Explainer

Reading a timber grade stamp

Every piece of structural timber carries a stamp, and once you know the four things it tells you, you can check in a second whether the board in your hand is the one the job needs.

Strength grade, use class, moisture condition and who graded it. That is the whole stamp.

Shop construction timber Pressure treatment explained

If you read nothing else

C16 or C24 is how strong it is. UC3 or UC4 is where it can go. Those two decide almost every buying question.

C16 is enough for most garden work. UC4 is non-negotiable for anything in the ground.

What each mark means

STRENGTH GRADE

C16 The standard structural softwood grade. Enough for most garden and domestic work

C24 Stronger and stiffer. Spans further at the same section size, and costs more

Which to buy C16 unless the span is over roughly 2.5m or the load is real

USE CLASS

UC1 / UC2 Internal, dry. Not for outdoor use

UC3 Outdoors but above ground — rails, cladding, deck boards

UC4 In the ground or permanently damp — posts, gravel boards, sleepers

MOISTURE CONDITION

KD or DRY Kiln dried. Graded at its finished moisture content and stable

WET Graded green. Sound, but it will shrink and move a little as it dries

THE REST OF THE STAMP

Standard The grading rule it was assessed against, usually BS EN 14081

Mill number Who graded it. This is what makes the stamp traceable

Species Often abbreviated — WPCS covers whitewood, redwood and pine

How to read one

Left to right, in four steps

The stamp is printed on the face or the end of the board. If you cannot find one, it has not been graded and it is not structural timber, whatever it is being sold as.

Find the C number C16 or C24. This is the strength grade and it is the first thing to check against your span.

Find the use class UC3 or UC4. If the timber is going into soil and the stamp says UC3, put it back.

Check the moisture mark KD or DRY means it is stable now. WET means allow for a little shrinkage and movement.

Check there is a mill number No mill number means no traceable grading. For anything structural, that matters.

Quick answers

The things people ask us most — you might not need to message at all.

Do I need C24, or will C16 do?

C16 for most garden work — fence rails, framing, light joists over short spans. Go to C24 when a joist is carrying real load or spanning beyond about 2.5m, where the extra stiffness stops the bounce.

What is the difference between a strength grade and a use class?

Strength grade is how much it can carry. Use class is where it can safely live. They are independent — a C24 board with a UC3 treatment is still the wrong thing to put in the ground.

My timber says WPCS. What is that?

Whitewood, pine, spruce — a species grouping rather than a single timber. It is normal on graded structural softwood and does not tell you anything about strength, which is what the C number is for.

The stamp is on the end I cut off.

Keep one stamped offcut from the pack. For anything that will be inspected, the stamp is the evidence the timber was graded, and once every piece is cut you have nothing to show.

Does the stamp tell me it is treated?

The use class does. UC3 and UC4 both mean it has been treated — the class tells you to what depth and for what exposure. Untreated structural timber carries no use class.

Is graded timber worth the extra?

For anything structural, it is not really optional. Ungraded timber has no known strength, so no span table applies to it and no building inspector will accept it.