C16 vs C24 Timber: When Homegrown UK Wood Fits the Job

The United Kingdom is a major timber importer, but that fact needs a defined measure. Forest Research recorded 6.3 million cubic metres of sawnwood imports in 2024, alongside 3.1 million cubic metres of imported wood-based panels. The total value of wood-product imports was £9.2 billion, of which £4.3 billion was pulp and paper.

Those categories should not be collapsed into an unsupported claim that one percentage describes all “timber.” A more precise policy-level statement comes from the UK government’s 2025 Timber in Construction Roadmap: it identifies the UK as the world’s third-largest net importer of timber and wood products.

The roadmap also points to a practical specification question. Most domestic softwood is graded to strength class C16, while the construction market is highly familiar with C24, a common grade for imported timber. The government argues that habitual C24 specification can exclude suitable homegrown material. That does not make C16 and C24 interchangeable.

What C16 and C24 mean

Under the EN 338 strength-class system, the letter C denotes coniferous timber and the number corresponds to characteristic bending strength in newtons per square millimetre. The class also carries other characteristic properties used in design, including stiffness and density values.

C24 has higher declared characteristic properties than C16. That statement does not tell a designer whether a particular member needs C24. Loads, span, section, spacing, restraint, service conditions, fire strategy and the wider structural system all affect the design.

The same section might work in either class in one application but not another. Elsewhere, a C16 option may need a different section, spacing or arrangement. An industry report published by the Alliance for Sustainable Building Products includes worked comparisons showing that the achievable span can change when the same section is assessed in the two grades under stated conditions. Those examples illustrate the design dependency; they are not reusable span tables for every project.

Only the responsible structural designer should approve the grade and any resulting geometry.

Strength class is not species, appearance or durability

C16 and C24 are performance classes, not tree species. Different softwoods can be graded into a class if they meet the applicable requirements. A piece should carry the required grading and product information rather than being accepted because it looks dense or has few visible knots.

A higher strength class does not automatically establish better appearance. Merchants may separately select stock for visual quality, but that is a commercial appearance specification.

Strength class also does not establish durability. Timber Development UK notes that durability requirements must be specified separately for the intended exposure or service environment. Species, sapwood content, moisture management and preservative treatment may all be relevant. C24 should not be described as inherently more durable than C16.

These distinctions protect both directions of the comparison: domestic C16 should not be dismissed as “low-quality,” and imported C24 should not be portrayed as unnecessary without calculation.

Ask the supply question early

The most useful time to test a homegrown option is before the design and procurement schedule are locked. The designer can ask suppliers what graded timber, sections and lengths are realistically available, then assess those options within the structural design.

An evidence-based conversation should cover:

• strength class and grading documentation;

• actual finished dimensions and available stock lengths;

• species or species group, where relevant;

• moisture condition, treatment and intended exposure;

• lead time, minimum order and machining;

• legality, responsible sourcing and chain-of-custody requirements;

• the effect of any changed section or spacing on the complete assembly.

“British timber” is not a complete product specification. Neither is “C16.” The procurement route still needs verified performance, dimensions, documentation and suitability for the project.

Turn the approved schedule into quantities

Once the structural option has been checked and the member schedule approved, volume and cost become useful coordination questions. Group identical members by actual thickness, width and length, and record the count. Keep C16 and C24 options separate if the design gives them different sections or spacing.

Use the Timber Volume, Weight & Cost Calculator to convert each approved schedule into total linear length, cubic metres, cubic feet and geometric board feet. A documented density can support an estimated timber mass, and a supplier’s unit price can support a material-cost comparison.

The tool does not decide whether C16 can replace C24. It does not calculate spans, loads, deflection, connections, fire resistance or code compliance. Entering the same geometry under a different label would conceal the design changes that may be required.

Cost also needs a matched basis. A quotation per cubic metre should use cubic metres; a piece-based quotation may reflect stock-length rounding, machining or minimum quantities that a continuous-volume estimate cannot reproduce. Transport, tax, treatment, connectors, labour and redesign are outside a timber-only material result unless explicitly added elsewhere.

What the import data does—and does not—prove

Forest Research’s 2024 figures demonstrate the scale and range of UK wood-product imports. They do not show that every imported product has a domestic substitute, or that choosing C16 will remove the need for international trade.

Some dimensions, species, appearance grades and engineered products may be more available from overseas. Domestic supply is also constrained by processing capacity, future softwood availability and the need for sustainable woodland management. The government roadmap treats homegrown timber as one part of a more resilient supply strategy, not a closed-border solution.

The defensible opportunity is therefore specific: where a verified design can use available C16, an automatic C24 default should not prevent that option from being considered.

Better specification starts with an honest requirement

C16 versus C24 is not a contest with one permanent winner. It is a request to separate habit from performance. Designers need to state the properties the project requires, suppliers need to show what is available and compliant, and estimators need to compare the resulting schedules rather than assuming identical geometry.

That sequence creates a fair test for homegrown timber: structural verification first, transparent quantity and price comparison second, and procurement claims that remain traceable to real products.

Sources

• Forest Research, “Forestry Statistics 2025—Chapter 3: Trade,” including corrected 2024 figures: https://www.forestresearch.gov.uk/tools-and-resources/statistics/publications/forestry-statistics/forestry-statistics-2025/2025-3-trade/

• UK Government, “Timber in Construction Roadmap 2025”: https://www.gov.uk/government/publications/timber-in-construction-roadmap-2025/timber-in-construction-roadmap-2025

• Timber Development UK, “C16 and C24 timber: your questions answered”: https://timberdevelopment.uk/c16-c24-timber-your-questions-answered/

• The Institution of Structural Engineers, “C16 timber: supporting a transition to more sustainable timber specification in the UK,” 2 July 2026: https://www.istructe.org/journal/volumes/volume-104-%282026%29/issue-7/c16-timber-more-sustainable-specification-uk/

• Alliance for Sustainable Building Products, “IMPACTT—Innovative Mapping and Processes to Advance Construction Timber Transparency,” 2025: https://asbp.org.uk/wp-content/uploads/2025/02/ASBP-Report-digital-paginated-version.pdf