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C24 is the stronger and stiffer grade, but it is not automatically the right choice for every project. C16 is suitable for most general structural work when the member size, span, spacing, load and connections are designed for it. C24 becomes useful when a design needs more bending capacity, a longer span, tighter deflection control or a smaller section. The grade that should be ordered is the grade permitted by the structural design and the applicable building rules—not simply the highest number available.
Both C16 and C24 are EN 338 strength classes for structural softwood. The “C” means coniferous timber; the number is the characteristic bending strength in newtons per square millimetre (N/mm²). Those are statistical design properties, not a guaranteed load for every piece of timber or every beam.
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What C16 and C24 mean
Structural softwood is strength graded before it is sold for load-bearing work. Grading can be visual or machine based under the EN 14081 framework. The graded material is then assigned to an EN 338 strength class. C16 and C24 are two of the principal classes used for structural softwood in the UK, within a wider EN 338 system.
The class number identifies the characteristic bending strength. In practical terms, C24 timber has a higher statistical resistance to bending than C16 timber of the same cross-section, assuming the other design conditions are comparable. It does not mean that every C24 board will carry exactly 50% more load, nor that changing the label alone makes an inadequately sized member safe.
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The figures behind the grades
| Property | C16 | C24 | What it means in design |
|---|---|---|---|
| EN 338 characteristic bending strength | 16 N/mm² | 24 N/mm² | C24 has the higher characteristic resistance to bending. |
| Mean bending stiffness | 8 kN/mm² | 11 kN/mm² | C24 generally deflects less than C16 under the same comparable loading and section. |
| Fifth-percentile density | 310 kg/m³ | 350 kg/m³ | The class values describe a lower-tail density property used in design; they are not a promise that every board has exactly this density. |
The bending-strength figures are reported for EN 338 classes by Timber Development UK and Forest Research. Forest Research’s comparison gives the mean stiffness and fifth-percentile density values shown above. As Nick Boulton of Timber Development UK put it, “it’s all about the strength!” That shorthand is useful, provided it is not taken to mean strength is the only design check.
Which is stronger: C16 or C24?
C24 is stronger. Its characteristic bending strength is 24 N/mm², compared with 16 N/mm² for C16. C24 also has the higher stated mean bending stiffness, 11 kN/mm² versus 8 kN/mm² for C16. That combination can give a design more capacity against bending and better control of movement under load.
However, the class is only one input to a structural calculation. A C24 joist with an unsuitable depth, excessive spacing, poor support or an inadequate connection can still fail to meet the design requirements. A correctly sized C16 member can be entirely appropriate where the span tables or engineering calculations allow it.
How the grade affects a real member
Bending strength
Floor joists, rafters, beams and deck members bend under imposed loads and their own weight. The characteristic bending value is used by the designer with the relevant safety factors and load combinations. Because C24 starts with a higher characteristic value, a design may be able to carry a greater load, span farther or use a smaller cross-section than an otherwise comparable C16 design.
Do not turn 16 or 24 N/mm² into a simple allowable load by multiplying it by a board’s width. The actual calculation also depends on the section modulus, effective length, load duration, moisture or service condition, lateral restraint, supports and connections.
Stiffness and deflection
Strength and stiffness answer different questions. Strength concerns whether the member has adequate resistance; stiffness concerns how much it moves. Excessive deflection can produce a springy floor, a sagging roof line, cracked finishes or ponding on a deck even when a basic strength check is satisfactory.
The published mean bending stiffness values are 8 kN/mm² for C16 and 11 kN/mm² for C24. A designer still has to check the actual span, load and serviceability limit. The higher grade can help where movement is the governing issue, but it does not remove the need for a deflection check.
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Density and handling
Forest Research reports fifth-percentile density values of 310 kg/m³ for C16 and 350 kg/m³ for C24. These are statistical class properties used in structural design, not a direct specification of the mass of every length. Actual weight also changes with moisture content and dimensions. Do not use the grade label alone to estimate lifting loads, transport weight or fixing performance.
When C16 is the sensible choice
C16 is the usual economical choice for general structural applications when the design permits it. It is commonly considered for framing, roof timbers, joists and similar work. The key condition is that a span table or engineering calculation confirms the selected size, spacing and support arrangement.
Typical C16 situations
- General domestic framing where the specified member size meets the design span and load.
- Roof timbers designed for the stated roof build-up, snow or maintenance loading and restraint.
- Floor joists where span tables confirm both bending strength and acceptable deflection.
- Raised-deck joists, beams and posts when the design uses at least the required strength class and provides suitable support and lateral stability.
- Projects prioritising the use of commonly available UK-grown structural softwood, where C16 is the grade most often produced.
Choosing C16 can avoid paying for capacity the design does not need. It can also make sourcing easier in regions where C16 is more readily stocked. The decision should be recorded in the design documentation rather than made solely by a merchant’s default stock grade.
When C24 is worth specifying
C24 is useful when the design needs structural reserve that C16 cannot provide within the proposed dimensions or layout. Common reasons include:
- Longer spans: a higher strength and stiffness class may allow a longer clear span, subject to the full calculation.
- Heavier loads: concentrated loads, higher occupancy loads or a heavier roof or floor build-up may make the additional capacity valuable.
- Tighter deflection limits: where finishes, glazing, partitions or user comfort make movement particularly sensitive.
- Smaller sections: a higher grade may allow a shallower or narrower member when the design, connections and serviceability checks all permit it.
- Restricted depth: renovation work or a fixed floor build-up may leave little room for a deeper C16 member.
C24 is not automatically safer, mandatory or better value. If the design already passes with C16, upgrading every member to C24 may add cost without improving the outcome that matters to the project.
C16 and C24 for decks
For a raised deck, the posts, beams and joists are structural components and need a sufficient strength grade. Timber Development Association guidance identifies C16 as the minimum acceptable class for raised decks. C24 can provide longer spans or allow slightly smaller sections, but the deck still needs a design appropriate to its dimensions and use.
Checks that remain necessary
- Confirm the span and spacing of joists and beams rather than substituting a grade without recalculation.
- Check posts, bearers, joist hangers, bolts, screws and other connections for the loads they transfer.
- Provide lateral restraint so members cannot roll or twist under load.
- Account for moisture exposure, drainage and the relevant service condition; strength grade is not the same thing as preservative-treatment class.
- Follow local building-control requirements for height, guarding, stairs, foundations and fire considerations where they apply.
A deck board’s appearance or species does not establish its structural grade. Ask for structural-grade identification for the load-bearing components and keep the specification with the project records.
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How to choose the grade: a practical process
- Define the member’s job. Identify whether it is a joist, rafter, beam, post, stud or deck component, and whether it carries a uniform or concentrated load.
- Measure the real geometry. Record clear span, bearing length, spacing, overhangs, openings and the proposed cross-section. Small changes can alter the result substantially.
- Establish the loads. Include permanent loads such as the structure and finishes, imposed occupancy or maintenance loads, and relevant environmental loads. A heavy floor build-up or roof covering can change the grade or size required.
- Check a recognised span table or obtain a structural calculation. Use a table that matches the member type, spacing, loading and service condition. For unusual layouts, alterations, long spans or concentrated loads, use a competent designer.
- Compare C16 and C24 on the same design basis. If C16 passes strength and deflection checks at an acceptable size, it may be the economical specification. If it fails or becomes impractically deep, assess C24 or another structural solution.
- Check stability and connections. Verify lateral restraint, bearing, notches, holes, hangers, fixings and load paths. A stronger grade does not compensate for a weak connection or poor support.
- Specify the delivered product. State the EN 338 class, dimensions, moisture or service requirements, treatment requirements where applicable, and any visual or machine-graded marking required for acceptance.
- Inspect on delivery. Confirm that the marking and paperwork identify the required structural class. Separate damaged, excessively distorted or unidentified pieces instead of assuming the grade label covers every defect.
Grade is not the same as species, appearance or treatment
C16 and C24 describe strength classes, not a single tree species. Two pieces can look similar while belonging to different grades, and a visually attractive board is not automatically structural. Conversely, a graded structural piece may contain normal features permitted by its grading rules.
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Strength grade also does not state whether timber is treated, how durable it is in a particular exposure, or what moisture condition it will experience. Treatment, durability, service class and fire performance are separate specification questions. They must be addressed alongside the EN 338 class rather than inferred from it.
Availability and the UK supply context
The Forestry Commission reports that C24 is often treated as a default construction grade, while UK-grown timber is most commonly graded C16. It also states that C16 is suitable for most structural applications and can support greater use of homegrown timber where the design specification permits it.
This distinction matters when ordering. A merchant may routinely stock C24 for a particular product line, while another may have more UK-grown C16 available. Availability does not change the engineering requirement: do not substitute one class for another without checking the design. If a specification calls for C24, obtain written approval for any proposed C16 substitution; if the design permits C16, there may be no technical benefit in paying for C24 simply because it is the default on the shelf.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Common mistakes and how to avoid them
Assuming C24 is always required
Higher grade does not mean every project needs it. Start with the span, load and deflection requirement. Many general structural applications can use C16 when the design allows it.
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Changing from C16 to C24 cannot correct inadequate bearing, excessive spacing, poor restraint, an undersized connection or a member with unacceptable notches and holes. Recalculate the complete arrangement.
Using characteristic strength as a working load
The 16 and 24 N/mm² figures are characteristic properties. They are not a direct allowable stress or a beam’s permitted load without the relevant design method, safety factors and service conditions.
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Confusing visual quality with structural classification
Colour, straightness and a clean appearance are not proof of C16 or C24. Require the correct structural grading mark or supplier documentation.
Ignoring service conditions
Moisture, exposure, duration of loading and fire requirements can affect the design and specification. A grade comparison cannot answer those questions on its own.
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Do not downgrade C24 to C16 because the latter is available, and do not assume an upgrade removes the need for approval. Record substitutions and have the responsible designer confirm that the revised member and connections remain compliant.
Buying certified C16 or C24 timber
Before placing an order, give the supplier a written schedule rather than asking for “strong timber.” Include:
- Required EN 338 strength class: C16, C24 or another class stated by the design.
- Finished dimensions and lengths, including any tolerances that matter to the installation.
- Member use and location, such as internal floor, roof, external raised deck or protected beam.
- Required treatment or durability specification, kept separate from the strength-class requirement.
- Any machine-graded marking, certification or delivery documentation needed for building control or project records.
- Whether substitutions require the designer’s written acceptance.
On delivery, compare the markings and paperwork with the schedule. Keep pieces with serious damage, unidentified grading or defects that conflict with the design out of the load-bearing work until they have been assessed.
Examples of the decision in practice
Standard domestic floor
A floor with a routine span and ordinary residential loading may be designed with C16 joists at a specified spacing. If the selected section satisfies both bending and deflection limits, C24 is not automatically necessary. The joist depth, bearing, restraint and openings still have to match the calculation.
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Longer open-plan span
Removing an internal support increases the clear span and can make deflection the governing issue. The designer may compare a deeper C16 section with a shallower C24 section, or may select a different beam solution. The correct choice depends on the complete calculation, not on the grade number alone.
Raised deck with limited depth
If a deck layout cannot accommodate deeper joists, C24 may offer a way to meet span and movement limits with a slightly smaller section. C16 remains acceptable where it is the minimum permitted class and the specified dimensions, spacing, supports and connections pass.
Heavy roof build-up
Tiles, insulation, ceilings, solar equipment or other permanent loads can change the rafter design. A higher grade may be part of the solution, but increased section size, closer spacing, additional supports or a revised load path may be more appropriate. The roof design should be checked as a system.
Quick Recap
Key takeaways for a specification
- C24 has a characteristic bending strength of 24 N/mm²; C16 has 16 N/mm².
- Published mean bending stiffness is 11 kN/mm² for C24 and 8 kN/mm² for C16.
- C16 is suitable for most general structural applications when the design permits it.
- C24 is most useful for longer spans, heavier loads, tighter deflection limits or smaller sections.
- For raised decks, C16 is identified as the minimum acceptable class in the cited guidance; C24 may extend spans or reduce section size.
- Neither class tells you the species, treatment, moisture condition, fire performance or final member size.
- Use the grade selected by the structural design, verify the marking on delivery and obtain approval before substituting.
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