Quick answer: CLS timber is smooth, planed softwood with eased edges, commonly used for internal stud walls, partitions and general framing. CLS stands for Canadian Lumber Standard. The important buying point is that trade sizes such as 3x2, 4x2 and 6x2 are nominal names: the timber is smaller after machining, so use the finished metric dimensions when planning your job.
If you already know CLS is the right timber, see our CLS timber product for the current 3x2, 4x2 and 6x2 sizes, lengths and prices.
What does CLS mean?
CLS stands for Canadian Lumber Standard. In UK building use, the term normally describes machined softwood with smooth faces and rounded or eased edges. That makes it comfortable to handle and gives a consistent section for internal framing.
The name tells you about the timber format and machining. It does not, by itself, tell you whether a piece is treated, what strength class it has, or whether it is suitable for a particular structural or external application. Those are separate parts of the timber specification.
CLS timber sizes: nominal size vs finished size
This is where CLS causes the most confusion. A customer asking for 3x2 CLS is not normally buying a piece that measures a full 3 inches by 2 inches. The trade name describes the nominal section before the timber is machined to its finished size.
| Common trade name | Finished CLS size we stock | Also written as |
|---|---|---|
| 3x2 CLS | 38 × 63mm | 2x3 CLS |
| 4x2 CLS | 38 × 89mm | 2x4 CLS |
| 6x2 CLS | 38 × 140mm | 2x6 CLS |
If you are checking wall depth, insulation space, door linings, sheet joints or whether framing will fit an existing opening, work from the 38 × 63mm, 38 × 89mm or 38 × 140mm finished dimension, not from the imperial nickname.
The live CLS timber product page shows the lengths currently available in each section.
Why is 3x2 CLS not actually 3x2?
The familiar 3x2, 4x2 and 6x2 names come from nominal timber sizes. CLS is machined and planed, which reduces the section from that original nominal size and produces the smoother faces and eased edges associated with CLS.
That is why the imperial trade name is useful when talking about the timber, but the finished metric size is the measurement to use when something has to fit.
What is CLS timber used for?
CLS is most at home in internal framing. Common uses include:
- Internal stud walls and partition walls
- Frames for plasterboard and sheet materials
- Boxing in pipework and services
- Service voids and simple internal frames
- Room-dividing and renovation framing
- Internal door openings where the wall framing is being formed in timber
For a complete stud-wall job, CLS commonly sits alongside plasterboard, insulation, drywall screws and, where there is an internal doorway, a suitable door lining.
When CLS is not the right timber
CLS is useful, but it should not become the default answer to every timber job.
- External or exposed work: the word CLS does not prove that the timber has the treatment required for outdoor exposure. Use a product specified for the conditions, such as suitable treated structural timber.
- Appearance-first joinery: if the timber will remain visible and you want a cleaner joinery section rather than framing timber, planed PSE timber may be the better starting point.
- Specification-led structural work: if drawings or calculations call for a particular strength class, section or treatment, follow that specification. “CLS” alone is not a structural grade.
- Ground contact or persistently damp conditions: choose timber specifically treated and suitable for that exposure rather than assuming CLS is appropriate.
Is CLS the same as C16 or C24 timber?
No. These terms describe different things.
CLS describes the machined timber format: planed faces, eased edges and a consistent finished section. C16 and C24 are structural strength classes.
A piece of CLS can be strength graded, but the word CLS does not automatically mean C16 or C24. Equally, strength-graded construction timber does not have to be CLS. Where the grade matters, check the exact timber supplied and the project specification rather than using the terms interchangeably.
Is CLS timber treated?
The term CLS does not tell you whether the timber is treated or untreated. Treatment is a separate product characteristic.
If treatment matters to the job, check the individual product specification. Do not assume that smooth, eased-edge timber is suitable outdoors simply because it is sold as CLS.
CLS timber vs planed timber
Both CLS and planed timber are machined smooth, but they are normally bought for different reasons.
CLS is primarily a practical framing timber: consistent sections, eased edges and sizes suited to studwork. Planed timber is more often chosen where the timber itself forms part of the visible finished work or where a wider choice of joinery sections is useful.
If the timber is disappearing inside a stud wall, CLS is usually the more relevant product family. If the timber remains visible, compare it with our planed PSE timber before deciding.
Which CLS size should I choose?
Choose the section from the finished wall or frame you need to build, not simply because one trade size is more familiar.
A 38 × 63mm, 38 × 89mm and 38 × 140mm frame gives three very different finished depths. The correct choice depends on the wall design, what has to fit within it, what boards will be fixed to it and whether the project has any structural, acoustic, fire or insulation requirements.
Where a drawing or tested wall system specifies the timber section and centres, follow that specification. For ordinary non-specified framing, start by working out the finished depth you actually need.
Before you order CLS timber
- Check the finished size: use 38 × 63mm, 38 × 89mm or 38 × 140mm rather than relying only on 3x2, 4x2 or 6x2.
- Check the length: choose the stocked length that suits the frame and sensible cutting plan.
- Check the environment: internal dry framing and exposed outdoor framing are different jobs.
- Check the specification: if the job calls for a strength class, treatment or tested wall build-up, follow it.
- Plan the rest of the wall: plasterboard, insulation, fixings, door openings and services can all affect the section you need.
CLS timber FAQs
What does CLS stand for?
CLS stands for Canadian Lumber Standard. In UK building use it generally refers to planed softwood with eased edges, commonly used for studwork and internal framing.
Why is 3x2 CLS only 38 × 63mm?
3x2 is the nominal trade name. The timber is machined and planed to its finished section, so the piece supplied is smaller than the nominal dimensions. For Wern-Wood’s 3x2 CLS, the finished section is 38 × 63mm.
What size is 4x2 CLS?
Our 4x2 CLS has a finished section of 38 × 89mm. Use that finished metric dimension when planning the wall or frame.
Can CLS timber be used outside?
Do not decide that from the word CLS alone. CLS describes the timber format, not its treatment or exposure suitability. For external work, choose timber specifically treated and specified for the conditions.
Is CLS the same as C16?
No. CLS describes the machined timber format; C16 describes a structural strength class. They are not interchangeable terms.
Is CLS timber treated?
CLS can be supplied in different specifications. The word CLS itself does not confirm treatment, so check the individual product before using it in damp or exposed conditions.
Choosing CLS timber
If CLS is right for the job, the next step is simply to choose the finished section and length you need. Our current range covers 38 × 63mm, 38 × 89mm and 38 × 140mm CLS, commonly called 3x2, 4x2 and 6x2.
