C24 is the most widely specified structural timber grade in European construction, defined by EN 338 as having a characteristic bending strength of 24 N/mm². For architects and structural engineers, understanding the differences between C16, C24, and C30 is essential for writing correct specifications and optimising structural design.
This guide explains the European structural timber grading system, compares the three most common strength classes, and provides practical guidance on when to specify each grade.
Understanding the EN 338 Strength Class System
The European standard EN 338 defines strength classes for structural timber based on three key properties:
| Property | C16 | C24 | C30 |
|---|---|---|---|
| Characteristic bending strength (fm,k) | 16 N/mm² | 24 N/mm² | 30 N/mm² |
| Mean modulus of elasticity (E0,mean) | 8,000 N/mm² | 11,000 N/mm² | 12,000 N/mm² |
| Characteristic density (ρk) | 310 kg/m³ | 350 kg/m³ | 380 kg/m³ |
| Mean density (ρmean) | 370 kg/m³ | 420 kg/m³ | 460 kg/m³ |
The strength class number (16, 24, 30) directly indicates the characteristic bending strength in N/mm². Higher numbers mean stronger timber capable of longer spans or heavier loads.
C16: Light Framing Grade
C16 is the lowest structural grade commonly available. Its applications are limited to:
- Non-load-bearing partitions and studwork
- Light roof battens and counter-battens
- Temporary works and formwork
- Agricultural buildings with low loading requirements
- Budget-driven projects where structural demands are minimal
When NOT to use C16: Floor joists, primary roof rafters, lintels, or any member where deflection limits are critical. The low stiffness of C16 (E = 8,000 N/mm²) means excessive deflection is often the governing design criterion before strength is reached.
C24: The Standard Structural Specification
C24 is the default specification for structural timber in European construction. Approximately 80% of structural softwood sold in the UK and Northern Europe is graded to C24.
Why C24 Dominates
- Optimal strength-to-cost ratio: C24 provides 50% more bending strength than C16 at approximately 10-15% higher cost
- Availability: Virtually all structural timber suppliers stock C24 in standard sections
- Code compliance: Building regulations and span tables are typically written around C24 performance
- Stiffness: The significantly higher modulus of elasticity (11,000 vs 8,000 N/mm²) means C24 deflects less under load, often the critical design criterion for floors
- Standardisation: Specifying C24 across a project simplifies procurement and reduces site errors
Standard C24 Sections (UK/EU Market)
| Width (mm) | Depths available (mm) |
|---|---|
| 38 | 89, 140, 184, 235 |
| 47 | 100, 125, 150, 175, 200, 225, 250, 300 |
| 63 | 150, 175, 200, 225, 250, 300 |
| 75 | 150, 175, 200, 225, 250, 300 |
Standard lengths: 2.4m to 6.0m in 300mm increments. Longer lengths (up to 7.2m) available on request with extended lead times.
C30: High-Performance Solid Timber
C30 is specified when C24 is structurally insufficient but the designer wants to maintain solid timber rather than switching to BSH glulam. Typical applications:
- Long-span floor joists (4.5–5.5m) where C24 section depth is constrained
- Primary roof rafters with heavy tile loading
- Exposed structural members where smaller sections are architecturally preferred
- Lintels and headers carrying concentrated loads
C30 Availability Considerations
C30 is less readily available than C24 because:
- Fewer trees in a harvest meet the higher grading threshold
- Not all sawmills stock C30 as a standard product
- Lead times are typically 1–2 weeks longer than C24
- Premium of 20–35% over C24 pricing
Specification tip: Before specifying C30, verify availability with your supplier. In many cases, increasing the C24 section depth by one increment (e.g. 200mm → 225mm) achieves the same structural result at lower cost and shorter lead time.
Machine Grading: How Quality is Assured
Modern structural timber grading uses automated machine stress grading (MSG) equipment that measures the stiffness of every piece passing through the production line.
How Machine Grading Works
- Each piece passes through rollers that apply a known bending force
- Deflection is measured precisely, calculating the modulus of elasticity
- Pieces are automatically sorted into strength classes based on measured stiffness
- Accepted pieces are ink-stamped with grade, standard reference, and grading body mark
- Rejected pieces are downgraded or diverted to non-structural use
Grade Stamp Information
A compliant grade stamp contains:
- Strength class: C24 (or C16, C30, etc.)
- Standard reference: EN 14081-1
- Grading body: Identification number of the certified grading organisation
- CE marking: Indicates compliance with the Construction Products Regulation
- Species or species group: e.g. "Spruce/Pine/Fir" or specific species
Specifying Timber Grade in Practice
For Tender Documents
A complete structural timber specification clause should include:
Structural softwood timber shall be machine stress graded to strength
class C24 in accordance with EN 14081-1, CE marked, and supplied with
a Declaration of Performance. Timber shall be kiln-dried to a moisture
content of 12% (±2%) at time of delivery. All timber shall carry FSC
or PEFC chain-of-custody certification.
Common Specification Errors
- Specifying "structural grade" without a class: Always state C16, C24, or C30 explicitly
- Omitting moisture content: Timber delivered at 20%+ MC will shrink and cause defects
- Not requiring CE marking: Without CE marking, there is no third-party verification of grade
- Mixing grades on site: Ensure different grades are clearly marked and stored separately
Cost Comparison
| Grade | Relative Cost (C24 = 100%) | Availability | Lead Time |
|---|---|---|---|
| C16 | 85–90% | Good | 3–5 days |
| C24 | 100% (baseline) | Excellent | 3–5 days |
| C30 | 120–135% | Limited | 1–3 weeks |
| GL24h (glulam) | 250–350% | Good | 1–3 weeks |
For most projects, C24 represents the optimal specification. The small premium over C16 is justified by significantly better structural performance, while the jump to C30 is rarely cost-effective compared to increasing section size in C24.
Related Resources
- Structural Timber Specification Guide — comprehensive pillar guide
- BSH Glulam Beams: Span Tables & Specification — for spans beyond C24 capability
- C24 Structural Construction Timber — product specification and quote request
- BSH Glued Laminated Beams — engineered alternative for long spans