7th Dec 2022 -

When buying ungraded timber you may find yourself receiving a substandard product, leading many to seek something of a higher quality. CLS timber is graded, planed, and treated, offering a strong and reliable beam that can be used throughout the construction of wooden buildings. In this guide from Building Materials Nationwide we’ll take a look at the definition of CLS timber, where it can be used, and the sizes it comes in.

What is CLS Timber?

CLS stands for “Canadian Lumber Standard”, as the beams were originally produced in Canada. Modern CLS timber merely implies a standard and finish rather than a point of origin. Most CLS timber sold within the UK will have originated from Scandinavia or central Europe. Conventionally made from pine, fir, or spruce softwood, CLS timber is defined as such by its strength and its unique planing. CLS is kiln-dried, treated, and planed, normally reaching C16 or C24 strength grading. All four corners of a CLS beam will be planed to give a rounded edge. Each side will also be smoothed. CLS timber benefits from its consistency and quality control. When buying a large volume of CLS beams there should be a far more reliable level of straightness compared to ungraded timber.

Looking For CLS Timber?


    • Ideal for wall partitioning and studwork
    • Kiln dried for extra stability

    • Ideal for wall partitioning and studwork
    • Kiln dried for extra stability

View All CLS Timber

Is CLS Timber Treated?

CLS timber will be treated as standard. This will give it benefits such as moisture resistance and insect resistance, allowing CLS beams to be used in a wide variety of settings where they may be at risk. The kiln-drying process will also help to remove moisture from each beam.

Can You Use CLS Timber Outside?

Thanks to its treatment and moisture resistance, CLS timber can be used in outside applications. There is a grading system for treated timber, called the use class system, and timber that will be in contact with water or soil will need to be treated to use class 4. CLS timber can be used for a wide variety of exterior uses. It will need to be maintained to keep its look during prolonged outdoor use, however, and there are a variety of different products that can aid this, such as preservatives, UV-blocking paints, and colour maintaining sealants.

Different varieties of caressing and cls timber materials including c16 timber and c24 timber in a pile one on top of the other.

What Sizes Does CLS Timber Come In?

CLS timber can come in a range of sizes. We stock beams in 75mm x 50mm and 100mm x 50mm (roughly equating to a 2x3 and a 2x4 on the conventional size chart), though it’s worth noting that due to the planing the beams will actually be smaller than this (63mm x 38mm and 88mm x 38mm respectively). They are supplied in lengths of 2400mm, 3000mm, and 4800mm. The prices of CLS timber will vary according to market rates, but it is usually on the more affordable end.

What Strength Grades Does CLS Timber Come In?

CLS timber will typically be sold at either a C16 or C24 strength grading. These are determined through a series of tests measuring the strength, consistency, and durability of timber beams. Below is a table showing the minimum requirements for both C16 and C24:

Strength ClassMeasurementC16C24
Bending Parallel to GrainN/mm25.37.5
Tension Parallel to GrainN/mm23.24.5
Compression Parallel to GrainN/mm26.87.9
Compression Perpendicular to GrainN/mm21.71.9
Shear Parallel to GrainN/mm20.670.71
Mean Modulus of ElasticityN/mm2880010800
Minimum Modulus of ElasticityN/mm258007200
Characteristic Densitykg/m3310350
Average Densitykg/m3370420

Whether you will require C16 or C24 will largely depend on what you are using it for. Carcassing CLS timber can benefit from the added strength of C24 when being used for load-bearing applications. Additionally, C24 timber will have a more uniform and attractive appearance, bringing benefits if you are planning to use CLS timber in a way that will be visible. In many cases, a cheaper C16 model will be perfectly fine for use, offering a solid level of strength. If cutting your CLS timber to reduce the height or width, you will need to have it re-graded before using it for any load-bearing operations, as sawn timber will possess a reduced strength.


What Can CLS Timber Be Used For?

CLS timber is commonly used for framing or internal work. Here are some examples of how it could be beneficial within a building project:

  • Constructing a Timber Frame Home: CLS timbers’ strength, durability, and consistency make them a great choice for building timber frame houses.
  • Carcassing: CLS timber makes an effective carcassing timber, whether you’re constructing a full building or merely making roofs, floors, or walls.
  • Interior Framing and Partitioning: CLS timber can make a great way to quickly erect strong framing and partitioning, allowing the creation of internal walls, doorways and windows.

Is CLS Timber Good For Studwork?

CLS is a popular choice for interior stud walling, constructed as either internal partition walls or batten walls. CLS studwork timber is reliably straight, meaning it can effectively be used to build a consistent stud wall. The moisture resistance offered by treated timber will also be useful when battening an external wall, as they can sometimes have issues with dampness.

Our Range of CLS Timber 

Building Materials Nationwide delivers quality CLS timber across the UK, perfect for construction projects of any size. Alongside our range of CLS timber, you’ll find an extensive selection of timber and joinery, including sheet materialsdecking, and cladding. As a company, we stock a comprehensive range of building supplies, working with clients across the country. Our trade accounts offer extra benefits for those in the industry, granting access to a dedicated account manager, our full product range and our express delivery options. Sign up for a trade account through our website today.