What is CLS timber and what is it for?

CLS became the default stud timber in British merchants because it solves the two things people hate about sawn carcassing: it is dry, so it stays roughly where you put it, and it is planed, so it is a consistent size and pleasant to handle. It is also usually spruce, and cheap. What it is not is a universal timber, and the rounded edges are a real limitation rather than a styling choice.

Step by step

1. Know what the name means.Canadian Lumber Standard — a North American sizing and grading convention that came to Britain with imported framing timber and stuck as a product name. Most CLS sold here now is European spruce made to the same profile.
2. Recognise it by the rounded edges.All four long edges are eased to a small radius. That is the giveaway, and it exists so the timber does not splinter when handled and does not tear plasterboard paper when it is fixed over.
3. Use it for what it is for: stud walls.Framing an internal partition, forming a service void, building out a wall, boxing in. It is dry so it will not shrink and crack your plasterboard joints, it is straight so the wall is flat, and the rounded arrises help rather than hurt.
4. Expect it to be dry, and keep it dry.CLS is normally kiln dried, which is the main reason it stays straight. Leaving it outside or in a damp garage for weeks undoes that — it will take up moisture and start to move like anything else.
5. Do not use it where the pieces must meet flush.The rounded arrises mean two pieces face to face leave a small groove along the joint, and a butt joint shows a chamfered line. For anything visible — a shelf, a frame, a piece of furniture — buy PAR timber with square arrises instead.
6. Check the grade before you use it structurally.Some CLS is strength graded and stamped C16 or C24; some is not graded at all. For a loadbearing wall, a joist, a lintel trimmer or anything carrying weight, you need graded timber and the size has to come from a span table or a designer, not from what looks about right.
7. Understand the sizes you will see.CLS is sold in a small number of standard sections — commonly 38 by 63, 38 by 89, 38 by 140 millimetres and similar. Those are the finished sizes, not nominal ones, which is one reason CLS is less confusing to buy than sawn timber.

Tips

• CLS is usually a good deal straighter than the sawn timber in the next rack for a similar price, which is why it is worth buying even for jobs where the rounded edges do not matter.
• If you need square arrises but want CLS's straightness, you can rip a few millimetres off one edge on a table saw or with a track saw, but at that point PAR is usually the cheaper answer.
• For a stud wall, CLS at 38mm thick gives a slightly narrower stud face than 47mm sawn carcassing. That is fine for plasterboard but leaves less to aim at when fixing, so mark your stud centres before boarding.

Common mistakes

• Using ungraded CLS as floor joists — Joists need a strength class and a size taken from a span table for that class, that span and that spacing. An ungraded piece has no class, and undersized joists produce a floor that bounces at best and fails at worst.
• Buying CLS for visible frames and then trying to fill the grooves — Filler in the groove between two eased arrises cracks and shows, especially under gloss. Buy square-edged PAR for visible work — it costs a little more and saves the whole problem.

Questions people ask