{"id":4645,"date":"2026-04-24T14:23:53","date_gmt":"2026-04-24T14:23:53","guid":{"rendered":"https:\/\/teknoprogettisrl.it\/?page_id=4645"},"modified":"2026-04-24T18:52:46","modified_gmt":"2026-04-24T18:52:46","slug":"non-destructive-testing-on-x-lam-timber-structures","status":"publish","type":"page","link":"https:\/\/teknoprogettisrl.it\/en\/services\/site-investigations\/non-destructive-testing-on-x-lam-timber-structures\/","title":{"rendered":"Non-destructive testing on X-Lam timber structures"},"content":{"rendered":"
\"Resistograph
IML Resistograph in use on X-Lam wall with metal connectors \u2014 school building under construction, Frassinoro (MO)<\/figcaption><\/figure>\n

<\/p>\n

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.<\/em><\/p>\n

1. Why X-Lam timber structures require non-destructive testing<\/h2>\n

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.<\/p>\n

Unlike concrete, in timber the most serious damage \u2014 density reduction, internal rot, layer delamination \u2014 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.<\/p>\n

NDT investigations on timber are explicitly required by applicable UNI standards “Cultural heritage \u2014 Wooden artefacts \u2014 Load-bearing structures of buildings \u2014 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.<\/p>\n

2. When NDT investigations on timber structures are necessary<\/h2>\n

There is no universal regulatory obligation, but there are contexts in which investigations are technically necessary or strongly recommended:<\/p>\n\n\n\n\n\n\n\n\n\n
Context<\/th>\nTechnical rationale<\/th>\nReference<\/th>\n<\/tr>\n<\/thead>\n
Building under construction \u2014 moisture detection from infiltration during construction<\/td>\nAssess whether degradation is superficial or has reached the core of the element before it is covered by finishes<\/td>\nStatic testing \/ Site supervisor<\/td>\n<\/tr>\n
Static testing of timber structures<\/td>\nProvide the structural inspector with objective instrumental data on the conservation state of load-bearing elements<\/td>\nNTC 2018<\/td>\n<\/tr>\n
Conservation restoration of historic buildings with timber beams<\/td>\nIdentify elements to be consolidated or replaced without destructive sampling<\/td>\nApplicable UNI standard<\/td>\n<\/tr>\n
Post-infiltration or post-fire checks<\/td>\nQuantify the extent of degradation to define the intervention plan<\/td>\nSite supervisor \/ Insurance survey<\/td>\n<\/tr>\n
Technical due diligence before purchase<\/td>\nAssess the actual condition of existing timber structures before a real estate transaction<\/td>\nTechnical survey<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

3. The three methodologies: principle, instrumentation and standards<\/h2>\n

3.1 Pin penetration tests on timber \u2014 Wood Pecker<\/h3>\n

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 \u2014 measured in millimetres with a resolution of 0.1 mm \u2014 can be correlated with the mechanical properties of the timber and its conservation state.<\/p>\n

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.<\/p>\n

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.<\/p>\n

What is measured: the needle penetration depth (in mm). A low penetration value indicates dense, sound timber. A high value signals density reduction \u2014 possible degradation. The heterogeneity coefficient Cs allows classification of response homogeneity (Excellent < 10% \/ Good 10\u201315% \/ Fair 15\u201320% \/ Poor > 20%).<\/p>\n

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.<\/em><\/p>\n

\"Pin
Pin penetration test (Wood Pecker) on X-Lam structural column<\/figcaption><\/figure>\n
\n
\n
\"Pin
Pin penetration test (Wood Pecker) on X-Lam structural column<\/figcaption><\/figure>\n<\/div>\n
\n
\"Reading
Reading the sclerometer dial: penetration depth measurement<\/figcaption><\/figure>\n<\/div>\n<\/div>\n

3.2 Electrical moisture measurement (according to applicable UNI standards)<\/h3>\n

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.<\/p>\n

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

Applicable DIN standards provide reference values for structural timber installed in different environmental conditions:<\/p>\n\n\n\n\n\n\n\n\n
Service condition<\/th>\nEquilibrium moisture content (% by weight)<\/th>\n<\/tr>\n<\/thead>\n
Enclosed building with heating<\/td>\n9 \u00b1 3%<\/td>\n<\/tr>\n
Enclosed building without heating<\/td>\n12 \u00b1 3% (maximum limit: 15%)<\/td>\n<\/tr>\n
Building with roof, no walls<\/td>\n15 \u00b1 3%<\/td>\n<\/tr>\n
Structures fully exposed to the elements<\/td>\n18 \u00b1 6%<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The electrical method does not provide the absolute internal moisture value but that of the surface layer at the electrode contact points.<\/em><\/p>\n

\"Moisture
Electrical moisture measurement at the base of an X-Lam panel (TESTO 606-2)<\/figcaption><\/figure>\n

Significant variations between adjacent measurements on the same element indicate localised anomalies to be investigated further with the Resistograph.<\/em><\/p>\n

3.3 Resistograph penetrometric inspections (according to applicable UNI standards)<\/h3>\n

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.<\/p>\n

Instrumentation: IML-RESI PD400 (s\/n PD400-0377), calibrated according to applicable ISO standards \u2014 the same instrument used for the investigation campaign documented in this article.<\/p>\n

How to read the resistogram: a flat, constant profile along the entire cross-section indicates homogeneous, sound timber. Sharp drops in the curve \u2014 especially at the core of the section \u2014 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.<\/p>\n

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.<\/p>\n

\"Resistograph
Resistograph IML penetrometric inspection on X-Lam wall panel<\/figcaption><\/figure>\n

4. Real case: NDT investigations on an X-Lam school building under construction (Frassinoro, MO)<\/h2>\n

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.<\/p>\n

4.1 Building description<\/h3>\n

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.<\/p>\n

\"Inspection
Timber-steel structural node: inspection in X-Lam school building under construction, Frassinoro (MO)<\/figcaption><\/figure>\n

4.2 Sampling plan<\/h3>\n

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.<\/p>\n

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).<\/p>\n

4.3 Results of pin penetration tests<\/h3>\n

The mean penetration value recorded across the sample was 18.75 mm, with overall limited scatter. Two elements showed anomalous values:<\/p>\n