CLS timber is a planed softwood used for internal framing such as stud walls and partitions. The term CLS stands for Canadian Lumber Standard, referring to timber that has been machine finished with smooth faces and rounded edges for safe handling and consistent installation.
It is typically kiln dried to reduce movement and is widely used where straight, reliable timber is needed for internal construction. CLS is designed to be quick to work with and is commonly used alongside sheet materials and fixings to build complete wall systems.
If you are building an internal frame, a stud wall, or boxing in pipework, CLS timber is usually the starting point. It is designed for accuracy, speed, and clean installation, especially when used alongside sheet materials and the right screws and fixings.
What is CLS Timber?
CLS timber stands for Canadian Lumber Standard. In practical terms, it refers to a type of softwood that has been planed all round and eased on the edges, giving it a clean finish that is easier and safer to handle than rough sawn timber.
It is most commonly used for internal carcassing and framing jobs where straightness, consistency, and ease of installation matter. Compared with rougher structural timber, CLS is cleaner to work with, which is why it is so popular for stud partitions, garage conversions, and general interior building work.
If you are comparing timber options, it also helps to look at the wider structural timber range so you are choosing the right material for the job rather than forcing one product to do everything.
View CLS Timber →How Does CLS Timber Work?
CLS timber works by forming the framework behind the finished surface. In a stud wall, for example, the CLS creates the structure, then boards are fixed to it, insulation is fitted between it, and the whole wall is finished over the top.
Because CLS is usually kiln dried and machined to a consistent finish, it is easier to set out accurately. That means straighter walls, cleaner boarding, and fewer problems during installation.
- Less movement after installation
- Cleaner lines for boarding and plastering
- Safer handling thanks to eased edges
- More consistent results on site
In most cases, CLS is paired with sheet materials such as OSB, plywood, or MDF, along with suitable timber screws and fixings.
Is CLS Timber Treated?
Standard CLS timber is usually supplied untreated and is designed for internal use. It is most commonly used for stud walls, framing, and internal carcassing where the timber is kept dry and protected.
Treated CLS timber does exist, but it is often not the most cost-effective option. In many cases, if a job requires treated timber, it makes more sense to use treated structural timber instead.
Because structural timber is produced and stocked in larger volumes, it is typically more readily available and better suited to outdoor or damp environments than CLS.
If you are working on an internal project, untreated CLS is the right choice. If the timber is going outside or may be exposed to moisture, switching to treated timber will give you better long-term performance and value.
Browse Structural Timber →What is CLS Timber Used For?
Stud Walls
This is the most common use. CLS is ideal for building internal stud walls because it is smooth, easy to work with, and consistent in size.
Internal Framing
CLS is widely used for framing out cupboards, service voids, boxing in pipework, and forming the internal structure of walls and light partitions.
Garage and Loft Projects
For garage conversions, loft spaces, and home office builds, CLS is a reliable option where you need straight, manageable timber for internal work.
DIY Builds
Shelving, storage frames, and simple workshop builds are all common CLS jobs.
See CLS Timber Sizes →FAQs
What does CLS stand for in timber?
CLS stands for Canadian Lumber Standard. In practice, it refers to smooth, planed timber used for internal framing.
Is CLS timber structural?
CLS can be used in structural internal applications where it is strength graded appropriately.
Can CLS timber be used outside?
Standard CLS is generally for internal use. For outdoor projects, treated timber is the better option.
