Choosing the right timber structural system is one of the most consequential decisions in a timber building project. The three primary systems — lightweight timber frame, Cross-Laminated Timber (CLT), and glulam post-and-beam — each have distinct structural characteristics, cost profiles, and optimal applications.
This guide provides architects with a rigorous technical comparison to inform system selection, covering structural performance, cost, programme, fire resistance, acoustics, and sustainability. It is written for the design stage when fundamental structural decisions are being made.
System Overview
Timber Frame (Platform Frame)
The most established timber construction method in Europe and North America. Load-bearing walls are assembled from C24 structural studs at regular centres, with structural sheathing (OSB or plywood) providing racking resistance.
Key characteristics:
- Stud sizes: 38 × 89 mm to 38 × 184 mm (walls); 47 × 200 mm to 75 × 300 mm (floors)
- Stud centres: 400 mm or 600 mm
- Panel sheathing: 9–15 mm OSB or plywood
- Insulation: Between studs + external (if required)
- Typical height: 2–7 storeys (platform frame)
Cross-Laminated Timber (CLT)
Engineered solid timber panels made from layers (typically 3, 5, or 7) of boards glued at right angles. CLT acts as a plate element — simultaneously load-bearing, bracing, and enclosing.
Key characteristics:
- Panel thickness: 60–300 mm (typically 100–200 mm for walls, 140–240 mm for floors)
- Panel dimensions: Up to 3.5 m wide × 16 m long (transport-limited)
- Layer thickness: 20–45 mm per layer
- Species: Typically Norway Spruce (C24 grade laminations)
- Typical height: 3–18 storeys
Glulam (Glued Laminated Timber)
Engineered beams and columns made from multiple layers of timber (laminations) glued together. Glulam creates linear structural elements — beams, columns, arches, and portal frames.
Key characteristics:
- Standard widths: 90, 115, 140, 160, 185, 200, 240 mm
- Depths: 200 mm to 2,000+ mm
- Lengths: Up to 40+ metres (transport-limited; site splicing for longer)
- Strength classes: GL24h to GL32h (homogeneous) or GL24c to GL32c (combined)
- Species: Norway Spruce, Scots Pine, Douglas Fir
- Typical spans: 6–40+ metres
Structural Performance Comparison
Load-Bearing Capacity
| Parameter | Timber Frame | CLT | Glulam |
|---|---|---|---|
| Wall load capacity | 20–80 kN/m (stud-dependent) | 100–500+ kN/m (thickness-dependent) | Column: 200–2000+ kN |
| Floor span (residential load) | 3.0–5.5 m | 4.0–7.5 m (one-way) | 6.0–20+ m (beam) |
| Floor span (office load) | 2.5–4.5 m | 3.5–6.5 m (one-way) | 5.0–15+ m (beam) |
| Maximum building height | 6–7 storeys | 10–18 storeys | No inherent limit |
| Lateral stability | Sheathed panels (racking) | Panel diaphragm action | Bracing/moment connections |
Stiffness and Deflection
| System | Floor Mass (kg/m²) | Typical Frequency (Hz) | Deflection Performance |
|---|---|---|---|
| Timber frame (joists) | 30–50 | 8–14 | Adequate with careful design |
| Timber frame + concrete topping | 80–120 | 12–18 | Good |
| CLT (140 mm, 5-layer) | 65–75 | 10–15 | Good |
| CLT (200 mm, 7-layer) | 95–105 | 12–18 | Very good |
| Glulam beam + CLT deck | 80–110 | 12–20 | Very good |
| Glulam beam + timber frame | 40–60 | 9–14 | Adequate |
Vibration Performance
Floor vibration is often the governing design criterion for timber floors (more critical than strength or static deflection). The three systems perform differently:
Timber frame floors: Lightweight construction is inherently susceptible to vibration. Mitigation strategies include:
- Deeper joists (increased stiffness)
- Blocking between joists (load distribution)
- Concrete or sand topping (added mass)
- Reduced joist spacing (400 mm vs 600 mm)
CLT floors: Higher mass provides better vibration performance inherently. A 200 mm CLT panel typically satisfies vibration criteria without additional measures for spans up to 5–6 m.
Glulam + deck: Performance depends on the deck system. Glulam beams with CLT deck panels provide excellent vibration performance for long spans.
Cost Comparison
Material Cost (Indicative, per m² of floor area)
| Component | Timber Frame | CLT | Glulam Frame |
|---|---|---|---|
| Structural walls | €45–75/m² | €120–200/m² | €80–150/m² (columns) |
| Floor structure | €55–85/m² | €140–220/m² | €100–180/m² (beams + deck) |
| Roof structure | €40–65/m² | €120–180/m² | €80–160/m² |
| Connections/fixings | €8–15/m² | €15–30/m² | €20–45/m² |
| Total structure | €150–240/m² | €400–630/m² | €280–535/m² |
Costs are indicative and vary significantly by region, project size, and market conditions.
Total Construction Cost (Including Follow-On Trades)
Material cost alone is misleading. When total construction cost is considered:
| Factor | Timber Frame | CLT | Glulam Frame |
|---|---|---|---|
| Material cost | Low | High | Medium–High |
| Site labour (erection) | Medium | Low | Medium |
| Crane time | Low | Medium | Medium |
| Follow-on trades | High (plasterboard, etc.) | Low (exposed CLT) | Medium |
| Programme savings | Moderate | Significant | Moderate |
| Defect/rework cost | Medium | Low | Low |
| Total relative cost | Baseline | +10–25% (low-rise) / –5–10% (mid-rise) | +15–30% |
Cost Sweet Spots
- Timber frame: 1–4 storey residential, repetitive layouts, cost-driven projects
- CLT: 4–10 storey residential/commercial, exposed timber aesthetic, fast programme
- Glulam: Long-span single-storey (sports, retail, education), architectural expression, hybrid structures
Programme Comparison
Typical Construction Programme (4-Storey Residential, 20 apartments)
| Phase | Timber Frame | CLT | Glulam + CLT |
|---|---|---|---|
| Design & engineering | 8–12 weeks | 10–14 weeks | 12–16 weeks |
| Manufacturing | 4–8 weeks | 8–12 weeks | 10–14 weeks |
| Foundation | 4–6 weeks | 4–6 weeks | 4–6 weeks |
| Structural erection | 6–8 weeks | 3–4 weeks | 5–7 weeks |
| Weathertight envelope | 2–3 weeks | 1–2 weeks | 2–3 weeks |
| Internal fit-out | 12–16 weeks | 8–12 weeks | 10–14 weeks |
| Total (design to completion) | 36–53 weeks | 34–50 weeks | 43–60 weeks |
CLT's programme advantage comes from:
- Fewer pieces to erect (large panels vs many studs)
- Immediate weathertightness (solid panels, not open frames)
- Reduced internal finishing (exposed CLT = no plasterboard)
- Factory precision reducing site rework
Fire Performance
Comparative Fire Resistance
| System | Inherent Fire Resistance | Strategy for REI 60 | Strategy for REI 90 |
|---|---|---|---|
| Timber frame | Minimal (studs char quickly) | 2 layers Type F plasterboard | 3 layers Type F plasterboard |
| CLT (160 mm, 5-layer) | ~45 min (3 layers before structural failure) | Exposed (with charring allowance) | 1 layer plasterboard or thicker panel |
| CLT (200 mm, 7-layer) | ~70 min | Exposed (inherent) | Exposed (with charring allowance) |
| Glulam (oversized) | 30–90 min (section-dependent) | Oversized section (charring method) | Larger oversizing or encapsulation |
Fire Design Implications
Timber frame: Always requires encapsulation (plasterboard) for fire resistance. The lightweight studs have insufficient mass for the charring method. Fire design is straightforward but adds cost and programme time for plasterboard installation.
CLT: Can achieve significant fire resistance through panel thickness alone. Exposed CLT soffits are possible with appropriate panel thickness and charring allowance. However, delamination at adhesive lines must be considered (adhesive-dependent).
Glulam: Large sections provide excellent fire resistance through the charring method. Exposed BSH glulam beams can achieve REI 60–90 without any applied protection, making them ideal for architecturally expressed structures.
Acoustic Performance
Airborne Sound Insulation (Separating Floors)
| System | Mass (kg/m²) | Rw (dB) without treatment | Rw (dB) with acoustic treatment |
|---|---|---|---|
| Timber frame (47 × 200 joists) | 35 | 35–40 | 50–55 (with resilient bars + 2× PB + mineral wool) |
| Timber frame + 50 mm screed | 120 | 45–50 | 55–60 (with floating screed + ceiling treatment) |
| CLT 140 mm (5-layer) | 65 | 38–42 | 52–58 (with floating floor + suspended ceiling) |
| CLT 200 mm (7-layer) | 95 | 42–46 | 56–62 (with floating floor + suspended ceiling) |
| CLT 200 mm + 80 mm screed | 190 | 52–56 | 60–65 (with floating screed) |
Impact Sound (Separating Floors)
| System | Ln,w (dB) without treatment | Ln,w (dB) with treatment | Treatment Required |
|---|---|---|---|
| Timber frame (bare) | 75–80 | 48–55 | Floating floor + resilient ceiling |
| CLT 140 mm (bare) | 70–75 | 45–52 | Floating floor + suspended ceiling |
| CLT 200 mm (bare) | 65–70 | 42–48 | Floating floor (ceiling optional) |
| CLT + concrete topping | 55–60 | 38–45 | Resilient layer under topping |
Key insight: CLT's mass advantage means less additional treatment is needed to meet acoustic regulations. For timber frame, achieving good acoustic performance requires significant additional build-up (mass, resilient layers, air gaps), which adds cost and reduces the floor-to-ceiling height advantage.
Sustainability Comparison
Embodied Carbon
| System | Carbon per m² floor (kgCO₂e/m²) | Sequestered Carbon (kgCO₂e/m²) | Net Carbon (kgCO₂e/m²) |
|---|---|---|---|
| Timber frame | 40–70 | –60 to –100 | –20 to –60 |
| CLT | 70–120 | –120 to –200 | –50 to –130 |
| Glulam frame + CLT | 80–130 | –130 to –210 | –50 to –130 |
| Steel frame (reference) | 150–250 | 0 | +150 to +250 |
| RC frame (reference) | 200–350 | 0 | +200 to +350 |
All timber systems achieve net-negative carbon footprints when biogenic sequestration is included. CLT and glulam systems sequester more carbon per m² due to higher timber volumes.
Material Efficiency
| System | Timber Volume (m³/m² floor) | Waste on Site | Recyclability |
|---|---|---|---|
| Timber frame | 0.04–0.07 | 5–10% | Good (if separated) |
| CLT | 0.08–0.14 | 2–5% (CNC precision) | Excellent (solid panels) |
| Glulam | 0.06–0.12 | 3–8% | Excellent (solid sections) |
Decision Matrix: Which System to Specify
By Building Type
| Building Type | Recommended System | Rationale |
|---|---|---|
| Detached housing (1–2 storey) | Timber frame | Cost-effective, fast, proven supply chain |
| Terraced housing (2–3 storey) | Timber frame | Repetitive, cost-driven, acoustic solutions available |
| Apartments (3–6 storey) | CLT or timber frame | CLT for speed/acoustics; frame for cost |
| Apartments (6–10 storey) | CLT | Mass, stiffness, fire resistance, acoustics |
| Office (open plan) | Glulam + CLT | Long spans, exposed structure, flexibility |
| Education | Glulam + CLT or CLT | Spans, acoustics, robustness |
| Sports/leisure | Glulam | Long spans (20–40 m), architectural expression |
| Retail (single storey) | Glulam | Long spans, clear heights |
| Healthcare | CLT | Acoustics, fire resistance, hygiene (sealed surfaces) |
By Priority
| If your priority is... | Choose... | Because... |
|---|---|---|
| Lowest cost (low-rise) | Timber frame | Cheapest material, established supply chain |
| Fastest programme | CLT | Fewest pieces, immediate weathertight, less finishing |
| Best acoustics | CLT (thick panels) | Mass provides inherent sound insulation |
| Longest spans | Glulam | Engineered for 20–40+ m spans |
| Exposed timber aesthetic | CLT or Glulam | Both provide finished structural surfaces |
| Maximum height | CLT | Proven to 18 storeys, mass provides stability |
| Design flexibility | Glulam frame | Column-free spaces, future adaptability |
| Lowest carbon | CLT | Highest timber volume = most sequestration |
Hybrid Solutions
The most sophisticated timber buildings often combine all three systems:
Example: 8-Storey Residential Tower
- Core: CLT walls (200 mm) — lateral stability, fire compartmentation
- Floors: CLT panels (180 mm, 5-layer) — acoustics, fire resistance, speed
- Columns: Glulam (240 × 240 mm GL28h) — load-bearing, exposed in lobby
- Roof: Glulam beams + CLT deck — long-span communal terrace
- Partitions: Timber frame (non-load-bearing) — lightweight, services-friendly
Example: University Teaching Building
- Primary frame: Glulam portal frames (span 12 m) — lecture halls
- Upper floors: CLT panels on glulam beams — classrooms, offices
- Internal walls: Timber frame — flexible, demountable for future reconfiguration
- Facade: Glulam mullions + timber cladding — architectural expression
Design for Disassembly and Reuse
An increasingly important consideration for all timber structural systems is end-of-life strategy. Timber's carbon sequestration benefit is maximised when the material is reused rather than burned or landfilled at end of life.
Disassembly Potential by System
| System | Disassembly Potential | Key Factors | Reuse Likelihood |
|---|---|---|---|
| Timber frame (nailed) | Low–Medium | Nailed connections difficult to separate | Downcycling (chipboard, biomass) |
| Timber frame (screwed) | Medium | Screwed connections reversible | Component reuse possible |
| CLT (screwed connections) | High | Large panels, reversible fixings | Panel reuse in new buildings |
| Glulam (bolted) | Very High | Bolted connections fully reversible | Direct member reuse |
| Glulam (welded steel plates) | Medium | Steel plates can be unbolted | Member reuse after plate removal |
Design Principles for Future Reuse
- Use mechanical connections: Bolts and screws rather than nails, glue, or welding
- Standardise member sizes: Standard sections are more likely to fit future projects
- Avoid composite action: Timber-concrete composite floors cannot be separated
- Document the structure: Provide as-built drawings with member sizes, grades, and connection details
- Protect from degradation: Ensure timber remains at low MC throughout service life
- Avoid irreversible treatments: Chemical treatments may limit future reuse options
Related Resources
- Structural Timber Specification Guide — comprehensive pillar guide
- Glulam Beam Span Tables — detailed span data for glulam specification
- Timber Fire Resistance — charring rates and Eurocode 5 fire design
- C24 Structural Construction Timber — standard framing timber
- BSH Glued Laminated Beams — engineered glulam for long spans
- Carbon Footprint of Timber — environmental comparison of structural systems
- Passive House Timber Construction — achieving ultra-low energy with timber systems