Softwood Timber Sizes Explained: 3×2, 4×2, CLS & What They’re For

What size is 3×2 and 4×2 timber really?

Ask for “4 by 2” at a merchant and you will not get a piece of timber measuring exactly 4 inches by 2 inches. This trips up more people than any other point in buying timber, so it is worth getting straight before anything else.

Timber is sold by its nominal size — the size it was before it was processed — not its finished size. “4×2” describes the rough-sawn dimension the timber started at. Once it is planed and regularised to a smooth, consistent section, material is removed and the finished piece is smaller. The name sticks; the dimensions shrink.

So the trade names (4×2, 3×2, 2×4 — the same thing said the other way round) are a category, not a measurement. Here is what they actually measure once finished.

The practical takeaway: when you are setting out stud centres or working out how a wall will sit, design to the finished size — 89×38mm for 4×2 CLS, 63×38mm for 3×2 CLS — not the nominal name. Getting this wrong by 11mm per stud adds up quickly across a wall.

CLS vs sawn vs planed (PSE) timber — what’s the difference?

These three terms describe how much the timber has been machined after sawing, and they matter because they change how the wood handles, how consistent it is, and what it is suited to.

Sawn (rough-sawn) carcassing is timber cut to size and left with a rough surface. Sometimes it is regularised — machined on the width so every piece is a consistent depth — but it is not planed smooth. It is the cheapest option and fine where the timber will be hidden and appearance does not matter.

Planed all round (PAR) or planed square edge (PSE) timber is sawn timber that has then been planed smooth on all faces. It is more accurate in dimension and cleaner to handle, and it is used where the finished face will be seen or where precision matters.

CLS (Canadian Lumber Standard) goes a step further again. It is kiln-dried first, then planed smooth on all four faces with the corners rounded off (eased edges). That combination — kiln-dried, planed, rounded arris — is what makes CLS the standard choice for stud walls. It is more pleasant to handle with no splinters or sharp edges, more consistent in dimension, and gives a clean, flat face for plasterboard to sit against.

Despite the Canadian name, CLS is an acknowledged grade now predominantly sourced from European softwood — typically whitewood (spruce or fir). Our CLS is graded to a minimum of C16, and we insist it is wane-free and straight, because both matter for framing that has to line up.

Timber strength grades explained: C16 vs C24

The “C” grades are strength classes. The number is, in effect, a measure of how much load the timber can carry — the higher the number, the stronger and stiffer the timber. For softwood framing in the UK you will almost only ever meet two: C16 and C24.

For the great majority of domestic internal framing — stud walls, non-loadbearing partitions, noggins, general carcassing — C16 is the correct and cost-effective grade, and it is what most CLS is supplied as. C24 comes into play when you are spanning further or carrying more load, most often on floor joists and rafters, and it is what an engineer will usually specify where the calculation demands it.

The rule of thumb: use C16 unless something (a span table, a building control requirement, or a structural engineer) tells you that you need C24. Do not pay for C24 on a partition wall that a C16 stud handles comfortably — and never drop to C16 on a structural member that has been specified as C24.

Treated (UC2) vs untreated — when you need treated timber

Treatment protects timber against rot, decay and insect attack by forcing preservative into the wood under pressure. Whether you need it comes down to one question: will the timber ever get damp?

Use-class 2 (UC2) treated timber is intended for internal use where there is a risk of occasional dampness — the kind of moisture that can occur inside a building fabric even when it is not directly exposed to the weather. It is the standard treatment for framing that needs a margin of protection.

As a practical guide:

• Internal, dry, well-ventilated stud walls — untreated C16 CLS is generally fine
• Framing in bathrooms, kitchens, utility rooms or anywhere with a raised moisture risk — choose treated
• Sole plates and any timber sitting on or near concrete or masonry — treated
• Garden rooms, outbuildings and anything in the building envelope exposed to potential damp — treated
• Roofing battens and any external or semi-external timber — always treated (UC2)
• Anywhere you are unsure — the cost difference is small, so treated is the safe default

Roofing battens are a special case: they must be treated, because they sit in the roof build-up where condensation and the occasional leak are facts of life. Our rough-sawn UC2 treated batten (25 × 50mm, PEFC certified) is made for exactly that.

Which timber size for which job

The choice between 3×2 and 4×2 on a stud wall is mostly about depth. A 4×2 wall (89mm finished) gives more room for insulation and for running services within the wall; a 3×2 wall (63mm finished) is shallower and saves space where floor area is tight. For non-loadbearing partitions, 4×2 C16 CLS is the industry standard, but 3×2 is perfectly sound for lighter, shallower walls.

How to work out how much timber you need

For a stud wall, a quick method that gives a reliable materials list:

• Measure the wall length and height.
• Studs: divide the length by your chosen centres (400mm or 600mm are standard) and add one for the final stud, plus one for each end. As a shortcut, wall length in metres × 2.5 gives roughly the number of studs at 400mm centres.
• Plates: you need a top and bottom plate, so add twice the wall length.
• Noggins: one or two rows depending on wall height — add roughly one wall-length of timber per row.
• Add 10% for waste, cutting and the odd bowed length you set aside.
• Convert to lengths or packs: divide your total linear metres by the pack length you are buying.

Our CLS is supplied in convenient 1.2m lengths in value packs — 5 lengths (6m total), 10 lengths (12m total) or 20 lengths (24m total) — which are easy to handle and well suited to smaller projects, repairs and tight spaces. Work out your total linear metres first, then pick the pack that covers it with a little spare.

Storing and handling softwood to avoid warping

Timber moves as its moisture content changes. Kiln-dried CLS starts stable, but poor storage on site will undo that in days, leaving you with bowed studs that fight you at every fixing.

• Store flat and fully supported — lay lengths on level bearers at regular intervals, not slung between two points where they will sag
• Keep it off the ground so it cannot wick up moisture from concrete or soil
• Keep it dry and covered, but allow air to circulate — sealing damp timber under plastic is worse than leaving it open
• Let it acclimatise — bring timber into the space where it will be used a few days before fixing so it settles to the local conditions
• Use the straightest lengths for the most visible or critical work and keep any slightly bowed pieces for noggins and short cuts
• Don’t buy far more than you need long before you need it — timber stored badly for weeks is timber wasted

Choosing close-grained, kiln-dried, wane-free material in the first place is half the battle — slow-grown, close-grained imported timber stays straighter and stronger along its length. Storing it properly is the other half.

Frequently asked questions

What size is 4×2 timber in mm?

Nominally 100 × 50mm. Once planed and regularised as CLS it finishes at about 89 × 38mm. Always design stud centres and wall depths to the finished size, not the 4×2 name.

Is CLS the same as 4×2?

CLS is available in a 4×2 section (finished 89 × 38mm), so “4×2 CLS” is a common product. But “4×2” is a size and “CLS” is a type — CLS is kiln-dried, planed and has rounded edges, whereas sawn 4×2 carcassing is rougher and left with square edges.

C16 or C24 — which do I need?

C16 for most internal, non-loadbearing framing such as stud walls and partitions. C24 for higher loads and longer spans, most commonly floor joists and rafters, or wherever a span table or engineer specifies it.

Do I need treated timber for a stud wall?

For a dry, well-ventilated internal wall, untreated C16 CLS is usually fine. Choose treated (UC2) for bathrooms, kitchens and utility rooms, for sole plates near masonry, and for any framing in a garden room or building envelope where damp is possible.

What’s the difference between sawn and planed timber?

Sawn timber is left rough from the saw; planed (PAR/PSE) timber has been machined smooth on its faces. CLS goes further still — kiln-dried, planed on all four faces and with rounded corners — which is why it is the standard for stud walls.

Products mentioned in this guide

• CLS Timber 50mm × 100mm C16 (finished 89 × 38mm, 4×2)
• CLS Timber 50mm × 75mm C16 (finished 63 × 38mm, 3×2) PEFC
• Rough Sawn UC2 Treated Batten 25 × 50mm PEFC
• General Building Supplies category

Related guides

• Insulating a Garden Room, Shed or Van (timber framing in practice)
• Roofing Felt Explained (battens in the roof build-up)