24th Jun 2024 -

Canadian Lumber Standard (CLS) grading rules ensure consistent quality. However, issues like knots, wanes and other defects can lead to inconsistencies that impact a project's structural integrity and aesthetics. 

For any timber structure, overcoming CLS timber quality inconsistencies is important for successful outcomes. This article outlines how to combat quality inconsistencies through proper inspection, handling, installation and defect remediation strategies.

What is CLS Timber Used For?

C16 timber and C24 timber are the 2 grades of CLS timber. C16 is a low grade for structural framing applications like wall studs, roof trusses and basic beam/post members. It allows more frequent knot sizes, wane and other defects compared to the higher C24 grade.

C24 is a premium grade with tighter restrictions on knots, wane, slope of grain and other quality attributes. This denser material is widely used as lumber components for tall load-bearing walls and larger-span beam applications.

With inconsistencies, C24 material is held to higher conformance standards during manufacturing because of its intended structural demand. However, both C16 and C24 suffer the same defects and issues if preventative measures are not taken. 

Explore more differences between C16 vs C24 timber with our guide.

6 Common CLS Timber Quality Inconsistencies

Understanding what constitutes a quality inconsistency is vital for working with CLS studwork timber. Below, we detail some areas where quality inconsistencies may occur.

1. Knots

CLS grades permit certain sizes and distributions of wood grain knots which can be entry points for moisture. However, larger knot sizes, the number of knots or clustering may compromise structural integrity. 

2. Wanes

Wanes are bark or lack of wood on portions of the piece's edges or faces. Excessive waning reduces the cross-sectional area available to carry loads. Deep wanes reduce the effective cross-sectional area and compromise structural capacity. 

3. Warp

This includes crooks, bows, twists and other forms of distortion in the timber. Too much warp makes CLS timber difficult to install properly. It may cause gaps, misalignment and structural deficiencies. 

4. Splits/shakes

Lengthwise separations in the form of splits or shakes beyond permitted sizes weaken the timber's capacity for tension. Larger splits act as potential failure planes, reduce load transfer ability and affect overall structural integrity. 

5. Decay

The presence of rot, fungal discolouration or other decay beyond small permissible quantities degrades the wood's strength and stiffness properties. Timber decay indicates the breakdown of the wood's cellular structure.

6. Density deviations

Lower-density wood than what the grade requires means reduced structural capacities. This may happen because of factors like species mix inconsistencies. Timber density relates directly to load-bearing ability.

What are the CLS Timber Quality Limits?

Various size CLS timer quality stacks on top of one another against a plain grey background.

Guidelines from the Timber Research and Development Association (BM TRADA) outline the acceptable levels of quality inconsistencies. Higher grades allow fewer defects to ensure adequate structural performance. The table below compares the maximum limits for common inconsistencies between the C16 and C24 grades.

InconsistencyC16 GradeC24 Grade
Knot size - Narrow facesMax 33% of cross-section width.Max 20% of cross-section width.
Knot size - Wide facesMax 25% of cross-section width.Max 33% of cross-section width.
Knot sum per 1.8m lengthMax 50% of cross-section width.Max 33% of cross-section width.
Wane depthMax 33% of piece thickness.Max 25% of piece thickness.
Crook per 2m lengthMax 12mm.Max 6mm.
Bow per 2m lengthMax 20mm.Max 10mm.
TwistLimits to prevent joint gaps.Limits to prevent joint gaps.
ShakesMax 16% of thickness penetration.Max 12% of thickness penetration.
SplitsMax 1/6 of width penetration.Max 1/8 of width penetration.
DecayNone permitted beyond minor streaks/pockets.None permitted beyond minor streaks/pockets.
Minimum density350 kg/m3.450 kg/m3.

How to Identify and Overcome CLS Timber Quality Issues

To ensure CLS timber meets grade specifications, you must properly identify each type of quality inconsistency through careful inspection. Here are some key methods for evaluating CLS timber pieces. 

Knots in CLS Timber

Carefully measure knot sizes using a ruler and map their precise locations along the full length of the timber. Check for any clusters of knots that exceed the maximum permissible quantities defined by the grade. 

Inspect knot areas on narrow faces or edges, as size limits are usually more restrictive there. Look for signs of decay, shakes/splits developing from the knots or knots that appear loose in the surrounding wood.


  • Cut out sections with excessive or oversized knot defects and replace them with wood inserts or joint reinforcement.
  • Use heavy-bodied structural wood fillers or epoxy repair compounds to re-bond and stabilise any loose knots.
  • Around knot clusters, reinforce with added framing anchors, adhesive solutions and closely spaced fasteners.
  • Avoid using pieces with significantly decayed knots for structural applications.

Wanes in CLS Timber

Measure the length and width of any wane areas or missing wood along all 4 faces of each timber piece using a ruler. Ensure the depth or penetration of wanes does not exceed the maximum allowed by the grade's specifications.

Pay close attention to areas where wanes on adjacent faces create narrow remaining cross-sections. Look for excessively diagonal wane slopes which can indicate excessive twisting of the log during sawing.


  • In areas exceeding wane limits, apply wood hardeners and plastic or epoxy fillers. 
  • Reinforce significantly waned sections with construction adhesives and additional timber pieces.
  • For deep wane pockets, cut out those segments and install reinforcements like plywood or steel plates. 
  • Place timber to ensure waned areas are not in high bending or vertical load locations.

Warp in CLS Timber

Lay timber on a level surface and use a straight edge to check for any curvature along its full length. Carefully check along the edges to detect bows, kinks or other lateral warping defects. 

Roll the timber to inspect for spiral twisting or winding along its length. Use a straight edge to identify the severity of any bows or crooks in terms of depth or magnitude.


  • Attempt mechanically straightening minor bows or twists using force with equipment such as laminated beam pressure boxes or crooked lags.
  • Cut out severely warped sections and install scarf or finger joints to re-straighten.
  • At framing connections, use strapping, bracing or ties to restrain any remaining curvature, bow, twist or crookedness. 
  • Account for remaining curvature by adjusting spans or in deflection/stability calculations.

Splits/shakes in CLS Timber

Inspect all 4 lengthwise faces for any separations in the wood grain such as splits or shakes. If present, carefully measure the length, width and depth of each using a probe. Check end sections for splits or shake extensions penetrating the end grain. Note whether any splits appear to fully extend through the full thickness between opposite faces.


  • For minor splits/shakes, apply structural adhesives or pourable wooden fillers.
  • Along larger splits, install supplementary reinforcement like added fasteners or ties. 
  • Cut-out pieces with splits penetrating deep into high-stress areas like beams.
  • Consider flexible membranes or tapes over minor cracks in moisture-prone areas.

Decay in CLS Timber

Closely inspect all surfaces and ends for any signs of fungal or insect decay. This includes discoloured rot pockets, areas with crumbly or desiccated wood texture or zones of obvious deterioration or cavities. 

Use a probe to test suspicious areas by penetrating the surface - decayed wood will easily dent or break apart. Pay particular attention to deterioration around knots which are typical entry points.


  • Remove all deteriorated wood down to where the internal wood is solid.
  • Reinforce with sistered, scabbed or laminated timber components using adhesives.
  • Apply preservative, borate or fungicidal treatment to exposed wood in decay areas.
  • For severe decay, remove and discard that timber, replacing it with all new material.

Density Irregularity in CLS Timber

Become familiar with the typical weight ranges for different CLS grade combinations by handling and comparing multiple pieces. Pick up and assess relative heaviness compared to other samples of similar size - denser wood will feel proportionately heavier. Use a Pilodyn wood tester which uses small probes to measure density through wood penetration.


  • For critical structural areas, use only higher grades like C24 timber.
  • With lower densities, account for reductions in stiffness, stress and stability.
  • Add reinforcements like flitch plates to increase effective section properties.
  • Avoid low-density pieces in areas requiring high lateral load resistance.

Can You Prevent CLS Quality Issues?

A female and male construction workers wearing yellow hard hats and hi-vis vests and grey t-shirts checking the CLS timber quality and writing on a clipboard.

Taking proactive measures to prevent quality issues from occurring in the first place is even more effective. CLS timber may develop inconsistencies at the manufacturing stage as well as during storage and construction work. Below, we explore the preventative measures to consider. 

Handling and protection

  • Use proper dunnage (solid structural supports) and stickers (spacers between layers) when storing CLS timber. This prevents ground contact and allows airflow.
  • Securely brace and strap bundles during transit to prevent mechanical damage like cracks, compression bruising and warp from shifting.
  • Practice 2-person lift techniques and carrying supports to avoid mishandling and impact damage.

Conditioning and acclimatisation

  • Allow CLS timber to acclimate to the local temperature and humidity conditions before installation for at least 24-48 hours.
  • If possible, order kiln-dried timber below 19% moisture content to prevent excess shrinking/swelling. This is particularly useful for interior applications like framing.
  • If exposed to weather, use approved accelerated drying methods like fans and heat to reach appropriate moisture levels before enclosing.
  • Pay attention to moisture levels of pieces that were exposed to precipitation before they can be properly enclosed.

Fabrication and modification

  • Avoid excessive ripping or cross-cutting that creates new areas of wane (missing wood) that may not meet grade requirements.
  • When drilling holes for utility runs or connections, maintain minimum distances from edges and ends.
  • Use supportive sawhorses or braces when cutting to length to stop new warp, twist or compression damage.
  • Apply wood preservative or borate treatment to any cut ends, drilled holes or damage that may happen during fabrication.

Shop Various CLS Timber Sizes Today

Building Materials Nationwide offers a wide range of CLS timberfor construction projects big and small. Our inventory includes popular sizes like 3x2 and 4x2 studs in various lengths. The 3x2 CLS is ideal for basic wall framing, while the 4x2 provides extra strength for taller walls or beams. Both sizes are available in C16 and C24 grades to meet your project requirements. 

Quickly open a trade account with us to streamline purchasing and job materials management. Our trade accounts have flexible credit options (subject to eligibility), quick UK delivery and one-on-one account management. We understand the importance of having quality CLS timber on hand when you need it. Contact us today about opening a trade account tailored to your company's needs or sign up now for instant access.

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