearing structures of buildings — On-site inspection for the diagnosis of timber members in service” and form an integral part of the static testing and site supervision process for timber structures subject to an investigation plan.

2. When NDT investigations on timber structures are necessary

There is no universal regulatory obligation, but there are contexts in which investigations are technically necessary or strongly recommended:

Context	Technical rationale	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 supervisor
Static testing of timber structures	Provide the structural 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	Applicable UNI standard
Post-infiltration or post-fire checks	Quantify the extent of degradation to define the intervention plan	Site supervisor / Insurance survey
Technical due diligence before purchase	Assess the actual condition of existing timber structures before a real estate transaction	Technical survey

3. The three methodologies: principle, instrumentation and standards

3.1 Pin penetration tests on timber — Wood Pecker

The timber sclerometer is a penetrometric instrument that drives a needle into the wood tissue with a preset number of impacts. The penetration depth of the needle — measured in millimetres with a resolution of 0.1 mm — can be correlated with 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 (EUROSIT, ref. SCL-04), periodically calibrated at SIT-accredited centres and verified with a reference sample at the start of each testing day.

What is measured: the needle penetration depth (in mm). A low penetration value indicates dense, sound timber. A high value signals density reduction — possible degradation. The heterogeneity coefficient Cs allows classification of response homogeneity (Excellent < 10% / Good 10–15% / Fair 15–20% / Poor > 20%).

Note: the pin penetration test on timber is an indirect test. Penetration values do not directly provide the mechanical strength of the element. They must be integrated 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 applicable UNI standards)

The electrical method measures the resistance between two electrodes placed in contact with the timber surface. Since the electrical conductivity of wood is strongly influenced by water content, resistance is correlated with moisture percentage by weight.

Instrumentation: TESTO resistance electrical hygrometer model 606-2 (ref. TER-07), with two electrodes at a fixed distance. Resolution 0.1%.

Applicable DIN standards provide reference values for structural timber installed in different environmental conditions:

Service condition	Equilibrium moisture content (% by weight)
Enclosed building with heating	9 ± 3%
Enclosed building without heating	12 ± 3% (maximum limit: 15%)
Building with roof, no walls	15 ± 3%
Structures fully exposed to the elements	18 ± 6%

The electrical method does not provide the absolute internal moisture value but that of the surface layer at the electrode contact points.

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 applicable UNI standards)

The Resistograph is the most sophisticated diagnostic tool of the three. A 3 mm diameter needle drills into the timber element to a depth of up to 40 cm, continuously measuring the resistance to drilling. Results are returned 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 according to applicable ISO standards — the same instrument used for the investigation campaign documented in this article.

How to read the resistogram: a flat, constant profile along the entire cross-section indicates homogeneous, sound timber. Sharp drops in the curve — especially at the core of the section — signal zones of density reduction: possible rot, cavities, biological attack, or layer separation in the case of X-Lam panels. Comparison between profiles on the same element (side 1 / side 2 / top / bottom) is essential for localising and quantifying the anomaly.

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 that allows diagnosis of internal degradation unreachable from the surface.

4. Real case: NDT investigations on an X-Lam school building under construction (Frassinoro, MO)

In March 2025, Teknoprogetti Engineering Srl was commissioned to carry out a campaign of instrumental investigations on structural timber elements of a school building under construction in the Municipality of Frassinoro (MO), client: Municipal Administration (Resolution No. 45 of 05/03/2025). The commission was requested by the Structural Inspector and the Site Supervisor following visual detection of moisture from infiltration on several load-bearing elements.

4.1 Building description

The building consists of a partially below-grade floor and one above-grade floor. The foundations and the central staircase-lift shaft are in reinforced concrete. The entire above-grade 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 sample elements, selected on the basis of their structural position (columns, walls, sill plates, portal beams, ridge beams, 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 (P1, P2, P8, P12, P16, P20), X-Lam walls (P3, P4), sill plates (P5, P6, P9, P10, P11), floor decking and joists (P7a, P7b, P7c), portal beams (P13, P14, P15), ridge beams (P17, P18, P19).

4.3 Results of pin penetration tests

The mean penetration value recorded across the sample was 18.75 mm, with overall limited scatter. Two elements showed anomalous values:

• P12 (column): one face of the column that remained exposed to rain and sun shows a higher penetration value, with degradation estimated at 10–15 mm in surface depth.
• P20 (window sill reveal): the zone below the window sill, where infiltrated moisture 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. It does not indicate a failure of the overall load-bearing capacity of the element, but signals a reduction in resistance near the exposed surface.

4.4 Results of moisture measurement

Almost all analysed elements showed moisture values below 15% by weight — the limit adopted for the service condition “enclosed building without heating”. Two significant exceptions:

• P7 (floor decking and joists): moisture detected between 28% and 38%. The element is in contact with a straw infill saturated with water from infiltration through the not-yet-completed roof.
• P20 (window sill reveal, zone below sill): moisture of 39.6%, confirming the anomaly already highlighted by the pin penetration test. The zone above the sill of the same element registers 10.3% — a value within the standard range.

These data confirm that the problem is strictly localised at specific points and not distributed across the entire structure.

4.5 Results of Resistograph inspections

All 31 Resistograph measurements returned constant resistographic profiles across the entire cross-section, with no drops in the curve indicating zones of density decay. This finding is particularly significant for elements that showed visible moisture: even in zones affected by infiltration, the profile was homogeneous from the very first millimetres of drilling (within 5 mm from the surface).

The data demonstrate that the infiltrations produced a localised increase in moisture without biological degradation (fungi, rot) having had time to compromise the internal structure of the element. The diagnosis made it possible to rule out element replacement and to limit recommendations to controlled drying measures and monitoring.

5. Common errors in interpreting NDT investigations on timber

• Using pin penetration testing as the sole diagnostic tool: the penetration value is a comparative index, not an absolute measure of mechanical strength. Alone it is not sufficient for making structural decisions.
• Confusing surface moisture with structural degradation: moisture of 35% in an element indicates a problem to be resolved, but does not imply that the load-bearing structure is compromised. Only the Resistograph can rule out internal degradation.
• Not considering the season and the climatic history of the building: moisture values must be interpreted in relation to the context (enclosed/open building, heated/unheated, construction phase/service).
• Insufficient sampling: the standard requires a representative sampling plan. Investigating only visually damaged elements does not allow construction of a reliable picture of the overall condition.
• Not distinguishing between X-Lam panels and traditional solid timber: in X-Lam panels the individual layers have orthogonal orientations. A Resistograph drilling parallel to the fibres of one layer passes through layers with different orientations, producing resistance variations that do not indicate degradation but the panel structure itself. Experienced technical interpretation is essential.

6. Comparison of NDT methodologies on timber

Method	Investigation depth	What it detects	Limitations	Relative cost
Pin penetration (Wood Pecker)	Superficial (a few mm)	Surface hardness, external degradation	Does not diagnose the core of the section	Low
Moisture measurement (electrical method)	Superficial (electrode contact)	Water content in the outer layer	Does not measure internal moisture. Relative value, not absolute	Low
Resistograph (penetrometry)	Entire cross-section (up to 40 cm)	Density variations, internal rot, cavities, delamination	Leaves a 3 mm hole (negligible). Requires experienced technician	Medium
Ultrasonic testing on timber	Entire section (indirect)	Elastic modulus, homogeneity	Difficult on irregular sections, highly sensitive to geometry	Medium-high

7. Design implications: how NDT investigations influence intervention decisions

The data provided by instrumental investigations are not an end in themselves, but a tool to guide the technical decisions of the Site Supervisor, the Structural Inspector and the Structural Engineer. In the Frassinoro case, the results made it possible to:

• Rule out replacement of structural elements: the homogeneous Resistograph profile demonstrated that degradation was exclusively superficial and that moisture had not compromised the load-bearing core of the X-Lam panels.
• Prescribe targeted drying measures: the Site Supervisor was able to specify precise procedures (forced ventilation, periodic hygrometric monitoring) rather than invasive interventions.
• Precisely locate anomalous zones: the combination of pin penetration testing and Resistograph on P20 made it possible to identify the problem on the sub-sill face only, ruling out the rest of the element.
• Provide objective documentation for the structural inspection: the technical report with resistograms and moisture tables forms part of the statutory inspection documentation, with legal and insurance value.

An NDT investigation campaign on timber produces value not only when it detects problems: a negative result — sound structure — is also a high-value technical finding that allows subsequent design or inspection phases to proceed with confidence.

 

9. Frequently asked questions (FAQ)

Are NDT investigations on timber structures legally mandatory?

There is no generalised obligation analogous to that for reinforced concrete structures under NTC 2018. However, in the context of a statutory static test of timber structures, the structural inspector may request instrumental investigations to support their assessment. NTC 2018 (§ 8.7) requires that knowledge of the structure be documented, and NDT tests are the primary means of reaching knowledge levels LC2 and LC3 without carrying out destructive tests.

What is the difference between the Resistograph and ultrasonic testing on timber?

Both investigate the entire cross-section, but with different principles. The Resistograph physically penetrates the timber and measures mechanical resistance to drilling — providing a direct density profile. Ultrasonic testing measures the propagation speed of a wave through the material — providing information on elastic modulus and homogeneity. The Resistograph is more suitable for identifying localised rot and cavities; ultrasonic testing is preferable for elastic modulus assessments on regular cross-sections. In many cases the two tests complement each other.

How many Resistograph measurements are needed for a reliable investigation plan?

There is no standardised minimum number: it depends on the structural type, the surface to be investigated and the required level of knowledge. As an operational reference, an investigation plan for a medium-sized X-Lam structure typically provides for at least 2 drillings per sample element (opposite faces of the cross-section), with a minimum sampling of 20–30% of main structural elements. For historic buildings or conservation restoration works, the plan may provide for inspections on all visible load-bearing elements.

What is the moisture limit above which a structural timber element is compromised?

Applicable DIN standards indicate equilibrium moisture values according to service condition. For an enclosed building without heating, the reference limit is 15% by weight. Values consistently above 20% promote the development of chromogenic fungi, while values above 28–30% sustained over time can lead to rot. However, moisture measured at a single point in time is not a sufficient data point: it must be interpreted together with the climatic history of the element, the presence of ventilation, and the Resistograph results, which is the only instrument capable of detecting internal degradation already underway.

Can NDT investigations on timber replace destructive tests?

In many cases yes, and this is precisely their main advantage. Applicable UNI standards explicitly recognise non-destructive tests as a method of in-situ diagnosis for timber structures. However, as applicable standards specify, penetrometric tests (pin penetration and Resistograph) should ideally be calibrated with destructive tests on sample elements taken on site, where possible. In high-value historic buildings or newly constructed X-Lam structures, the exclusive use of NDT is standard practice and technically accepted.