C16 and C24 timber are both strength-graded structural softwoods, with C24 the stronger grade. The number refers to the strength as measured in Newtons per square millimetre: C16 has a characteristic bending strength of 16 N/mm², while C24 has a characteristic bending strength of 24 N/mm².

In practical terms, C24 is generally better for longer spans, heavier loads and more demanding structural work, while C16 is often suitable for general construction, studwork, floor joists, roof timbers and shorter-span applications where the specification allows it. The C in the designation refers to “Coniferous”, as this wood is all coniferous softwood.

For many domestic and trade projects, the right choice will depend on the structural design, span, load, timber size and whether the job has been specified by an architect, engineer or Building Control.

What does C16 timber mean?

C16 timber is a structural softwood strength class commonly used in UK construction. The “C” refers to coniferous softwood, while the number refers to its characteristic bending strength. C16 timber has a bending strength of 16 N/mm², making it suitable for many everyday structural uses.

C16 timber is commonly used for:

• Internal stud walls
• General carcassing
• Shorter-span floor joists
• Roof timbers
• Timber framing
• Decking subframes, where suitable weather treatment is applied

It is often chosen because it offers a good balance between structural performance and cost. For many standard building jobs, C16 will be sufficient, provided it matches the project’s required span and load.

What does C24 timber mean?

C24 timber is a high-strength structural softwood strength class commonly used in UK construction where strength is needed. The “C” refers to coniferous softwood, while the number refers to its characteristic bending strength. C24 timber has a bending strength of 24 N/mm², making it suitable for many everyday structural uses.

C24 timber is often used for:

• Longer-span floor joists
• Roof structures
• Rafters
• Structural framing
• Load-bearing applications

C24 timber usually has fewer strength-reducing defects, such as large knots, and may be selected where a more reliable or higher-performing structural grade is required. It is sometimes sourced for this specifically, as it is very uniform in appearance.

Is C24 timber stronger than C16?

Yes, C24 timber is stronger than C16 timber. The number in the strength class indicates the timber’s characteristic bending strength, so C24 is rated higher than C16. C16 is rated at 16 N/mm², while C24 is rated at 24 N/mm², so whilst both are graded timber, C24 is rated at higher strength.

That does not mean C16 is “weak” or unsuitable. It simply means C24 has a higher structural strength class. In many cases, C16 is perfectly suitable for general construction work. However, C24 may be needed where the timber has to span further, carry more weight or meet a specific structural design.

C16 vs C24 Timber: At a Glance

• C16: 16 N/mm² bending strength, suitable for general construction and shorter spans.
• C24: 24 N/mm² bending strength, suitable for heavier loads and longer spans.
• C16: Usually the more economical option.
• C24: Often specified for structural framing and demanding applications.

When should you use C16 timber?

You should use C16 building grade timber when the project specification allows it and the timber is suitable for the required span and load.

C16 is commonly suitable for:

• General structural carcassing
• Stud walls
• Shorter joist spans
• Roof timbers in standard domestic applications
• Low load framing work

When should you use C24 timber?

You should use C24 timber when the project needs a higher strength grade, when the drawings specify it, or when the timber is being used in a more demanding structural position.

C24 is often the better choice for:

• Longer floor joist spans
• Heavier load-bearing applications
• Roof structures
• Structural beams and framing
• Projects where an architect or engineer has specified C24
• Jobs where a higher-grade structural timber is required for compliance

If a plan, span table or specification calls for C24, you should not substitute it with C16 without professional approval. Although both are structural grades, they are not automatically interchangeable.

Can you use C16 instead of C24?

You should not use C16 instead of C24 if the plans, span tables or structural calculations specify C24. C16 has a lower strength rating, so swapping C24 for C16 could reduce the load-bearing capacity of the structure.

However, C16 may be suitable where:

• The design allows C16
• The span is shorter
• The load is lighter
• Building Control or the project specification accepts C16
• The timber size has been selected to match the required performance

If you are unsure, check the structural drawings, span tables or ask a qualified builder, structural engineer or timber supplier before buying.

Can you use C24 instead of C16?

In many cases, C24 can be used instead of C16 because it is the higher strength grade. However, you should still check the required size, treatment, moisture content and specification before substituting timber.

C24 may cost more than C16, so there is not always a benefit to upgrading if C16 already meets the job requirements. The best option is usually the timber grade that matches the project specification without unnecessary extra cost.

Is C16 or C24 better for joists?

C24 is generally better for longer joist spans or heavier loads, while C16 may be suitable for shorter spans and standard domestic applications.

For floor joists, the correct choice depends on:

• Joist size
• Span length
• Spacing between joists
• Expected load
• Timber grade
• Building Regulation requirements
• Whether the floor is domestic or commercial

Is C16 or C24 better for decking?

C16 is often suitable for decking subframes, but C24 may be preferred for larger decks, heavier loads or longer spans. The grade is only one part of the decision. For outdoor use, the timber also needs to be appropriately treated for durability.

For decking frames, consider:

• Structural grade
• Joist spacing
• Span between supports
• Ground clearance
• Drainage and ventilation
• Preservative treatment
• Fixings and installation quality

Treatment helps protect timber from decay, but it does not increase the structural strength grade. A treated C16 timber remains C16; a treated C24 timber remains C24.

What is regularised timber?

Regularised timber has been machined to a more consistent finished size, making it easier to work with on site. It often has eased edges, which makes handling safer and more comfortable.

Regularised timber is useful for:

• Joists
• Studwork
• Roof timbers
• Carcassing
• Framing
• General structural work

What is kiln-dried timber?

Kiln-dried timber has been dried in controlled conditions to reduce moisture content and improve stability. This helps reduce movement, twisting and shrinkage after installation.

For structural projects, kiln-dried timber is often preferred because it is more stable and predictable than wetter timber.

How can you tell if timber is C16 or C24?

Strength-graded structural timber should usually carry a grade stamp or marking. This helps identify the timber’s grade, standard and grading information. UK construction timber is commonly machine or visually graded, with C16 and C24 being two of the main softwood strength classes used in the UK.

When buying timber, check:

• The grade: C16 or C24
• The size: nominal and finished size
• Whether it is treated or untreated
• Whether it is kiln-dried
• Whether it is regularised
• Whether it matches the project specification

If a project requires structural timber, avoid using ungraded timber unless a qualified professional has confirmed it is suitable.

C16 vs C24 timber: which should you buy?

Buy C16 timber if you need a cost-effective structural timber for general construction and the project specification allows it. Buy C24 timber if you need a stronger grade for longer spans, heavier loads or specified structural work.

As a simple rule:

• Choose C16 for general-purpose structural work where the design allows it.
• Choose C24 for higher-strength applications, longer spans and specified load-bearing work.
• Follow the drawings or structural calculations where provided.
• Do not downgrade from C24 to C16 without approval.
• For outdoor or exposed work, choose correctly treated timber as well as the correct strength grade.

Harlow Bros supplies a range of graded timber, including C16 and C24 options for construction projects, with treated, regularised and kiln-dried products available across the range, all of which has been green pressure treated.