What do the stamps and markings on timber and sheet materials mean?
Decoding the strength grade, the drying state, the treatment class, the board type code and the certification marks — and which of them actually change what you can do with the piece.
- Difficulty
- beginner
- Time
- 9 min
- Read
- 4 min
Short answer
On structural timber, look for the strength class (C16, C24), the species code, the drying state (DRY or KD, or WET), a grader or machine mark and the certifying body. On sheet materials, look for the board type and class (OSB/3, P5, MDF.MR), the standard it is made to and the formaldehyde class. On treated timber, look for the use class. If a piece carries no mark at all, treat it as ungraded and do not use it structurally.
Timber and boards arrive covered in printing that almost nobody reads, and it contains exactly the information you need to know whether a piece is fit for the job. The marks are not decoration; on structural timber they are the only evidence that the piece has been graded at all.
Step by step
- Find the strength class on structural timber.C16 and C24 are the two you will see, printed or stamped along the face. C24 is stiffer and stronger and spans further for the same section. The class is what a span table is written against, so a piece without one cannot be sized from a table.
- Read the drying state.DRY or KD means dried to a specified moisture content and graded in that state. WET or a green marking means graded wet, at whatever moisture content it was, and it will shrink as it dries in place. Both are legitimate; they tell you how much movement to expect.
- Note the grader or machine mark and the certification body.Structural timber must be graded either visually by a qualified grader or by machine, and the mark identifies who did it and under whose scheme. Together with the standard reference, this is what building control looks for on site.
- Find the use class on treated timber.Treated stock should be marked or labelled with the use class it has been treated to — most usefully whether it is suitable for ground contact. This is the single most important marking on garden timber and the one most often missing.
- Read the board type code on sheet materials.OSB/2, OSB/3, OSB/4 for oriented strand board, in rising order of moisture and load performance. P2 to P7 for chipboard, where P5 is the moisture-resistant structural flooring grade. MDF, MDF.MR for moisture resistant, MDF.H for high density. Plywood carries a bond class and a face grade instead.
- Look for the formaldehyde class.E1 is the standard emission class for boards sold in Britain and is what you should expect on MDF, chipboard and most plywood. Lower-emission products are marked accordingly. It is worth checking on anything going into a bedroom or a nursery.
- Understand the certification marks.FSC and PEFC are chain-of-custody schemes about where the timber came from, not about its quality or grade. CE and UKCA marks show the product meets the relevant construction product requirements. None of them tells you the strength class or the treatment.
- Treat unmarked material as ungraded.Timber with no strength mark has not been graded, and no amount of it looking sound makes it structural. Use graded material for anything carrying load, and keep the unmarked stock for battens, noggins and jigs.
Tips
- The marks are usually printed on one face only and are often on the face that ends up hidden. Check both faces of a board before you decide there is no mark.
- Merchants can normally supply a delivery note or a certificate stating the grade of what they sold you, and building control may ask for it. Keep it with the job paperwork.
- On sheet goods, the printed stamp is often on the back face near a corner. If the sheets are stacked face up in the rack, ask to see the back of one before you buy.
Common mistakes
- Treating an FSC logo as a quality mark — It certifies the supply chain of the timber, nothing about strength, treatment or moisture. A perfectly certified board can still be ungraded, untreated and unsuitable.
- Cutting the marked end off and losing the evidence — If building control needs to see the grade, the printed face is the evidence. Keep an offcut with the mark, or the delivery note, until the job is signed off.
- Assuming green timber is treated to a ground-contact class — The colour is dye. Ground contact is a treatment level and it is stated in the marking or the pack label, not in the shade of green.
Questions people ask
What does 'wet graded' mean and is it a problem?
It means the piece was strength graded at a moisture content above about 20 per cent, so the grading rules used allowed for that. It is completely standard for carcassing. It matters practically because that timber will shrink in service, so it is not what you want for internal joinery or for anything you are finishing immediately.
Is there a marking that tells me a board is waterproof?
No board is waterproof. What the markings tell you is the moisture class the board is rated for — OSB/3 for humid conditions, P5 for moisture-resistant flooring, MDF.MR for moisture resistance, and a bond class on plywood. All of those describe resistance to humidity and occasional wetting, not immersion.
What if the timber I have is unmarked but obviously fine?
Use it for anything non-structural. For a joist, a rafter, a beam, a lintel trimmer or a loadbearing stud, buy graded timber. It is not expensive, and the grade is the only reason a span table applies to it.