Static and Conservative Restoration of Wooden Trusses: Techniques, Materials and Prevention of Degradation

Wood was one of the first materials used by man for construction, thanks to its availability, workability and excellent mechanical properties. The trusses, often exposed to atmospheric and biological agents, undergo degradation processes over time that compromise their stability.

Understanding the characteristics of the wood used, such as its porosity, density and resistance to degrading agents, is essential for planning adequate restoration interventions.

What are wooden trusses and what are they for?

Wooden trusses are key structural elements in architecture, used to support the roofs of buildings. They are composed of a set of wooden beams and uprights, connected to each other to form a single structural triangle or series of triangles. This triangular configuration gives the trusses great strength and stability, allowing them to distribute the weight of the roof along the entire structure of the building, right down to the foundations.

Functions of Wooden Trusses

Structural Support: The main function of trusses is to provide structural support to roofs, allowing the creation of large internal spaces without the need for columns or intermediate load-bearing walls. This makes them particularly useful in large buildings, such as warehouses, warehouses, gyms, churches, and in residential structures.

Load Distribution: Thanks to their triangular shape, the trusses are able to effectively distribute vertical and horizontal loads (such as the weight of the roof, snow, wind) along the entire structure, reducing the risk of deformations or collapses.

Material Efficiency: The design of the trusses allows for efficient use of wood, a renewable and sustainable building material, maximizing its structural performance with minimal use of resources. This makes them an economically advantageous and ecologically sustainable solution.

Versatility and Aesthetics: The trusses are not only functional but can also be characterized by a high aesthetic value. They can adapt to a variety of architectural styles, from traditional to modern, and can be left exposed inside buildings to create spectacular visual effects.

Diagnosis Techniques for the Degradation of Wooden Trusses

The accurate diagnosis of the deterioration of wooden trusses is the first critical step for an effective restoration intervention. This process requires a methodical approach that combines visual, instrumental, non-destructive techniques and laboratory analysis.

Visual Techniques

Visual inspections are essential to identify obvious signs of deterioration, such as cracks, distortions, xylophagous insect attacks and rot. This initial evaluation can be performed with the naked eye or with the aid of magnifying glasses and handheld microscopes for finer details.

Non-destructive instrumental techniques

The use of advanced technologies allows you to obtain detailed information on the internal condition of the trusses without damaging them. Some common methods include:

Infrared Thermography: Detects temperature variations that may indicate the presence of humidity or internal voids.

Ultrasonic Tomography: Provides cross-sectional images of wood, revealing internal cracks and areas of decay.

Resistography: Measures the resistance of wood to penetration by a fine needle, offering clues to density and the presence of rot.

Laboratory analysis

Wood samples can be taken for detailed analysis in the laboratory. These tests may include:

Scanning Electron Microscopy (SEM): For the identification of wood species and the characterization of biological attacks.

Chemical Analysis: To determine the composition of the wood and the presence of preservative substances.

Mechanical Strength Test: To evaluate the residual load-bearing capacity of the material.

Principles of Conservative Restoration

The conservative restoration of wooden trusses requires a holistic approach that respects the history and material integrity of the architectural elements. At the heart of this approach are some fundamental principles:

Intervention Philosophies

Conservation aims to maintain the authenticity and historical integrity of the trusses, favoring minimal interventions. The restoration philosophy must balance structural needs with the conservation of historical surfaces, choosing solutions that do not irreversibly alter the cultural asset.

Reversibility Criteria

A key principle is the reversibility of the interventions: every action carried out on the trusses should, ideally, be removable without damaging the original substrate. This allows for future revisions or restorations in the face of new techniques or materials.

Minimum Intervention and Historical Reading

The concept of minimum intervention urges us to do as little as possible but as much as necessary to ensure conservation. At the same time, the historical reading of the artefact should guide the restoration, maintaining and enhancing the signs of time as evidence of the life path of the building.

Intervention techniques on wooden trusses

Work on wooden trusses must be carefully planned to ensure their structural stability while maintaining their original characteristics. The intervention techniques vary based on the condition of the wood and the specific objectives of the restoration.

Structural Consolidation

Structural consolidation is essential to ensure the safety and stability of wooden trusses. This process may include various techniques, depending on the specific conditions and structural problems identified.

Insertion of Tie Rods and Metal Brackets

The consolidation of wooden trusses with the use of tie rods and metal brackets is a widespread technique in structural restoration, especially for strengthening historic buildings or structures that have suffered damage or decay over time. This method involves the use of metal components to connect and support the wooden parts of the trusses, improving their ability to withstand loads and stresses.

Principle of operation

The idea behind consolidation with tie rods and metal brackets is to introduce elements that can redistribute loads between the various parts of the truss, limiting stress on weakened areas and preventing further deformation or damage. Metal ties, brackets, connection plates and bolts are used to create an integrated bracing "system" that works in synergy with the wood to increase the strength and stability of the entire structure.

Phases of the Intervention

Evaluation and Design: Before proceeding with any intervention, it is crucial to carry out a detailed evaluation of the state of the trusses, including the characteristics of the wood, the weak points, and the stresses to which the structure is subject. Based on this analysis, the consolidation system is designed, determining the type and optimal arrangement of the metal components.

Preparation of the Structure: Cleaning of wooden surfaces and removal of any rotten or excessively degraded parts. This step may include treating the wood with preservatives or biocides to improve its durability.

Installation of Tie Rods and Metal Brackets

Tie rods: They are used to connect opposite parts of the truss to each other, exerting a compressive force that counteracts tensile stresses, common in structures subjected to high loads or in the presence of structural damage.

Connection Brackets and Plates: They serve to connect the different elements of the truss, providing additional support in critical points or where the wooden connections are weakened or damaged.

Fixing and Adjustment: Metal components are fixed to the wood using special bolts, screws or nails, designed to minimize damage to the original material. The tension of the tie rods can be adjusted to optimize the distribution of loads within the structure.

Control and Maintenance: After installation, it is important to carry out periodic checks to verify the integrity of the metal components and the correct tension of the tie rods, ensuring that the consolidation system maintains its performance over time.

Advantages

Structural Efficiency: Significant increase in the load-bearing capacity and stability of the trusses.

Versatility: Adaptability to different types of structures and deterioration conditions.

Reversibility: Possibility to remove or modify the reinforcement system without permanent alterations to the wooden structure.

Use of Epoxy Resins

Epoxy resins are often used for consolidating cracks and reconnecting fragmented parts, thanks to their strong adhesion and compatibility with wood.

The consolidation of wooden trusses with epoxy resins represents one of the most effective and respectful techniques for the structural restoration of historic architectural elements. Epoxy resins, thanks to their high adhesion and compatibility with wood, are particularly suitable for strengthening, consolidating cracks, and reconstructing missing or damaged parts.

Principles of Consolidation with Epoxy Resins

The process of consolidating wooden trusses with epoxy resins is based on the application of these polymeric substances which, once hardened, form a composite material with the wood, restoring or increasing the original mechanical resistance of the structural element without significantly altering its I wait.

Phases of the Intervention

Diagnosis and Planning: Before proceeding with the consolidation, it is essential to carry out an accurate diagnosis of the structure to identify the damaged areas and define the extent and type of intervention necessary.

Preparation of the Intervention Area: Remove debris, dust and rotten or damaged parts to ensure perfect adhesion of the resin to the healthy wood. This may include mechanical or chemical cleaning of surfaces.

Application of Epoxy Resin

The resin can be applied in various ways depending on the type of damage:

Injection into Cracks and Fissures: For narrow cracks or crevices in wood, the resin is injected under pressure to ensure complete filling and consolidation of the damaged area.

Impregnation of Damaged Areas: In the presence of widespread degradation, the wood can be impregnated with low viscosity epoxy resin to improve its internal cohesion.

Reconstruction of Missing Parts: Epoxy resins, mixed with wood flours or other fillers, can be molded to reconstruct missing or heavily damaged parts.

Curing and Cure: After application, the resin requires an adequate period for curing. Environmental conditions, such as temperature and humidity, can affect the timing and effectiveness of curing.

Finishing: Once the resin is fully cured, the treated surfaces can be finished to restore the original appearance of the wood. This may include sanding, painting or varnishing.

Advantages

High Strength: Epoxy resins have excellent mechanical properties, improving the strength and durability of treated trusses.

Compatibility with Wood: These resins adhere effectively to wood, forming a homogeneous compound that respects its flexibility and characteristics.

Versatility: Suitable for a wide range of applications, from micro-cracks to large part reconstruction.

Durability: Epoxy resins are resistant to chemical and biological agents, offering long-lasting protection to the wood.

Microcement injections

In cases of advanced degradation, microcement injections can provide internal support, increasing cohesion.

The injection of microcements represents an innovative technique in the field of structural consolidation of wooden trusses. This method is particularly useful for strengthening wooden elements that have suffered damage or decay, without compromising the aesthetics or historical integrity of the building. Microcements are materials composed of high-performance cements, with very small aggregates and specific additives that improve their workability and mechanical properties.

Principle of operation

The consolidation of the trusses through the injection of microcements is based on the insertion of this material inside the voids, cracks or degraded areas of the wood. Microcements, thanks to their high fluidity and penetration capacity, completely fill these spaces, subsequently hardening and forming a single body with the wood. This process not only increases the mechanical resistance of the damaged parts but also improves the distribution of loads within the structure, contributing to its overall stability.

Phases of the Intervention

Preliminary Diagnosis: In-depth assessment of the state of the trusses to identify the areas that require consolidation. This phase may include visual analysis, instrumental surveys and non-destructive testing.

Preparation: Cleaning of the affected areas and, if necessary, removal of irreparably compromised parts of the wood. Make holes or channels for injecting the microcement into the areas to be consolidated.

Microcement Injection: Use of specific injection pumps to introduce the microcement into the prepared holes. The operation must be carried out with care to ensure uniform distribution of the material and adequate filling of the voids.

Cure and Curing: After injection, microcement requires a period of time to harden completely. During this time, it is important to protect the treated areas from excessive humidity or temperature changes which could compromise the hardening process.

Control and Finishes: Verification of the effectiveness of the consolidation through visual inspections and specific tests. Any aesthetic finishing work can be carried out to harmonize the treated areas with the rest of the structure.

Advantages

Structural Strengthening: Significant improvement in the mechanical resistance of the trusses, with an increase in load-bearing capacity and stiffness.

Durability: Microcements offer resistance to humidity, chemical agents and freeze/thaw cycles, ensuring long-lasting consolidation over time.

Versatility: Ability to use this technique in a wide range of situations, including complex or historically valuable structures without compromising the aesthetic appearance.

The fiberglass bars

Also known as Glass Fiber Reinforced Polymer (GFRP) bars, they are composite materials used in the restoration and structural reinforcement sector for their excellent mechanical properties, including high tensile strength, lightness and corrosion resistance. These characteristics make them particularly suitable for the consolidation of wooden structural elements, such as trusses, in contexts where maintaining lightness and visual integrity is fundamental.

Fiberglass bars are used to reinforce and consolidate wooden trusses that have undergone degradation or that require an increase in their load-bearing capacity.

The consolidation process with GFRP bars can be described in the following stages:

Diagnosis and Design: Before proceeding with any intervention, it is necessary to carry out an accurate diagnosis of the state of the trusses, identifying the damaged or weakened areas and evaluating the need for intervention. Subsequently, the reinforcement is designed based on the specific structural needs.

Preparation of the Trusses: The surface of the truss where the bars will be applied must be adequately prepared. This includes cleaning the wood and, if necessary, removing rotten or damaged parts.

Installation of GFRP Bars: Fiberglass bars can be installed by applying them to the surface of wooden beams or by inserting them into previously drilled holes. Surface installation requires the use of epoxy adhesives to secure the bars to the truss. The internal insertion, however, involves anchoring the bars inside the holes filled with epoxy resin, which acts as glue.

Finishing: Once the GFRP bars have been installed and the adhesive has cured, we proceed with the finishing operations. This may include covering the bars for aesthetic reasons or protecting the treated wood with specific impregnators.

Advantages of Using GFRP Bars

Non-Invasiveness: The method of reinforcement with GFRP bars is less invasive than other consolidation techniques, preserving the original appearance of the trusses.

Resistance and Durability: GFRP bars offer high resistance and durability over time, even in environments with high humidity or the presence of chemical agents.

Compatibility with Wood: The flexibility and lightness of GFRP bars make them compatible with the mechanical characteristics of wood, allowing effective reinforcement without overloading the structure.

Wood Treatment and Protection

The treatment of the wood aims to protect the trusses from further damage caused by biotic factors (such as xylophagous insects and fungi) and abiotic factors (humidity, thermal variations).

Application of Biocides: Biocidal products can be applied to eliminate active infestations and prevent new attacks.

Impregnation with Preservatives: Wood can be impregnated with preservative substances which increase its resistance to humidity and biological agents, while preserving breathability.

Protective Coatings: Breathable protective coatings can be applied to protect the wood from the elements, maintaining its ability to "breathe" and preventing moisture trapping.

Integration of Missing Parts

When parts of the trusses are missing or unrecoverable, integration with new material becomes necessary. These additions must be made in such a way as to be recognizable but harmonious with the original.

Replacement 'in General': New parts, made with techniques and materials similar to the original ones, can be inserted to replace the missing sections, following the principles of reversibility and differentiation.

'Anastylosis' techniques: Where possible, anastylosis, i.e. the reintegration of recovered original fragments, is preferred to maintain the historical authenticity of the structure as much as possible.

Final Considerations

The restoration of wooden trusses requires a delicate balance between maintaining historical integrity and applying techniques that ensure their functionality over time. Each intervention must be preceded by an accurate diagnosis and detailed planning, always considering the impact on the mechanical and aesthetic properties of the trusses. The goal is to preserve not only the physical structure, but also the historical and cultural value that the trusses represent.

Innovative and Traditional Materials

The restoration of wooden trusses sees the use of both traditional materials and innovative solutions. The choice between these depends on several factors, including compatibility with the original substrate, long-term effectiveness, and aesthetic impact.

Traditional Materials

Traditional materials, such as wood of specific essences, lime, clay and natural resins, have been used for centuries in the conservation of wooden structures. These materials are preferred for their physical and chemical compatibility with existing structures, as well as for their reversibility and minimal visual impact.

Wood: Selection of compatible essences for additions or replacements.

Lime and Clay: Used for surface treatments that regulate humidity and protect from external agents.

Natural Resins: For consolidations and surface finishes.

Innovative materials

Innovations in building materials offer new possibilities for restoration, including greater durability, energy conservation efficiency and improvements in mechanical properties.

Nanotechnologies: Nanoparticles for the consolidation of wood and protection from humidity and biological degradation.

Biopolymers: Environmentally friendly materials for conservative treatment, offering good integration with ancient wood.

Carbon and Glass Fibres: For structural reinforcement, offering discreet and reversible solutions.

Prevention of Degradation

Scheduled maintenance

Scheduled maintenance is essential to preserving the integrity of wooden trusses. This approach involves regular interventions based on a predefined timetable, with the aim of preventing serious problems from arising.

Regular Inspections: Conduct periodic visual and instrumental inspections to identify early signs of degradation or damage.

Cleaning: Keep the trusses free of debris and dust accumulations that can retain moisture or encourage insects to settle.

Humidity Control: Ensure adequate ventilation and control humidity levels in the surrounding environment to prevent wood rot.

Constant monitoring

Advanced monitoring technologies can be used to monitor the health of the trusses in real time, allowing timely intervention at the first report of anomalies.

Humidity Sensors: Install sensors to track moisture levels in the wood and the environment, identifying changes that could indicate problems.

Thermography: Periodically use thermography to detect hidden areas of moisture or structural defects not visible to the naked eye.

Structural Monitoring Systems: Employ systems that measure the deformation or displacement of trusses to detect excessive stress or failure.

Prevention of Degradation

The adoption of targeted preventive measures can significantly reduce the risk of deterioration of wooden trusses.

Pesticide Treatments: Apply environmentally friendly treatments to protect the wood from xylophagous insects and fungi.

Protection from atmospheric agents: If exposed, treat the surfaces of the trusses with paints or impregnating agents that offer protection from rain, sun and thermal variations.

Structural Reinforcements: In case of high mechanical stresses or identified structural weaknesses, specific reinforcements may be necessary to avoid future damage.

Implementation Strategies

To be effective, a deterioration prevention program must be customized based on the specifics of the building and its wooden trusses. A multidisciplinary approach involving restorers, structural engineers and conservators is important, in order to develop a maintenance and monitoring plan that takes into account all aspects of conservation, from structural stability to aesthetic preservation.

Prevention is always more advantageous than restoration, both in terms of costs and conservation of the historical and cultural value of the building. By implementing proactive strategies, you can safeguard your timber trusses, ensuring they continue to serve their structural function and tell the story of the buildings they support for many future generations.

Books

"Consolidation and restoration of wooden structures": This volume focuses on the wooden structures present in historical artefacts, offering specific attention to trusses. It is a useful reference for professionals and operators in the sector, delving into the design process issues from problem recognition to consolidation interventions.

"Restoration of wooden structures. Structural consolidation techniques": Another resource that provides details on structural consolidation techniques specifically for wooden structures, including trusses