How to Retrofit Existing Structures Using Treated Timber

The use of treated timber has emerged as a game-changer for retrofit projects. Here, we look at the transformative potential of treated timber in retrofitting. We explore its applications and benefits for sustainable building practices. 

By examining treated timber in retrofitting, this article outlines its role in revamping existing structures while aligning with sustainability goals. 

Why is treated timber used in retrofitting?

When you undertake a retrofit project, materials such as treated carcassing timber stand out as a material of choice for structural components. The unique properties offer a range of benefits as detailed below.

1. Environmental sustainability

Using treated timber aligns seamlessly with sustainability goals. The treatment processes extend the longevity of the timber. It reduces the need for frequent replacements and minimises resource consumption over the retrofit's lifespan. As a result, it lowers its environmental impact.

2. Structural integrity and durability

Treated timber's enhanced resistance to weathering, insects and decay is particularly valuable in retrofitting projects. The material withstands the difficulties of integration with existing structures. Its improved durability ensures structural integrity to strengthen the retrofitted elements for years to come.

3. Energy efficiency

The improved insulative properties of treated timber are particularly beneficial in retrofits aimed at boosting energy efficiency. These unique characteristics help optimise thermal performance, reducing energy consumption and costs.

4. Acoustic performance

For spaces that require superior acoustics, treated timber offers inherent sound-absorbing qualities. This is compared to untreated wood which has less density and is less effective at blocking the transmission of airborne sound waves. 

5. Cost-effectiveness

Treated timber’s durability and low maintenance translate into lower lifecycle costs than untreated wood. This long-term cost benefit is particularly valuable in retrofits, where minimising future maintenance is important.

How to retrofit with treated timber

Retrofitting requires a meticulous approach to seamlessly integrate the new elements while preserving the structural integrity of the original building. The following steps outline the best practices and techniques for a successful retrofitting project.

1. Comprehensive assessment

Evaluate moisture levels, insect activity and decay in existing timber elements. This will determine suitability for integration with newly treated components. Assess load paths and identify areas that require reinforcement or load transfer from old to new components.

Accessing concealed areas for a thorough inspection can be difficult. Use non-destructive testing methods like thermal imaging or moisture meters.

2. Design and planning

Specify treatment types based on exposure and performance requirements. Design connections that allow for wood shrinkage or expansion. Plan temporary bracing during installation. 

Integrating newly treated timber within existing structural constraints can be a challenge. Make sure to use 3D modelling and clash detection software to identify potential conflicts.

3. Selective demolition

Carefully remove deteriorated sections, creating pathways for new treated timber installation while protecting the remaining structure. Temporary shoring may be required to support loads during this phase. 

Containing dust and debris from demolition activities is difficult. Be sure to enforce proper containment measures.

4. Structural reinforcement

When joining treated timber with existing members, ensure proper surface preparation and use of structural adhesives or mechanical fasteners. You may find it challenging to achieve uniform pressure distribution in post-tensioned connections. 

Therefore, you should use load-distributing elements like steel plates or specialised anchorage systems.

5. Connections and joinery

Ensure proper wood moisture content and use of appropriate adhesives or mechanical fasteners. Using corrosion-resistant fasteners is essential for treated timber. This is because regular nails or screws can corrode over time due to the preservative chemicals. 

It can be a challenge to accommodate differential movement between new and existing components. Use slotted or oversize holes for fasteners, allowing for wood shrinkage or expansion.

6. Finishing and detailing

If exposing treated timber, apply appropriate finishes such as stain, paint or sealant which is compatible with the specific treatment type. This increases protection against elements such as moisture and wet weather conditions. Conceal connections and joints with trim or cladding for aesthetic purposes. 

Achieving a seamless visual transition between new and existing elements can sometimes be difficult. Be careful with material selection and detailing to match existing finishes and profiles.

7. Testing and inspection

Post installation, carry out load tests by gradually applying weights and monitoring deflections. Use moisture meters to verify that treated timber is within acceptable range. Inspect connections for proper installation and tightness. 

Accessing connections in concealed or hard-to-reach areas can prove challenging. Plan for inspection access during the design phase and use borescopes if needed.

What is the best treated timber for retrofitting?

The choice between treated timbers depends on several factors. These include the application, environmental conditions and the nature of the existing structure. Here is an overview of which timber treating material may be the best choice for retrofitting. 

1. Pressure treated timber

Pressure treated timber (also known as tanalised timber) is often an excellent choice for retrofitting projects. Copper preservatives like alkaline copper quaternary (ACQ) or copper azole (CA-B) provide robust protection against fungal decay, insect infestation and moisture damage. This makes pressure treated timber well-suited for outdoor applications.

Timber that is pressure treated is beneficial for replacing structural elements such as joists, beams and posts. It is also suitable for exterior components including a garden fence, deck, path edge and landscaping features. The deep penetration of the preservatives ensures long-lasting protection. 

However, pressure-treated timber may not be the best option for interior applications. This is because potential leaching of preservatives could be a concern, such as near water sources.

2. Heat treated timber

Heat treated timber can be a sustainable choice for retrofitting projects, particularly for interior applications or exterior cladding and trim. The heat treatment process enhances the wood's dimensional stability and resistance to rot and insects without chemical preservatives. 

It is well-suited for replacing interior flooring, wall panelling, window and door frames and exterior siding or trim elements. Its improved dimensional stability is beneficial as it minimises the risk of warping or shrinking over time.

However, heat-treated timber may not be the best choice for exterior applications involving ground contact or exposure to termites. The heat treatment alone may not provide sufficient protection in these scenarios.

3. Acetylated timber

Acetylated timber is another sustainable option for retrofitting projects, specifically for exterior applications where durability and dimensional stability are key. The acetylation process enhances the wood's resistance to decay, moisture absorption and dimensional changes. 

This makes it an excellent choice for replacing exterior cladding, trim, decking and outdoor furniture. Acetylated timber is also a suitable option for interior applications, such as flooring and wall panelling. Its enhanced stability and resistance to moisture is beneficial.

One potential drawback of acetylated timber is its relatively higher cost and limited availability compared to other treated timber options. This makes it less practical for large-scale retrofitting projects.

Is sawn treated timber suitable for retrofitting? 

Treated sawn timber can be a suitable choice. However, it is important to consider several factors to ensure its effectiveness and longevity. Below is a detailed look at its suitability for retrofitting projects. 

Structural integrity

It is often used for structural retrofit applications, such as roof and wall framing, joists and beams. The sawing process potentially creates small cracks or openings in the wood. It may allow moisture and insects to penetrate more easily. 

This is compared to planed treated timber battens or boards. The outermost layer of the wood is removed to create a smooth surface that may be less prone to cracks or openings. 

Preservative penetration

The effectiveness of the preservative treatment largely depends on its ability to penetrate the wood fibres. With rough sawn treated timber, the sawing process exposes the end grains. This can facilitate better penetration of the preservative than the side grains.

However, if the sawing process is not followed by a proper post-treatment process, the exposed end grains may be more prone to moisture absorption.

Retention levels

For structural applications, it is important to use sawn treated timber that meets the required retention levels of the preservative. Building codes and standards such as  BS8417 specify this.

Retention levels refer to the amount of preservative present in the wood, measured in kilograms per cubic metre (kg/m³). Higher retention levels are generally recommended for critical structural components. You will find higher retention in types including carcass, C16 treated timber and C24 treated timber.

End grain sealing

Proper end grain sealing is crucial for sawn treated timber to prevent moisture penetration and degradation. Applying a suitable end grain sealant or preservative can help mitigate the potential risks associated with exposed end grains.

Failure to properly seal the end grains leads to premature deterioration and potentially compromises the structural integrity of the retrofitted components.

Corrosion resistance

When using sawn treated timber, it is essential to use corrosion-resistant fasteners and connectors, such as hot-dipped galvanised or stainless steel. The preservative chemicals used in treated timber can boost corrosion in regular steel fasteners. 

As a result, this weakens the connections and compromises the overall structural performance.