{"id":4678,"date":"2026-04-24T13:32:57","date_gmt":"2026-04-24T13:32:57","guid":{"rendered":"https:\/\/teknoprogettisrl.it\/services\/site-investigations\/non-destructive-testing-on-x-lam-timber-structures-2\/"},"modified":"2026-05-07T12:53:07","modified_gmt":"2026-05-07T12:53:07","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 variations, water infiltration, biological degradation from 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 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. <\/p>\n

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

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

There are various contexts in which investigations are technically necessary or strongly recommended:<\/p>\n\n\n\n\n\n\n\n\n\n
Context<\/th>\nTechnical justification<\/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 supervision<\/td>\n<\/tr>\n
Static testing of timber structures<\/td>\nProvide the 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>\nCurrent UNI standard<\/td>\n<\/tr>\n
Post-infiltration or post-fire inspections<\/td>\nQuantify the extent of degradation to define the intervention plan<\/td>\nSite supervision \/ Insurance report<\/td>\n<\/tr>\n
DTechnical due diligence before purchase<\/td>\nAssess the actual condition of existing timber structures before a real estate transaction<\/td>\nTechnical repor<\/td>\n

t
\n<\/tr>\n<\/tbody>\n<\/table>\n

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

Preliminary note: Visual Analysis<\/h3>\n

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. <\/p>\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 timber tissue with a preset number of blows. The penetration depth of the needle \u2014 measured in millimetres with a resolution of 0.1 mm \u2014 can be correlated to 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<\/strong>: Wood Pecker mechanical sclerometer.
\nWhat is measured<\/strong>: the needle penetration depth (in mm). A low penetration value indicates dense, intact timber. A high value signals density reduction \u2014 possible degradation. The heterogeneity coefficient Cs classifies the homogeneity of the response (< < 10% \/ Good 10\u201315% \/ Fair 15\u201320% \/ > > 20%). <\/p>\n

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\"Pin
Reading the sclerometer dial during a test on a structural timber beam<\/figcaption><\/figure>\n
\"Lettura
Lettura del quadrante sclerometrico su trave lignea<\/figcaption><\/figure>\n<\/figure>\n

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

3.2 Electrical moisture measurement (according to 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 timber is strongly influenced by its water content, the resistance is correlated to the percentage moisture content by weight. <\/p>\n

Instrumentation<\/strong>: TESTO 606-2 electrical resistance hygrometer, with two electrodes at a preset distance. Resolution 0.1%.
\nMoisture measurement at the base of an X-Lam wall with TESTO electrical hygrometer: infiltration anomaly<\/p>\n\n\n\n\n\n\n\n\n
Service condition<\/th>\nEquilibrium moisture content<\/th>\n

(% by weight)
\n<\/tr>\n<\/thead>\n

Enclosed building on all sides with heating<\/td>\n9 \u00b1 3%<\/td>\n<\/tr>\n
Enclosed building on all sides without heating<\/td>\n12 \u00b1 3% (max limit: 15%)<\/td>\n<\/tr>\n
Building with roof covering, without walls<\/td>\n15 \u00b1 3%<\/td>\n<\/tr>\n
Open-air structures exposed to weather on all sides<\/td>\n18 \u00b1 6%<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

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

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

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. <\/p>\n

Instrumentation<\/strong>: IML-RESI PD400 (s\/n PD400-0377), calibrated to DIN EN ISO 9001 \u2014 the same instrument used for the investigation campaign documented in this article.<\/p>\n

How to read the resistogram<\/strong>: a flat, consistent profile along the entire cross-section indicates homogeneous, intact timber. Sharp drops in the curve \u2014 especially at the core of the section \u2014 signal zones of density reduction: possible rot, voids, biological attack, or layer separation in the case of X-Lam. <\/p>\n

Difference from the sclerometer<\/strong>: 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. <\/p>\n

\"Resistograph
Resistograph IML at the base of an X-Lam wall \u2014 digital display and RES-01 label visible<\/figcaption><\/figure>\n
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4. Real case: investigations on an X-Lam school building under construction (Frassinoro, MO)<\/h2>\n

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. <\/p>\n

4.1 Building description<\/h3>\n

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. <\/p>\n

\"Investigation
Indagine su nodo legno-acciaio nell’edificio scolastico X-Lam di Frassinoro (MO)<\/figcaption><\/figure>\n

4.2 Sampling plan<\/h3>\n

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. <\/p>\n

Types of elements investigated: columns, X-Lam walls, sole plates, floor decking and joists, portal beams, ridge purlins.<\/p>\n

4.3 Pin penetration test results<\/h3>\n

The average penetration value recorded across the sample as a whole was 18.75 mm, with overall limited dispersion. Two elements showed anomalous values: <\/p>\n