A Guide to CLS Timber for Maximum Structural Integrity

20th Jun 2024 -

Structural integrity is key for all construction projects. An important element is the choice of timber used. Among the various options, Canadian Lumber Standard (CLS) timber stands out as a superior choice for strength, durability and reliability.

In this article, we look at the essentials of CLS timber and explore its unique characteristics. We detail the key benefits of using CLS timber and provide tips for choosing the best timber for your needs. 

What is CLS Timber?

While initially of Canadian origin as the name suggests, CLS timber is now often sourced from various regions around the globe including Scandinavia. Production begins with the harvesting of softwood species suitable for construction, such as spruce, pine and fir.

Logs are then transported to sawmills for debarking and breakdown into rough-sawn lumber. The rough-sawn lumber is then kiln-dried to reduce its moisture content to appropriate levels, typically between 15-19% for most applications.

The drying process is crucial to prevent issues like warping, twisting and shrinkage. These issues may happen with changes in moisture levels. Lumber undergoes various tests to evaluate its strength properties, including bending, compression and stiffness. Test results must meet the specific requirements for C16 and C24 studwork timber grades. 

Read our detailed guide to learn more about CLS timber.

C16 vs C24 Timber: What are the Differences?

Understanding the strength grades is essential for selecting the right material for your project's structural requirements. The 2 common strength grades are C16 and C24. Each grade is suited for specific applications based on their load-bearing capacities.

C16 timber

C16 timber is a lower-strength class of structural timber, suitable for applications where moderate load-bearing capacity is required. This grade of timber is commonly used in the following applications.

• Stud walls: C16 timber is used for non-load-bearing interior stud walls in residential and light commercial construction.
• Floor joists: For moderate-span floor systems or in areas with lower load requirements, C16 timber can be used for floor joists.
• Roof trusses: In smaller residential or low-rise commercial buildings, C16 timber is used for the construction of roof trusses.
• Exterior applications: When treated with preservatives, C16 timber is used for outside spaces such as decking and fencing. 

C24 timber

C24 timber is a higher-strength grade of structural timber, designed to withstand greater loads and stresses. It is typically used in the below applications where increased structural integrity and load-bearing capacity are required.

• Load-bearing walls: C24 timber is installed for load-bearing walls in residential and commercial construction. It provides the necessary strength to support the weight of the structure.
• Floor joists: For longer spans or areas with higher load requirements, C24 timber is used for floor joists to ensure good support. 
• Roof trusses: In larger buildings, C24 timber is installed for roof trusses. It supports the weight of the roofing materials and any potential wind loads.
• Beams and headers: C24 timber is suitable for use as structural beams, headers and lintels, where higher load-bearing capacity is necessary.
• Engineered wood products: C24 timber is a common component in engineered wood products like laminated veneer lumber (LVL) and glued-laminated timber (Glulam). Both are used in various structural applications like columns and walkways.

Discover more about the differences between C16 and C24 timber with our comprehensive guide.

5 Benefits of CLS Timber

Selecting the right timber is crucial for ensuring structural integrity and durability. CLS timber stands out as an excellent choice, offering a range of benefits that make it a preferred option. Below, we outline 5 reasons why CLS timber is best for your project. 

1. Structural reliability

A primary benefit is its structural reliability. As mentioned above, the grading process evaluates factors like moisture content, visual characteristics, strength and stiffness. This ensures that each grade meets specific performance criteria. It allows you to buy the appropriate grade based on load-bearing requirements, span lengths and other design considerations.

2. Consistent performance

Unlike some ungraded timber products, CLS timber adheres to strict grading standards. This results in consistent performance across all grades, ensuring that the lumber meets or exceeds the specified design values. You can rely on these standards for precise calculations and material specifications.

3. Design flexibility

From the moderate-strength C16 to the high-strength C24, CLS timber offers design flexibility to meet diverse project requirements. Architects and engineers can optimise material usage while maintaining safety and durability standards, potentially reducing overall construction costs.

4. Sustainability

CLS timber is sourced from responsibly managed forests, promoting sustainable forestry practices and reducing environmental impact. Manufacturers work closely with organisations like the Forestry Stewardship Council (FSC) to ensure that timber complies with sustainability guidelines.

5. Longevity

When properly treated and maintained, CLS timber exhibits excellent durability and resistance to moisture, decay and insect damage. This longevity extends the lifespan of structures and minimises the need for frequent repairs or replacements.

What are the Common CLS Timber Sizes?

CLS timber is available in a wide range of standard sizes, catering to various structural requirements and applications. The most common dimensions are 38x89mm (2x4"), 38x140mm (2x6"), 38x184mm (2x8"), 38x235mm (2x10"), 38x28 mm (2x12") and 76x50mm (3x2”). These dimensions are commonly used in framing, joists and rafters.

Larger sizes like 89x89mm (4x4"), 89x140mm (4x6"), 89x184mm (4x8") and 89x235mm (4x10") are commonly used for greater load-bearing capacity structures.

The dimensions of CLS timber help determine its structural integrity and performance. Below, we detail how the size affects structural capacity.

• Cross-sectional area: The larger the cross-sectional area (width x depth), the greater the timber's ability to withstand compressive and bending forces. For example, a 38x235 mm (2x10") joist spans longer distances and supports heavier loads than a 38x89 mm (2x4") joist.
• Bending resistance: The moment of inertia, a measure of a beam's resistance to bending, increases with the depth of the timber. A deeper beam, such as a 38x286 mm (2x12"), has a significantly higher moment of inertia than a shallower one.
• Stability: Wider members, like 89x235mm (4x10") beams, offer improved lateral stability compared to narrower ones, reducing the risk of buckling or twisting under load. Larger cross-sections distribute loads more evenly, reducing localised stress concentrations and the potential for premature failure.

Shop our range of various CLS timber dimensions for your project.

How to Tackle CLS Timber Issues

Despite its exceptional quality, working with C16 and C24 timber presents challenges. Being aware of these common issues and having strategies in place to address them helps ensure a successful project. Here are some of the most prevalent issues and their corresponding solutions. 

1. Load capacity limitations (C16)

As the lower strength class, C16 timber has reduced load-bearing capacity compared to C24. This may lead to potential issues like excessive deflection, cracking or even structural failure under heavy loads.

Solution: Conduct thorough load calculations based on intended use and span lengths. For areas with higher load requirements or longer spans, upgrading to C24 is recommended for the necessary strength and stiffness.

2. Moisture-related issues (C16 and C24)

Both grades are susceptible to moisture-related problems like warping, twisting, splitting and dimensional instability. Excessive moisture may cause the wood to swell, while dry conditions can lead to shrinkage and cracking. 

Solution: Proper drying and acclimatisation to the job site's humidity levels before installation are crucial. Use vapour barriers, dehumidifiers or HVAC systems to maintain consistent moisture levels during and after construction.

3. Environmental exposure (C16 and C24)

Outdoor applications or damp indoor environments increase the risk of decay, insect infestations and general deterioration for both grades. 

Solution: Apply preservative treatments like copper azole (CA) or alkaline copper quaternary (ACQ) to enhance resistance. Also, ensure proper site drainage, ventilation and moisture control and introduce regular maintenance and inspections.

4. Surface defects (C16)

As a lower structural grade, C16 often contains more knots, wane and other surface imperfections than C24 timber. Overall, this may impact structural integrity as well as appearance. 

Solution: Be sure to carefully inspect and remove lumber with excessive defects for structural use. For visible applications like exposed beams or trim, consider working with higher grades like C24 timber. However, you can also use defect-filling techniques like wood putty or epoxy filler.

5. Strength variability (C24)

While C24 is engineered for higher strength, there can still be variations within this grade because of natural wood characteristics. This variability may affect load calculations and safety factors. 

Solution: You should conduct material strength testing and introduce redundant load paths by distributing loads across multiple structural members. Also, consider quality control and inspection processes during construction. 

How Long Does CLS Timber Last?

CLS timber longevity varies depending on several factors, including the species, grade, treatment methods and environmental conditions. With proper maintenance and care, CLS timber offers a service life spanning decades.

Untreated CLS timber

When used in dry, interior applications with minimal moisture exposure, untreated CLS timber may last 30-50 years. However, untreated CLS outdoors is highly prone to decay, insect damage and weathering. It potentially reduces its lifespan to as little as 5-10 years.

CLS treated timber

Applying preservative treatments can extend the lifespan of CLS timber used in damp conditions. Treatments include alkaline copper quaternary (ACQ), copper azole (CA) or chromated copper arsenate (CCA). Properly treated CLS offers a service life of 20-40 years or longer. This depends on the treatment quality, exposure levels and maintenance practices.

Maintenance for extended lifespan

To boost longevity, regular maintenance is essential, especially for outdoor applications. Here are some recommended practices.

• Inspections: Conduct regular visual inspections (at least annually) for signs of damage, decay, insect activity or surface degradation.
• Cleaning: Gently clean surfaces with mild detergents and a soft-bristle brush to remove dirt and debris that promotes decay.
• Refinishing: Apply fresh coats of water-repellent finishes, stain or paint coatings every 2-5 years or as needed based on weathering.
• Repair: Quickly address areas showing signs of decay or damage by replacing affected sections or applying appropriate wood preservatives.
• Moisture management: Ensure proper drainage and ventilation. Avoid direct ground contact to minimise moisture accumulation.