The most common “timber mistake” is not choosing the wrong species — it’s assuming the size on the label is the size you actually have. Rough-sawn, planed, PAR, CLS, S4S, 2×4… these terms decide whether your joints close, your floor sits level, and your project ends up the size you drew.
When timber leaves the sawmill it is rough, slightly oversized, and rarely perfectly straight. Most timber is then dried and machined. Every one of those steps changes the dimensions.
This guide will help you:
- understand rough-sawn vs planed timber
- avoid the trap of nominal vs finished sizes
- choose the right stock for accuracy, speed, and yield
- plan allowances for flattening, thicknessing, and movement
The Two Realities of Timber Size
Timber has two “sizes” at the same time:
- What it is called (nominal size)
- What it measures (actual finished size)
Nominal size
Nominal size is the name of the section.
- It is often historical.
- It often refers to the size before drying and planing.
- It exists because it is convenient for trade.
Actual (finished) size
Actual size is what you can measure with a tape.
- It is smaller because:
- timber shrinks as it dries
- timber is planed to remove saw marks and straighten faces
<aside> 📌
Nominal sizes are a language, not a measurement. Always check the actual finished size if accuracy matters.
</aside>
Rough-Sawn vs Planed: What You’re Really Buying
Rough-sawn (as-sawn / sawn)
Rough-sawn timber comes straight off the saw.
Characteristics:
- saw marks and a slightly furry surface
- dimensions are closer to the sawn target size (usually a little oversized)
- faces are not guaranteed square or parallel
- edges may include wane
Best for:
- beams and heavy sections where you will not machine much
- projects where you will plane/flatten everything yourself
- situations where you want extra thickness to work with
Downside:
- you are paying for wood you may cut away
- it is slower to build with if you need accuracy
Planed (PAR / dressed / surfaced)
Planed timber has been machined so the faces are smooth and more consistent.
You will see terms like:
- PAR (Planed All Round)
- S4S (Surfaced Four Sides)
- Dressed or finished
Characteristics:
- smooth faces
- more consistent thickness and width
- more likely to be square and straight (but not guaranteed)
Best for:
- joinery and furniture parts
- framing where speed matters
- projects where you want predictable dimensions
Downside:
- less “meat” left to flatten if it moves after purchase
- still not truly “final dimension” if you will joint and thickness it
Why Dimensions Change: The Three Main Reasons
1) Shrinkage during drying
As timber dries, it shrinks across the grain. (Track 2.)
- tangential shrinkage is greater than radial shrinkage
- uneven shrinkage contributes to cupping and distortion
A board sawn at a target size green will almost always be smaller once dry.
2) Machining allowances
Planing removes material:
- to remove saw marks
- to bring boards to standardised dimensions
- to improve straightness
3) Standardisation and grading
Merchants want timber that stacks, trades, and builds predictably.
Standard sizes simplify:
- structural calculations
- wall and floor layouts
- sheet material alignment
Common Product Terms (What They Usually Mean)
(Naming varies by country and merchant, but these definitions will keep you safe.)
CLS (Canadian Lumber Standard)
Often used for framing timber.
- usually kiln dried
- usually planed
- corners often slightly eased/rounded
- sold by a nominal “regularised” size
Regularised / eased
- Regularised: machined to consistent dimensions
- Eased: corners slightly rounded to reduce splintering
Green vs KD (kiln dried) vs AD (air dried)
- Green: high moisture content, will shrink and move significantly
- KD: kiln dried to a target MC, more stable for indoor work
- AD: air dried, often not as uniform as kiln dried but excellent for many uses
<aside> 💡
For accurate joinery, “dry + planed” usually matters more than the species name printed on the rack.
</aside>
Rough-Sawn Sizes, Planed Sizes, and the “Hidden” Loss
A practical way to think about buying timber:
If you buy rough-sawn
You are buying potential.
- You can flatten and true it.
- You can choose your final dimensions.
- You must expect waste.
If you buy planed
You are buying convenience.
- It will be closer to usable size immediately.
- It may still need jointing/thicknessing.
- If it moves, you have less thickness to rescue it.
Rule of thumb: keep an allowance
For joinery and furniture:
- allow extra thickness for flattening
- allow extra width for jointing edges
- allow extra length for end checks and snipe
You do not need the exact numbers to be safe.
You need the habit of leaving yourself room.
How to Choose: A Simple Decision Guide
Buy planed when:
- you need speed
- you need consistent framing stock
- you are building from a cut list that assumes standard sizes
Buy rough-sawn when:
- you need maximum stability control (you will mill it yourself)
- you want thicker finished parts
- you are matching boards for grain and colour
Buy “oversize” when:
- the project is high-precision
- the timber has been stored badly
- you are unsure of its moisture condition
Common Traps (and How to Avoid Them)
- Designing from nominal sizes
- Fix: take one board off the rack and measure it before committing.
- Mixing planed and rough in one build without adjusting
- Fix: decide your “reference thickness” early.
- Assuming planed timber is straight
- Fix: sight down boards, reject twist, and store properly (Track 4 Guide 6).
- Skipping acclimation
- Fix: let timber settle in the workshop before final milling.
Media and Image Recommendations
- Side-by-side photo: rough-sawn vs planed surface
- Diagram: nominal vs actual dimension (labelled)
- Photo: eased corners on framing timber vs sharp corners on rough-sawn
- Simple table: “You need X, so buy Y” (allowance concept)
The Key Idea
<aside> 💡
Timber size is a process, not a number. Rough-sawn sizes are closer to the sawmill target. Planed sizes are smaller and standardised. Nominal sizes are names. Measure actual stock, leave allowances, and you will stop losing projects to surprise dimensions.
</aside>
What’s Next
In Guide 9, we look at what happens when processing goes one step further: treatment and protection — how preservation treatments work, what the treatment labels mean, and how treatment interacts with moisture, movement, and durability outdoors.
🔗 Knowledge Network
Glossary Terms
- Nominal size
- Finished size
- Rough-sawn
- Planed (PAR)
- S4S / S2S
- Regularised timber
- Eased edges
- Green timber
- Kiln dried (KD)
- Air dried (AD)
Calculators
- None for this guide