Structural timber is the engineered backbone of modern timber construction, encompassing solid graded sections (C16, C24, C30), glued laminated beams (BSH/Glulam), and advanced engineered products like CLT and LVL. This guide covers everything architects and structural engineers need to specify timber correctly for load-bearing applications.
The European structural timber market is governed by harmonised standards (EN 338, EN 14081, EN 14080) that define strength classes, grading rules, and quality assurance requirements. Understanding these standards is essential for writing compliant specifications and ensuring structural performance.
Strength Classes and Grading Systems
European structural timber is classified into strength classes defined by EN 338. The most common classes for softwood construction are:
| Strength Class | Bending Strength (N/mm²) | Typical Use | Species |
|---|---|---|---|
| C16 | 16 | Light framing, non-structural | Spruce, Pine |
| C24 | 24 | General structural (most common) | Spruce, Pine, Larch |
| C30 | 30 | Heavy structural, long spans | Selected Spruce, Douglas Fir |
| D30 | 30 | Hardwood structural | Oak, Beech |
| D40 | 40 | High-performance hardwood | Oak, Ash |
C24 accounts for approximately 80% of structural timber specified in European construction. It offers the optimal balance between structural performance, availability, and cost. Higher grades (C30, C35) are specified only where engineering calculations demand additional capacity.
Machine Grading vs Visual Grading
Structural timber is graded by two methods:
Machine stress grading (MSG) uses automated equipment to measure stiffness (modulus of elasticity) of every piece. This is the preferred method for consistent quality and is required for CE marking under EN 14081-1.
Visual grading relies on trained graders assessing knot size, slope of grain, rate of growth, and other visual characteristics. While still permitted under EN 14081-2, visual grading produces more conservative results and higher rejection rates.
For architectural specifications, always specify machine-graded timber to ensure consistent structural performance and reduce site rejection rates.
BSH Glulam: Engineered Structural Timber
Glued laminated timber (BSH — Brettschichtholz) is manufactured by bonding multiple layers (laminations) of strength-graded timber with structural adhesive under controlled factory conditions. The result is an engineered product that exceeds the performance of solid timber in every structural metric.
Glulam Strength Classes
| GL Class | Bending Strength | Typical Span Range | Application |
|---|---|---|---|
| GL24h | 24 N/mm² | 6–12 m | Standard beams, purlins |
| GL28h | 28 N/mm² | 8–16 m | Primary beams, portal frames |
| GL32h | 32 N/mm² | 12–25 m | Long-span roofs, bridges |
| GL36h | 36 N/mm² | 15–30+ m | Sports halls, swimming pools |
The "h" suffix indicates homogeneous layup (all laminations same grade). Combined layup ("c" suffix, e.g. GL28c) uses higher-grade laminations in the outer zones where bending stress is greatest, optimising material use.
Why Specify Glulam Over Solid Timber?
- Longer spans: Glulam beams routinely span 12–25 metres without intermediate support
- Larger sections: Cross-sections up to 2000mm deep are achievable (solid timber limited to ~300mm)
- Dimensional stability: Factory-controlled moisture content (typically 12%) minimises in-service movement
- Architectural expression: Exposed glulam beams are a design feature, not hidden structure
- Curved forms: Glulam can be manufactured in curved profiles for architectural roofs and arches
- Predictable fire performance: Large sections char predictably, maintaining structural capacity
Specifying Glulam to EN 14080
A complete glulam specification includes:
- Strength class (GL24h, GL28h, GL32h, etc.)
- Service class (1, 2, or 3 per EN 1995-1-1)
- Appearance class (Industrial, Standard, or Architectural)
- Species (typically Norway Spruce or Scots Pine for softwood glulam)
- Adhesive type (Type I for all service classes, Type II for service class 1 only)
- Surface treatment (if required — typically left natural for interior exposed applications)
- Dimensional tolerances per EN 14080 Table 3
- CE marking and Declaration of Performance reference
Span Tables and Structural Design
Preliminary sizing of timber members can be estimated from span tables, but final design must always be verified by a structural engineer to EN 1995-1-1 (Eurocode 5).
Indicative Spans for C24 Softwood (Domestic Floor Joists)
| Section (mm) | Span at 400mm centres | Span at 600mm centres |
|---|---|---|
| 47 × 150 | 2.7 m | 2.4 m |
| 47 × 175 | 3.2 m | 2.8 m |
| 47 × 200 | 3.7 m | 3.2 m |
| 47 × 225 | 4.2 m | 3.6 m |
| 75 × 200 | 4.5 m | 4.0 m |
| 75 × 225 | 5.0 m | 4.5 m |
Based on 1.5 kN/m² imposed load + 0.5 kN/m² dead load (excluding self-weight). Verify with structural calculations.
Indicative Spans for Glulam Beams (Simply Supported)
| Section (mm) | GL24h Span | GL28h Span | GL32h Span |
|---|---|---|---|
| 120 × 300 | 6.0 m | 6.5 m | 7.0 m |
| 140 × 400 | 8.5 m | 9.5 m | 10.5 m |
| 160 × 500 | 11.0 m | 12.5 m | 13.5 m |
| 180 × 600 | 14.0 m | 15.5 m | 17.0 m |
| 200 × 800 | 18.0 m | 20.0 m | 22.0 m |
Indicative only — based on UDL from typical roof loading. Verify with structural calculations to EN 1995-1-1.
Fire Resistance of Structural Timber
Contrary to common perception, large-section structural timber performs predictably in fire. The charring rate method defined in EN 1995-1-2 allows engineers to calculate residual cross-section and structural capacity at any point during a fire.
Charring Rates (EN 1995-1-2)
| Timber Type | Charring Rate (mm/min) |
|---|---|
| Softwood (solid) | 0.65 |
| Softwood (glulam) | 0.70 |
| Hardwood (solid, density ≥ 450 kg/m³) | 0.50 |
| LVL (laminated veneer lumber) | 0.70 |
Design Approach
For a 60-minute fire resistance requirement:
- Charring depth = 0.70 × 60 = 42mm per exposed face
- Add 7mm zero-strength layer below char
- Residual section = original section minus (42 + 7) mm per exposed face
- Verify residual section capacity under fire-condition loading (reduced load factors)
A 200×400mm glulam beam exposed on three sides retains a residual section of approximately 102×302mm after 60 minutes — typically sufficient for fire-condition loading.
Comparison with Steel
Unprotected structural steel loses 50% of its strength at approximately 550°C, which can be reached within 15–20 minutes in a standard fire. Steel requires applied fire protection (intumescent paint, board encasement, or concrete encasement) to achieve fire ratings.
Timber's charring mechanism provides inherent fire resistance without applied protection, making it architecturally advantageous where exposed structure is desired.
Moisture Content and Dimensional Stability
Timber is hygroscopic — it absorbs and releases moisture in response to ambient humidity. Dimensional movement is proportional to moisture content change, occurring primarily across the grain (width and thickness) rather than along the grain (length).
Target Moisture Content by Application
| Application | Target MC | Tolerance |
|---|---|---|
| Internal structural (heated buildings) | 12% | ±2% |
| External structural (covered, ventilated) | 16% | ±2% |
| External structural (exposed) | 18–20% | ±3% |
| Glulam (all applications) | 12% | ±2% |
Why Moisture Content Matters
Timber installed above its equilibrium moisture content will shrink as it dries in service, causing:
- Joint opening and connection loosening
- Floor deflection increase
- Cracking and checking of exposed surfaces
- Risk of fungal decay if sustained above 20%
Always specify kiln-dried timber and verify moisture content on delivery using a calibrated pin-type or resistance meter. Reject deliveries exceeding the specified tolerance.
Connections and Fasteners
Timber connections are typically the critical design element — the timber itself rarely fails before its connections. Modern timber connection design uses:
Connection Types
| Type | Capacity | Application |
|---|---|---|
| Nails (annular ring) | Low–Medium | Sheathing, light framing |
| Screws (structural) | Medium–High | Beam-to-beam, joist hangers |
| Dowels (steel) | High | Portal frame joints, moment connections |
| Steel plates (concealed) | Very High | Glulam connections, architectural joints |
| Glued-in rods | Very High | Moment connections, hidden joints |
Design Principles
- Minimum edge and end distances must be maintained to prevent splitting (EN 1995-1-1, Section 8)
- Group effects reduce capacity when multiple fasteners are used in a row parallel to grain
- Moisture class affects connection capacity — service class 3 (external exposed) requires stainless steel fasteners
- Fire design of connections requires additional cover or protection to maintain capacity during fire exposure
Sustainability and Certification
Structural timber is the only major construction material that is renewable, carbon-sequestering, and available from certified sustainable sources. Specifying certified timber demonstrates environmental responsibility and supports tender compliance.
Certification Schemes
| Scheme | Coverage | Recognition |
|---|---|---|
| FSC (Forest Stewardship Council) | Global | Highest recognition in architecture |
| PEFC (Programme for Endorsement of Forest Certification) | Global (largest by area) | Strong in Europe |
| SFI (Sustainable Forestry Initiative) | North America | Limited European relevance |
Carbon Benefits
Structural timber sequesters approximately 1.6 tonnes of CO₂ per cubic metre. A typical timber-frame house stores 20–30 tonnes of CO₂ in its structure — equivalent to 10–15 years of household energy emissions.
When compared to steel or concrete alternatives, timber structural systems typically demonstrate 50–75% lower embodied carbon over the building lifecycle.
Procurement and Lead Times
European Supply Chain
The majority of structural softwood timber used in European construction originates from Scandinavian (Sweden, Finland) and Central European (Germany, Austria, Czech Republic) managed forests. Supply chains are well-established with typical lead times of:
| Product | Standard Stock | Custom Specification |
|---|---|---|
| C24 solid timber (standard sections) | 3–5 working days | 2–3 weeks |
| BSH glulam (standard sections) | 1–2 weeks | 3–6 weeks |
| BSH glulam (curved or large section) | 4–8 weeks | 6–12 weeks |
| CLT panels | 4–8 weeks | 8–16 weeks |
Specification Tips for Procurement
- Specify early: Glulam and CLT have longer lead times than steel — include in procurement schedule from RIBA Stage 4
- Standardise sections: Using standard available sections reduces cost and lead time vs. custom dimensions
- Consider transport: Maximum transport length is typically 13.6m (standard trailer). Longer elements require special transport arrangements
- Storage on site: Timber must be stored off the ground, covered, and ventilated. Specify storage requirements in the contract
Related Product Specifications
PuurModulair supplies the following structural timber products to architectural specification:
- C24 Structural Construction Timber — machine-graded softwood for general structural applications
- BSH Glued Laminated Beams — engineered glulam for long spans and architectural expression
- Glued Laminated Turned Logs — structural log elements for timber-frame and log-home construction
All products are supplied with CE marking, chain-of-custody documentation, and full technical data sheets. Request a custom specification quote for your project.