Buy structural timber to the design, not by nominal size or the C16/C24 label alone. Before ordering, confirm the finished section, length, strength class, moisture condition, treatment or Use Class and required quantity. On delivery, check those details against the drawings and order before the pack is distributed or signed for. Standard structural dimensions are referenced to 20% moisture content; timber delivered wetter can shrink as it dries, so the size you measure on the lorry may not be the size that fits after acclimatisation.
For most UK work, C16 is adequate when the design specifies it. C24 is stronger for the same section, but it is not automatically better value or necessary. The engineer’s calculations, not appearance or habit, determine the required grade and section.
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Start with the information on the drawings
A timber order is only correct when it matches the design information. Read the drawings, schedules and engineer’s calculations together, then give the merchant a written specification. “47 by 100” is incomplete if the order does not also state the length, grade, moisture condition, treatment and quantity.
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Record these fields for every item
- Product description and species or product type shown in the specification.
- Finished section, including whether the dimensions are nominal, regularised or planed.
- Cut length and number of pieces or pack quantity.
- Structural class, such as C16, C24 or TR26, and the grading standard required by the design.
- Dry, kiln-dried or other moisture condition, including any “DRY” or “KD” marking.
- Preservative treatment and the Use Class required for the exposure.
- Any special straightness, defect, certification, identification or delivery requirements.
Ask the merchant to state whether a quoted dimension is the target size or the finished measured size. That distinction matters when a joist, rafter, stud or trimmer has to fit an opening or connect to a proprietary bracket.
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Common structural timber sizes and lengths
Dimension timber is available in many combinations, but merchants commonly stock a core range. The following examples come from the UK dimension-timber range described by Timber Development UK (TDUK); availability and finished dimensions vary by supplier and machining process.
| Nominal section or dimension listed | How to use the information |
|---|---|
| 47 × 50 mm | A commonly listed small section; verify the actual finished measurement and grade. |
| 75, 100, 125, 150, 175, 200, 225 and 250 mm | Common width or depth dimensions used in larger sections; the drawing must identify the other dimension. |
| 75 × 150 mm | Common rectangular section. |
| 75 × 175 mm | Common rectangular section. |
| 75 × 200 mm | Common rectangular section. |
| 75 × 225 mm | Common rectangular section. |
| 100 × 200 mm | Common larger rectangular section. |
Stock lengths commonly increase in 0.3 m increments from 2.4 m to 4.8 m. Longer lengths may be available to order, but lead time, transport and handling can change. Do not assume that a nominal 4.8 m piece will be supplied at exactly 4,800 mm after machining or drying.
Allow for tolerances
BS EN 336:2012 defines dimensional tolerances for structural timber. Sawn timber is supplied under Tolerance Class 1; planed or machined timber is supplied under Tolerance Class 2. These classes describe permitted variation, so two pieces sold under the same nominal description are not guaranteed to measure identically at every point.
TDUK’s standard structural-timber dimensions use a total moisture content of 20% as the reference condition. Measure the section when tolerances affect the work, especially at junctions, truss seats, stair openings and prefabricated panels. A pack label is not a substitute for checking the finished piece.
Choosing C16, C24 or TR26
C16 and C24 are the two principal strength classes of structural softwood used by UK designers and buyers. The “C” identifies a coniferous timber class and the number is its characteristic bending-strength class: C16 is 16 N/mm² and C24 is 24 N/mm². For an identical section, C24 has greater characteristic bending strength and may provide more capacity or span, but the section, loading, support conditions and connection details still control the design.
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| Class | Characteristic bending strength | Typical decision |
|---|---|---|
| C16 | 16 N/mm² | Often suitable for ordinary floors, walls, roofs and framing when the design specifies it. |
| C24 | 24 N/mm² | Used where calculations require higher capacity or a longer span for the selected section. |
| TR26 | Higher structural class associated with trussed-rafter material; no numerical bending value is specified for TR26 in this guide. | Use only where the truss or structural specification calls for TR26. |
The UK Government’s 2025 Timber in Construction Roadmap cautions that familiarity with imported C24 can lead to overspecification. Paying for C24 does not correct an undersized section, poor support or an unsuitable connection. Conversely, substituting C16 where C24 is specified can reduce capacity and invalidate the design.
Use the grade on the design, not visual judgement
Strength grading is not a reliable visual “looks strong” assessment. Knots, slope of grain, splits and other features are assessed under the applicable grading rules. Confirm the grade stamp and supplier documentation. Where specified, check that the product is graded and marked to BS 4978 or BS EN 14081. Never upgrade or downgrade a member because the stamp is missing, a piece looks straighter, or another class is temporarily unavailable; obtain design approval first.
Moisture content changes both size and sequence
Structural dimensions are referenced to 20% moisture content. Sawn timber produced wetter than that can reduce in width and thickness while it dries. Timber milled below 20% can take up moisture and expand later. Consequently, a piece that fits a gauge or opening on delivery can move after installation, while a piece that appears undersize may have been measured in a different moisture state.
What to ask the supplier
- Was the timber kiln dried or otherwise dry graded?
- Is it marked “DRY” or “KD” where the specification requires that condition?
- Are the quoted dimensions nominal target sizes or finished machined sizes?
- Was the timber graded and marked at the moisture condition required by the design?
A pin moisture meter is useful for screening a delivery and finding unusually wet pieces. Take several readings from representative members rather than relying on one surface reading. Meter readings are affected by temperature, species, grain direction and contact, and a single result cannot replace the specified grading evidence or an engineer’s acceptance.
Do not close up wet work
Keep timber covered, off the ground and ventilated. Let it equalise with the project environment before enclosing it in a wall, floor or roof, while following the project specification. Closing wet members can lock in movement and create fit, finish or durability problems. Acclimatisation is not permission to use a lower grade or an incorrect treatment; it only manages moisture movement in material that already meets the design.
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Treatment and Use Class are separate from strength
Strength grade answers how a member performs structurally. Treatment and Use Class address exposure and decay risk. A C24 stamp does not mean that timber is suitable outdoors, and pressure treatment does not turn an under-strength member into a stronger one.
Confirm the Use Class for the actual location: internal dry conditions, a risk of intermittent damp, ground contact or external exposure may require different protection. Do not assume indoor-rated Use Class 2 timber is suitable for exposed external work. Ask what preservative was used, whether the treatment is factory applied and what restrictions apply after cutting.
When treated timber is cut, follow the treatment manufacturer’s or project specification’s requirement for end-grain protection. Keep treated and untreated packs identified so that pieces are not swapped during installation.
Inspect every delivery before accepting it
NHBC guidance says materials should be checked on delivery for conformity with the design. Make the inspection while the pack is accessible, not after it has been spread through the building. Photograph discrepancies before unloading or before signing a delivery note that says “received in good condition”, and follow the merchant’s written claims window.
Delivery inspection sequence
- Match the paperwork. Compare the delivery note, order and drawings for product description, section, length, grade, dry or kiln-dried condition, treatment, Use Class and quantity.
- Check the pack identity. Find the visible grade stamp or label showing the class, producer or identification mark and any dry or treatment condition. Confirm the stated grading standard where the specification names one.
- Measure critical sections. Use a tape or calliper on representative pieces when a finished dimension matters. Record whether your measurement is being made on sawn, planed or machined faces.
- Check lengths and count. Verify the requested cut lengths and count the pieces or bundles. Separate short, damaged or mislabelled members before they are mixed with accepted stock.
- Look for moisture problems. Identify fresh saturation, standing water, mould or a pack that is visibly much wetter than the rest. Use a meter for screening if required, but do not treat one reading as a grading certificate.
- Inspect straightness and defects. Look along each critical member for severe bow, twist, cup or spring. Check for splits, loose or oversized knots, insect damage and crushed ends where they could affect the design or connection.
- Verify treatment. Confirm that the treatment and Use Class match the location. Check that cut ends or damaged surfaces have not lost the required protection.
- Document and quarantine discrepancies. Photograph marks, defects, labels and the pack number. Keep disputed timber separate and notify the merchant within the stated claims period.
Defects that need a design decision
Not every knot or small surface check makes a member unusable; grading rules allow defined characteristics. The practical question is whether the defect exceeds the specified grade or affects the member’s role. A severe bow may prevent a stud from lining up, while a crushed end can reduce bearing or connection area. If a questionable piece is part of a designed beam, truss, lintel or heavily loaded joist, stop and ask the designer or engineer rather than trimming it until it “looks right”.
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Store accepted timber so it can stabilise
Place packs on level bearers or other supports that keep them off the ground. Use stickers or spacers so air can circulate through the pack, cover the top against rain and leave the sides ventilated. Avoid wrapping wet timber in impermeable film or stacking it directly on slabs where moisture can be trapped.
Keep packs level to limit additional bowing, and protect ends from impact during handling. Store treated material so its identification remains visible. Before closing a wall, floor or roof, confirm that the timber has reached the moisture condition allowed by the project specification and that movement will not compromise finishes, insulation or airtightness.
Compare products only after they meet the design
When two offers appear to satisfy the drawings, compare the attributes that affect installed performance and risk rather than choosing the cheaper grade label.
| Comparison point | Question to ask | Why it matters |
|---|---|---|
| Structural class and section | Does the member have the specified class and calculated section capacity? | A stronger label cannot compensate for an incorrect size or support condition. |
| Finished dimensions and tolerances | Are dimensions nominal, regularised, planed or machined, and to which tolerance class? | Fit and connection details depend on the actual section. |
| Moisture condition | Is it dry graded, kiln dried, marked DRY/KD, or supplied wetter? | Drying or moisture uptake can change dimensions. |
| Treatment and Use Class | Does protection match the exposure, and how are cut ends treated? | Indoor and external durability requirements differ. |
| Straightness and defect allowance | What grading rules apply and how are disputed pieces handled? | Defects can affect fit, bearing and strength. |
| Grade mark and identification | Will every pack carry a visible class and producer mark? | Traceability supports acceptance and future claims. |
| Species and source | Does the specified species or product description match the order? | Design assumptions and treatment instructions may depend on the product. |
| Pack quantity and lengths | Are there enough usable pieces in the required increments? | Shortages can delay framing or force unapproved joints. |
| Lead time and delivery access | Can the vehicle reach the site and deliver when the structure is ready? | Long or awkward packs need a safe unloading plan. |
| Returns and claims | What evidence and timescale apply to rejected timber? | Clear records protect the project if material is non-conforming. |
Common buying decisions on site
“The drawing says C16, but C24 is available today”
Do not change the class unilaterally. Ask the designer whether the alternative is acceptable and whether connections, deflection limits or detailing change. If both classes are permitted, compare the delivered cost, availability, straightness, moisture condition and the cost of handling or rework; C24 is not automatically the economical choice.
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“The timber is the right nominal size but will not fit”
Check whether one piece is sawn and the other is planed, and record the moisture condition. Confirm the tolerance class and measure several pieces. Do not remove substantial material from a structural member without design approval. A merchant may have supplied the correct nominal section even though the finished size differs from the dimension assumed by a prefabricated detail.
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“A pack arrived wet or mouldy”
Photograph the condition, record moisture screening results and keep the pack ventilated and separate. Ask the merchant whether the timber was specified as dry graded or kiln dried. Do not seal it into the structure while the project specification requires a drier condition. Surface mould, persistent saturation or dimensional movement may justify rejection; the decision should follow the supplier’s grading evidence and the engineer’s acceptance.
“The stamp is missing or unreadable”
Quarantine the affected members. Request traceability documents or replacement stock with a legible grade mark. Appearance alone cannot establish C16, C24 or TR26, and an unmarked piece should not be substituted into a designed structural position simply because it looks straight.
“Treatment was cut off at the end”
Check the treatment specification and apply the required end-grain protection before installation. If the timber’s Use Class or treatment cannot be verified, stop using it in the exposed location and obtain written confirmation or replacement material.
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For each delivery, keep the order, drawings, delivery note, pack labels, photographs and inspection notes together. Record the date, weather or storage condition, pieces quarantined, meter readings if taken, and the person who accepted or rejected the material. This creates a traceable link between the installed member and its specified class, dimensions, moisture condition and treatment.
The reliable buying rule is simple: specify the section and class from the design, confirm the 20% moisture reference and treatment requirement, then inspect the actual pack for marks, dimensions, defects and quantity before acceptance. Store accepted timber off the ground with ventilation, allow it to equalise as specified, and obtain design approval for every substitution.
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