Fire Resistance and Safety: Using CLS Timber in Construction

21st Jun 2024 -

Timber is widely valued for its strength, workability and natural appearance. However, concerns about fire safety have led to scrutiny surrounding the use of timber in modern projects. Canadian Lumber Standard (CLS) timber emerges as a promising solution, offering fire resistance properties that prevent risks and boost safety.

This article explores the fire resistance benefits of CLS timber. We delve into the relevant fire safety regulations and discuss treatments and construction techniques that improve fire resistance capabilities.

6 Fire Resistant Benefits of CLS Timber

CLS studwork timber offers superb fire resistance properties. This section explores the 6 various fire resistance benefits of CLS timber and how it contributes to resilient building structures.

1. Predictable charring rate

CLS timber undergoes extensive fire testing following rigorous standards, allowing for accurate determination of its charring rate. This measures the speed at which the wood burns and forms an insulating char layer (a carbon-rich residue). 

The predictability allows you to calculate the residual load-bearing capacity of CLS timber during a fire with precision. It ensures the structure maintains its integrity. 

2. Inherent thermal insulation

The natural insulating properties of wood are a significant benefit when it comes to fire resistance. As the outer layers char, the char layer acts as an insulating barrier, preventing heat transfer to the inner core. This slows down the combustion process, prolonging the timber's structural performance.

3. Large cross-sections

CLS timber is often used in large cross-sections, such as beams, columns and heavy timber construction. These larger dimensions provide a significant advantage in fire resistance because of their increased mass and surface area ratio. 

The outer layers char and insulate the inner core. This allows timber to maintain its load-bearing capacity for an extended period, even in the event of a severe fire.

4. Robust connections and joints

CLS timber construction typically involves robust connections and joints, such as mortise and tenon, dovetail or heavy steel connections. These connections are engineered to maintain their integrity during a fire, preventing premature failure and ensuring overall stability. This is particularly important in load-bearing elements like beams and columns.

5. Low combustibility

Compared to other construction materials like steel or concrete, CLS timber has a relatively low combustibility rate. It does not release toxic fumes or contribute significantly to the spread of fire. 

Overall, this makes it a safer choice in terms of fire safety and evacuation. Also, the char layer formed during a fire acts as a barrier, stopping the spread of flames to unaffected areas.

6. Moisture content control

CLS timber is seasoned and kiln-dried to a specific moisture content, typically between 12% and 19%. This controlled moisture content reduces the risk of excessive shrinkage or swelling. Both issues compromise the timber fire resistance by creating gaps or cracks that allow fire to penetrate more easily.

Read our detailed guide on CLS timber properties and benefits.

C16 vs C24 Timber: Fire Resistant Differences

​​It is important to understand the potential differences between the C16 timber and C24 timber grades. The following table compares the key factors that may impact fire resistance between these 2 common timber grades.

Factor	C16	C24
Density	Lower density ranges from 350-500 kilogram per cubic metre (kg/m³). Its lower density can result in a slightly faster charring rate compared to C24 timber.	Higher density ranges from 420-590 kg/m³. Increased density creates a more compact char layer, slowing heat transfer and combustion.
Structural integrity	C16 has lower strength and stiffness. It may not contain load capacity as long during a fire event.	Higher strength and stiffness which means it can contain load capacity for longer before failure.
Cross-section size	Often used in smaller cross-sections like joists and studs. A smaller mass/surface area ratio makes them more vulnerable to heat transfer.	Commonly used in larger cross-sections like beams and columns. A larger mass/surface area ratio provides better insulation and resistance.
Moisture content	Typically 12-19% moisture content range, but may have slightly higher average moisture content. Higher moisture increases the cracking risk and compromises the char layer.	Same moisture content range, but may have slightly lower average moisture content. Lower moisture contributes to better fire resistance by reducing cracking/splitting risk.

Learn more about the differences between C16 timber and C24 timber with our article.

What are the CLS Timber Fire Regulations?

Regulations and codes outline specific guidelines to ensure CLS timber meets strict standards, protecting from the risks of fire. The following details the UK building regulations and codes that cover CLS timber and its fire resistance performance. 

Approved Document B (Fire Safety)

• Guides on meeting the requirements of The Building Regulations 2010 Part B.
• Tables and calculations determine the required ratings for timber based on their intended use, occupancy and height of the building.
• The document guides CLS timber use in project types, such as traditional timber framing and heavy timber construction.
• It outlines acceptable test methods and standards for determining the fire resistance of timber elements. This includes the use of charring rate calculations and fire resistance furnace tests.

BS 9999: Code of Practice for Fire Safety in the Design of Buildings

• This code of practice guides the design, management and use of buildings to achieve reasonable standards of fire safety.
• It includes sections on the use of timber in construction, with recommendations for fire resistance, dividing structures and protection measures.
• The code addresses the use of CLS timber in different construction types. It also guides the design and detailing of timber to meet fire resistance requirements.

Timber Eurocode 5 (BS EN 1995-1-2)

• This European standard provides design rules for fire resistance.
• It includes calculations and methods for determining the load-bearing capacity and fire resistance of timber elements under fire conditions.
• The standard covers CLS timber and guides the design of timber structures to meet specific fire resistance requirements.

Can You Enhance CLS Fire Safety Design? 

Meticulous detailing and design of timber connections, joints and assemblies are key for ensuring optimal fire resistance performance. Well-designed connections and details maintain the structural integrity during a fire event, adhering to the above fire standards.

Robust connections

• Apply robust connection methods such as mortise and tenon joints, steel brackets or concealed hangers. These connections are less susceptible to failure under fire conditions, providing superior load transfer and stability.
• Properly designed and detailed connections prevent separation or dislodgement of timber elements. This ensures the overall structural integrity of the assembly during a fire event.

Minimising penetrations and openings

• Carefully minimise the number of penetrations and openings in load-bearing CLS timber elements. These can create potential pathways for fire and smoke spread.
• Avoid unnecessary holes, notches or cuts in structural timber members. They may compromise fire resistance and load-bearing capacity.
• When penetrations are unavoidable, ensure proper sealing and fire-stopping measures are implemented. It maintains the fire resistance rating of the assembly.

Continuous load paths

• Design CLS timber structures with continuous and uninterrupted load paths. This minimises the need for load transfers or discontinuities in structural elements.
• Continuous load paths reduce the risk of localised failures. Also, they ensure loads are efficiently transferred through the timber assembly during a fire event.

Redundancy and alternate load paths

• Introduce redundancy and alternate load paths into the design. They offer further safety measures in case of localised failures or damage during a fire.
• This is achieved through the use of added load-bearing members or bracing systems. 

Fire resistant detailing

• Use detailing techniques that enhance fire resistance. For example, using thicker cross-sections for structural timber elements, increasing member sizes or incorporating charring allowances in the design calculations.
• Consider fire resistant materials or coatings for connections, fasteners and hardware to improve their performance under fire conditions.

How to Treat CLS Timber to Boost Fire Resistance

Various treatments can be applied to boost CLS timber performance and provide a layer of fire protection. These treatments improve the timber's ability to resist ignition, slow the rate of combustion and prolong structural integrity.

1. Fire-retardant coatings

Fire-retardant coatings and impregnations are among the most effective treatments for boosting fire resistance. These involve applying a chemical solution or compound to the timber surface or impregnating the wood fibres with fire-retardant substances. 

Common fire-retardant chemicals include borates, phosphates and inorganic salts. They effectively prevent combustion and insulate the timber from heat exposure. Application methods may include brushing, spraying, dipping or pressure impregnation processes.

2. Intumescent coatings

Intumescent coatings are specially formulated to swell and form an insulating char layer when exposed to high temperatures or fire. These coatings are typically applied to the surface of the timber using brushes, rollers or spraying techniques. 

As the intumescent coating expands and chars, it creates a protective barrier that insulates the underlying timber from heat. This effectively increases the fire resistance rating of the element.

3. Encapsulation systems

Encapsulation systems involve enclosing the CLS timber element within a fire-resistant cladding or enclosure material. Common encapsulation materials include gypsum boards, mineral wool insulation or specialised fire resistant panels. 

These systems create a physical barrier between the CLS timber and fire, protecting the structural integrity for an extended period.

4. Heat and fire barriers

Heat and fire barriers prevent or delay the spread of fire and heat within a building. Examples include fire-rated membranes, intumescent strips or fire-resistant sealants applied to joints, gaps and penetrations in timber assemblies.

These barriers effectively divide structures, limit the spread of fire and allow CLS timber elements to maintain their load-bearing capacity.

5. Timber modification

Certain timber modification processes, such as chemical or thermal treatments, may enhance the fire resistance of CLS timber. Chemical modifications, like acetylation or furfurylation, involve reacting the wood with chemicals that replace or modify the wood's components. This reduces its combustibility and improves its resistance to flames.

Thermal modification processes, like heat treatment or torrefaction, involve heating the CLS timber to specific temperatures in controlled environments. These treatments alter the chemical composition of the wood and make it less susceptible to combustion.