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What is SLS / CLS timber?

The Wood Guide · Calculating with wood

Structural timber

SLS and CLS timber

Two abbreviations that are often used interchangeably, while they describe something quite different. The skeleton of timber-frame construction, and probably of your garden shed too.

  • SLS = spruce, pine, fir
  • CLS = Canadian Lumber Standard
  • Rounded edges, 18% moisture
Picea · Pinus · Abies
Stack of planed CLS studs with rounded edges
Stack of planed CLS studs with rounded edges
Timber-frame house under construction: the skeleton of studs and rails (Wellington, New Zealand)
Timber-frame house under construction: the skeleton of studs and railsWellington, New Zealand
Summary

SLS and CLS at a glance

The confusion is persistent, so let's be clear straight away: one abbreviation is about what it is, the other about how it has been cut.

SLS stands for spruce, pine, fir — spruce, pine and fir. It is a trade group: North American softwoods with comparable strength properties that are traded and graded together. So you are not buying one tree species, but a performance class.

CLS stands for Canadian Lumber Standard — a size and finish standard. CLS timber is planed on all four sides, has rounded edges and is dried to a moisture content of about 18%. Hence the characteristic smooth stud with rounded arrises that you see in every DIY store.

In practice most CLS timber is also SLS timber, which is why the terms get mixed up. Strictly speaking: SLS is the species group, CLS is the processing.

Why the rounded edges

Not for decoration. They reduce the risk of splinters during handling and stop a coat of paint or stain pulling away from a sharp edge. In timber-frame construction, where hundreds of studs are carried by hand, that really makes a difference.

SLS
spruce, pine, fir
CLS
Canadian Lumber Standard — size standard
Origin
Canada and the northern United States
Moisture content
about 18% on delivery
Finish
planed on four sides, rounded edges
Durability
class 4–5 — indoors only or treated

To be fair: untreated, this timber does not belong outdoors. It is softwood in durability class 4 to 5; without impregnation or a dry, ventilated location it will rot within a few years.

In pictures · SLS is what, CLS is how1 / 7
What it is

Spruce, pine, fir

SLS stands for spruce, pine, fir. It is a trade group: North American softwoods of comparable strength that are traded and graded together. So you are not buying one tree species, but a performance class.

Click a chapter or use the arrows · sizes to scale (dotted: nominal inch size), processing speeded up.

What you get

The usual sizes

CLS is standardised, and that is exactly its point: every board from every batch is the same thickness.

Nominal size Actual size Typical use
2×4 38 × 89 mm Studs in timber-frame construction, the most widely used size
2×6 38 × 140 mm External walls with extra insulation thickness
2×8 38 × 184 mm Floor joists, roof structure
2×10 38 × 235 mm Heavier spans
Nominal is not actual

A "2×4" does not measure 2 by 4 inches. The nominal size refers to the rough sawn size before drying and planing; what you buy is 38 × 89 mm. Always calculate with the actual size, or your stud spacing will be off.

Measurable

Why it is used so much

Strong for its weight

High strength-to-weight ratio: one person easily lifts a three-metre 2×4, and yet it carries a storey.

Dimensionally stable

Dried to 18%, so it hardly shrinks or warps after fitting. Crucial when you screw plasterboard onto it.

Predictable

Graded by strength class, so an engineer can calculate with it without assessing every board.

Easy to work

Sawing, drilling and screwing can be done with ordinary hand tools. No pre-drilling needed as with hardwood.

Uses

What you use it for

Yes

This is where it belongs

  • Timber-frame construction: walls, floors, roof structure
  • Internal partitions and plasterboard studs
  • Garden shed and carport frames, provided they are sheltered
  • Scaffolding, formwork and temporary structures
  • Furniture frames that are hidden from view
No

This is where it doesn't belong

  • Untreated and exposed to the weather
  • Ground contact of any kind
  • Decking boards or garden furniture
  • Visible furniture work where the figure counts
Outdoors? Then pressure-treated

If you want softwood that can go outdoors, pressure-treated spruce is the answer — the same kind of wood, but preserved under pressure. That is also what goes into our picnic tables.

Quick answer

Frequently asked questions about SLS and CLS timber

What is the difference between SLS and CLS?

SLS describes the wood species (spruce, pine, fir), CLS describes the size and finish standard (planed on four sides, rounded edges, about 18% moisture). Most CLS timber is SLS timber, hence the confusion.

Can CLS timber go outdoors?

Not untreated. It is in durability class 4 to 5. For outdoors you choose pressure-treated wood or a naturally durable species.

Why doesn't a 2×4 measure 2 by 4 inches?

The nominal size refers to the rough sawn size before drying and planing. What remains is 38 × 89 mm. Always calculate with the actual size.

Is SLS the same as spruce?

Partly. The "S" of spruce is what we call spruce . But SLS can also be pine or fir — it is a group, not a species.

More in The Wood Guide

SLS and CLS in perspective

Structural timber you rarely see and always use.

To The Wood Guide What is spruce? Pressure-treated wood

Have a question about your project? Just ask — we're happy to help.

Photos via Wikimedia Commons: Stack of planed CLS studs with rounded edges — Coldupnorth, CC BY-SA 4.0 · Wellington, New Zealand — L Maule, CC BY-SA 4.0

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