The Millimetre Matters: Ordering Timber by What Actually Arrives
Photo: Drewshotz/andrew stephen, CC BY-SA 4.0, via Wikimedia Commons
If you have ever measured a so-called 4x2 and found it closer to 38x89mm, you have not been fleeced. You have met nominal sizing. In the UK timber trade, the name on the rack is often a shorthand, not a finished measurement. That is fine once you know the rules. It is expensive when you find out after you have cut.
The shorthand is not a measurement
A 4x2 starts life as a rough-sawn imperial size, around 100mm by 50mm. By the time it has been planed, regularised or graded to CLS, it is smaller. A rough-sawn 100x50 may arrive at 100x50. A planed all round, or PAR, version might finish around 95x45. Regularised carcassing might be nearer 94x44. CLS framing timber is commonly 38x89. The exact figure depends on the mill, the grade and the process.
The same applies up and down the rack. A 6x2 is not a finished 150x50. It could be a 145x45 PAR, a 47x150 regularised joist, or a 38x140 CLS. If the job depends on the measurement, ask for the finished size in millimetres. Do not order from the trade name alone.
Where the millimetres go
Planing removes material from each face. Take a couple of millimetres off each side and a 100mm board is no longer 100mm. Regularising rounds the corners and sizes the timber for consistency. CLS is a standardised softwood for framing, with its own fixed finished sizes. Saw kerf takes another few millimetres every time you cut, and wastage adds up across a cut list.
Moisture matters too. Timber shrinks as it dries, and treated carcassing can arrive wet. A length that measures 95x45 in the yard may not stay exactly that once it has been in a warm shed for a fortnight. For framing that is usually fine. For joinery, it is another reason to buy a little oversize and machine it yourself.
Three places it catches people out
Framing
Studs at 400mm or 600mm centres are designed around actual widths. If you assume a 4x2 is 50mm wide and it turns out to be 38mm, insulation, noggins and fixings can all be out. Draw the frame using the finished size you will actually receive. If you are setting out a shed or partition, check the timber before you mark the plates.
Decking
Decking boards are often sold by a nominal width, but the actual coverage is narrower. If you calculate the area using the nominal figure, you will be short. Allow for the gap between boards as well. A simple plan with the real board width, the real gap and a sensible waste allowance will save a return trip.
Joinery
PAR and planed square edge sizes are finished sizes. If you are making a door, a drawer box or a frame, order oversize if you plan to machine it further. Do not order the exact finished size and then take another 2mm off each face. There will be nothing left to trim, and the fit will suffer.
How to order accurately
Use millimetres for anything critical. Say whether you need rough sawn, regularised, PAR, CLS or PSE. Ask for the finished dimensions and the tolerance. Give a cut list with finished sizes, quantities and a note about which faces need planing. Allow for saw kerf and a sensible waste margin: five to ten per cent on simple jobs, more on complex ones with lots of short cuts.
If you are unsure, bring a sketch or a sample. A good woodyard would rather spend five minutes checking than have you return a length that has been cut and cannot be sold again.
At the woodyard
At Walsh Woodyard, 4x2 can mean more than one rack. We will ask what it is for: studwork, decking joists, garden planters, a shed repair. That changes what we pull. If you need 95x45 PAR, say so. If you need 38x89 CLS, say so. If you are cutting joints, tell us and we will leave you some meat to machine.
Check before you cut. Measure a few pieces from the stack. If something looks off, call before the saw goes through it. Most suppliers would rather swap a length than hear about a ruined project after the event.
Nominal sizes are a language, not a lie. Learn the finished sizes and the ordering gets simple. Your 4x2 can still be a 4x2 in the trade. Just do not expect it to measure 4x2 on the tape.
