What do PAR, PSE and sawn mean on a timber price list?

This is the single most confusing thing about buying timber in Britain, and it produces the classic complaint that the 50 by 100 you bought measures 44 by 94. Nothing is wrong. The label is the size before planing, the planing removes material from every face, and the trade has worked that way for a century.

Step by step

1. Understand sawn.Straight off the saw with a rough, fibrous surface and square but ragged arrises. It is the full nominal size, or close to it, it is the cheapest and it is what carcassing, joists, studs, formwork and fence rails are sold as.
2. Understand regularised.A halfway house you will see on structural timber: sawn on the faces but machined to a consistent thickness and width so joists come out level with each other. It is still rough, but the size is reliable. Most modern carcassing is regularised even when the label says sawn.
3. Understand PAR.Planed All Round. All four faces machined smooth and the arrises left square or very slightly eased. It is dearer per metre, it is a few millimetres smaller in both dimensions than the nominal size, and it is the correct choice for anything that will be seen, touched or painted.
4. Understand PSE.Planed Square Edge — planed on one face and both edges, or on other partial combinations, leaving the back rough. It exists for things like shelving and cladding where one face is never seen. It is much less common than PAR in general merchants and often has to be ordered.
5. Apply the sizing rule when you buy.Assume PAR loses a few millimetres from each dimension against the nominal size. A nominal 25 by 150 PAR board is commonly about 20 by 144. The exact loss varies between mills, so measure rather than assume if the number matters.
6. Buy by the finished size when it has to fit.If a piece has to slot into a gap or match an existing moulding, take the actual measurement to the merchant and check the stock with a tape before you pay. Do not order from a nominal size and hope.
7. Buy all matching pieces at once.Two pieces of PAR from different batches can differ by a millimetre or two, which shows as a step at every joint. Buy everything for one job in one visit from one rack.

Tips

• Planed timber is dearer per metre but often cheaper overall for visible work, because the alternative is buying sawn and spending an afternoon planing and sanding it — and it will still not be as consistent.
• PAR softwood is usually redwood, which is why it stains and finishes better than the whitewood used for most CLS and carcassing.
• If you only need a smooth face on one side, PSE or a light sand of sawn stock is a legitimate money-saver. Under paint, sawn timber sanded through two grits is perfectly presentable.

Common mistakes

• Designing to nominal sizes and building with PAR — A carcass drawn round "50mm uprights" and built from 44mm PAR is 12mm out across three uprights. Draw with the real measured size of the timber you have actually bought.
• Buying sawn to save money on visible work — The saw marks show through paint, the arrises splinter, and getting it smooth by hand takes longer than the price difference is worth. Sawn is for behind plasterboard.

Questions people ask