Cross-laminated timber has fundamentally changed what is possible in timber construction. Where traditional timber frame was limited to low-rise residential buildings, CLT enables architects to design multi-storey structures with the speed of modular construction and the architectural freedom of bespoke design. The material's inherent strength, dimensional stability, and factory precision make it the structural system of choice for a growing number of residential and commercial projects across Europe.
This guide explains how CLT modular homes are manufactured, delivered, and erected — from raw timber to completed building — with practical guidance on design, cost, and programme for architects and builders considering the system for the first time.
What Is Cross-Laminated Timber?
CLT is an engineered wood panel made from layers of structural timber boards glued together at right angles. This cross-lamination creates a panel that is:
- Structurally bidirectional — capable of spanning in two directions like a concrete slab
- Dimensionally stable — cross-layers restrain shrinkage and swelling
- Strong in-plane and out-of-plane — functions as both wall and floor
- Rigid — high in-plane stiffness for lateral stability (shear walls)
CLT Panel Composition
| Layer Configuration | Typical Thickness | Primary Application |
|---|---|---|
| 3-layer (3s) | 60–120 mm | Internal walls, single-storey floors |
| 5-layer (5s) | 100–200 mm | External walls, multi-storey floors |
| 7-layer (7s) | 180–280 mm | Long-span floors, shear walls |
| 9-layer (9s) | 240–400 mm | Very long spans, high-rise cores |
Each layer uses boards graded to minimum C24 (longitudinal layers) or C16 (transverse layers), typically 20–40 mm thick and 80–240 mm wide. The boards are finger-jointed to achieve panel lengths up to 16 m.
Manufacturing Process
- Timber selection and grading: Boards are machine-stress-graded to C24/C16 and kiln-dried to 12% ±2% moisture content
- Finger jointing: Boards are end-jointed to create continuous laminations of the required length
- Planing: All boards planed to precise thickness (±0.1 mm)
- Lay-up: Boards arranged in alternating grain directions on a vacuum table
- Adhesive application: Polyurethane (PUR) adhesive applied — formaldehyde-free
- Pressing: Hydraulic press applies 0.6–1.0 N/mm² for 2–4 hours
- CNC machining: Panels cut to exact dimensions, openings routed, connection details machined
- Quality control: Delamination testing, dimensional verification, visual grading
- Packaging and transport: Panels wrapped in weatherproof membrane, stacked on transport frames
CLT vs Volumetric Modular: Understanding the Difference
The term "modular" is often used loosely. It is important to distinguish between CLT panelised construction and volumetric modular:
| Aspect | CLT Panelised | Volumetric Modular |
|---|---|---|
| Factory product | Flat panels (walls, floors, roof) | Complete 3D room modules |
| Site assembly | Panels craned and connected | Modules stacked and joined |
| Architectural freedom | High — any plan form possible | Limited — rectangular modules |
| Transport efficiency | High — panels stack flat | Low — transporting air inside modules |
| Maximum building height | 18+ storeys (proven) | 12 storeys (current maximum) |
| Internal finish | Completed on site | Factory-finished |
| Services | Installed on site (or in cassettes) | Factory-installed |
| Typical programme saving | 30–40% vs traditional | 40–50% vs traditional |
| Cost premium (shell) | 5–15% vs traditional | 10–25% vs traditional |
| Design flexibility | Bespoke designs possible | Repetitive layouts preferred |
CLT panelised construction occupies the middle ground between traditional site-built and fully volumetric modular — offering significant programme benefits while maintaining architectural flexibility.
Design Principles for CLT Homes
Structural System
CLT buildings use a platform construction method:
- Ground floor panels placed on foundation (concrete slab or screw piles)
- Wall panels erected on floor panels
- Next floor panels placed on top of wall panels
- Process repeats for each storey
- Roof panels placed on top-storey walls
This creates a simple, repetitive load path where each floor acts as a platform for the storey above. Vertical loads transfer through bearing walls in direct compression — CLT's strongest load direction.
Lateral Stability
CLT panels provide excellent lateral stability (resistance to wind and seismic forces) through their high in-plane shear stiffness. A CLT building typically requires:
- Minimum two shear walls in each orthogonal direction per storey
- Shear walls connected to floors with angle brackets or hold-down connectors
- Floor panels acting as rigid diaphragms to distribute lateral loads
- Connections designed for combined shear and uplift forces
Acoustic Design
Acoustic performance is a critical design consideration for CLT buildings, particularly multi-storey residential:
| Element | Typical CLT Performance | Regulation Requirement | Solution |
|---|---|---|---|
| Party wall (airborne) | 40–45 dB Rw (bare CLT) | 53–56 dB DnT,w | Double-leaf CLT with cavity |
| Party floor (airborne) | 38–42 dB Rw (bare CLT) | 53–56 dB DnT,w | Floating floor + ceiling |
| Party floor (impact) | 75–80 dB Ln,w (bare CLT) | ≤58 dB L'nT,w | Floating floor essential |
The key acoustic strategies for CLT buildings are:
- Floating floors: Concrete screed (50–70 mm) on resilient layer on CLT floor panel
- Suspended ceilings: Independent ceiling on resilient hangers below CLT floor
- Double-leaf walls: Two separate CLT panels with air gap (no rigid connections)
- Flanking isolation: Resilient strips between wall and floor panels at junctions
Thermal Performance
A typical CLT external wall build-up achieving U = 0.15 W/m²K:
| Layer | Thickness | Function |
|---|---|---|
| Internal CLT panel | 100 mm | Structure + thermal mass |
| Vapour control layer | — | Moisture management |
| Service void (optional) | 50 mm | Services without penetrating CLT |
| Insulation (wood fibre) | 160 mm | Thermal performance |
| Breather membrane | — | Weather protection |
| Ventilated cavity | 25 mm | Moisture drainage |
| External cladding | 20–25 mm | Weather screen |
| Total | ~360 mm | U = 0.15 W/m²K |
Construction Sequence
Phase 1: Foundation (Weeks 1–4)
CLT buildings are significantly lighter than concrete or masonry equivalents:
- Single-storey CLT home: ~150 kg/m² (vs ~400 kg/m² masonry)
- Three-storey CLT building: ~250 kg/m² (vs ~600 kg/m² masonry)
This weight reduction allows:
- Reduced foundation depth and width
- Screw pile foundations (avoiding wet trades entirely)
- Construction on sites with poor bearing capacity
- Reduced excavation and concrete volumes
Foundation must be level to ±2 mm across the building footprint — CLT panels cannot accommodate the irregularities that masonry can absorb through mortar joints.
Phase 2: Ground Floor Installation (Day 1)
Ground floor CLT panels are placed on the foundation with:
- DPM (damp-proof membrane) between concrete and timber
- Compressible foam strip to level minor irregularities (±2 mm)
- Panels connected to each other with half-lap joints or spline connections
- Panels anchored to foundation with hold-down brackets
Phase 3: Wall Erection (Days 2–3)
Wall panels are craned into position in a planned sequence:
- External walls erected first (starting from corners)
- Panels temporarily braced until adjacent panels connected
- Internal load-bearing walls erected
- All panels connected with angle brackets (shear) and hold-down connectors (uplift)
- Panel-to-panel joints sealed with compressible tape
A typical wall panel (3.0 × 2.8 m × 100 mm) weighs approximately 400 kg — easily handled by a standard mobile crane.
Phase 4: Upper Floors (Days 3–4)
Floor panels placed on wall panels:
- Bearing length minimum 60 mm on supporting walls
- Panels connected to each other with half-lap or surface spline
- Edge connections to walls with angle brackets
- Temporary propping for long spans during installation
Phase 5: Roof (Days 4–5)
Roof panels installed:
- Flat roof: CLT panels with falls created by tapered insulation
- Pitched roof: CLT panels on glulam ridge beam and purlins
- Weathertight membrane applied immediately after panel installation
Phase 6: Weathertight Envelope (Week 2)
With the structural shell complete in under a week, the building is made weathertight:
- Breather membrane applied to external walls
- Roof membrane completed
- Temporary window covers or permanent windows installed
- All panel joints sealed
From this point, internal trades can work in dry, protected conditions regardless of weather.
Phase 7: Services and Finishes (Weeks 3–12)
Internal fit-out proceeds conventionally:
- Electrical and plumbing first fix (in service voids or surface-mounted)
- Insulation installation
- Internal linings (plasterboard where required, exposed CLT elsewhere)
- Second fix services
- Decoration and finishes
Cost Analysis
Shell Cost Comparison (per m² gross internal floor area)
| System | Structure Cost | Programme | Foundation Saving | Net Position |
|---|---|---|---|---|
| Traditional masonry | €180–220/m² | Baseline | Baseline | Baseline |
| Timber frame | €200–240/m² | −20% | −10% | +5% to −5% |
| CLT panelised | €240–300/m² | −35% | −30% | +5% to −10% |
| Volumetric modular | €280–350/m² | −45% | −30% | +10% to −5% |
Where CLT Becomes Cost-Competitive
CLT is most competitive when:
- Programme is critical: Each week saved reduces site prelims (crane hire, scaffolding, site management, welfare) by €5,000–15,000 depending on project size
- Site is constrained: Reduced deliveries, less storage, quieter construction
- Building is 3+ storeys: Foundation savings increase with building weight
- Labour is scarce: CLT requires fewer site operatives (4–6 vs 15–20 for masonry)
- Quality is paramount: Factory precision eliminates site variability
- Sustainability targets are demanding: CLT stores carbon, reducing embodied carbon
Cost Breakdown for a Typical CLT Home (180 m²)
| Element | Cost Range |
|---|---|
| CLT panels (walls, floors, roof) | €45,000–60,000 |
| Glulam beams and columns | €8,000–15,000 |
| Connections and metalwork | €5,000–8,000 |
| CNC machining and factory finishing | €12,000–18,000 |
| Transport to site | €3,000–6,000 |
| Crane hire and erection | €4,000–8,000 |
| Total structural shell | €77,000–115,000 |
Case Studies
Single-Family Home — Rural Site
- Size: 185 m² over two storeys
- System: 5-layer CLT walls (140 mm), 5-layer CLT floors (160 mm)
- Erection time: 4 days to weathertight shell
- Total programme: 5 months (land to completion)
- Energy performance: Passive House certified (15 kWh/m²a heating demand)
- Key decisions: Exposed CLT ceilings throughout, external wood fibre insulation, triple-glazed timber windows
Multi-Storey Residential — Urban Infill
- Size: 24 apartments over 6 storeys, 2,400 m² GIA
- System: CLT walls and floors with glulam transfer beams at ground floor
- Erection time: 6 weeks (one floor per week)
- Programme saving: 14 weeks vs concrete frame equivalent
- Foundation: Piled raft — 40% less concrete than for equivalent concrete frame
- Acoustic strategy: Floating concrete screed (65 mm) + suspended ceilings
Education Building — School Extension
- Size: 800 m² single-storey classroom block
- System: CLT roof panels spanning 7.2 m on glulam beams
- Erection time: 3 days
- Key benefit: Construction during school holidays — minimal disruption
- Internal finish: Exposed CLT ceilings for acoustic absorption and biophilic design
Design for Manufacture and Assembly (DfMA)
Successful CLT projects require early engagement between architect, engineer, and manufacturer. Key DfMA principles:
Panel Optimisation
- Design to standard panel widths (1.2 m, 2.4 m, 3.0 m) to minimise waste
- Limit panel weight to 5,000 kg for standard crane capacity
- Consider transport constraints: maximum 3.0 m width, 12.0 m length for standard road transport
- Minimise the number of unique panel types — repetition reduces cost
Connection Strategy
- Standardise connection details across the project
- Design connections for site tolerance (±5 mm adjustment)
- Minimise the number of different connector types
- Consider assembly sequence — can each panel be placed without moving previously installed panels?
Service Integration
- Plan service routes before panel design — CNC can pre-rout channels
- Avoid penetrating CLT panels where possible (use service voids)
- Coordinate electrical back-boxes with panel machining
- Design plumbing risers in non-structural zones
Quality and Tolerances
CLT construction demands higher design precision than traditional building:
| Parameter | CLT Tolerance | Traditional Tolerance |
|---|---|---|
| Foundation level | ±2 mm | ±10 mm |
| Wall plumb | ±3 mm per storey | ±10 mm per storey |
| Floor level | ±3 mm over 5 m | ±10 mm over 5 m |
| Opening sizes | ±2 mm | ±5 mm |
| Panel dimensions | ±1 mm | N/A |
Sustainability Credentials
CLT construction offers significant environmental advantages:
Carbon Storage
Every cubic metre of CLT stores approximately 700 kg of CO₂ (biogenic carbon captured during tree growth). A typical CLT home stores 20–40 tonnes of CO₂ in its structure — equivalent to 10–20 years of operational carbon emissions.
Embodied Carbon
CLT structures typically achieve embodied carbon of 200–350 kgCO₂e/m² (Modules A1–A5), compared to:
- Steel frame: 400–600 kgCO₂e/m²
- Concrete frame: 350–550 kgCO₂e/m²
- Masonry: 300–450 kgCO₂e/m²
When biogenic carbon storage (Module D) is included, CLT buildings can achieve net-negative embodied carbon.
End of Life
CLT panels can be:
- Reused in new structures (if connections allow disassembly)
- Recycled into particleboard or other wood products
- Used as biomass fuel (recovering stored energy)
- Composted (returning carbon to soil)
Procurement Considerations
Lead Times
| Item | Typical Lead Time |
|---|---|
| CLT panels (standard) | 10–14 weeks from order |
| CLT panels (complex/large) | 14–20 weeks |
| Glulam beams | 6–10 weeks |
| Metalwork connections | 4–6 weeks |
| Design and engineering | 6–12 weeks (before manufacturing) |
| Total from design freeze | 16–32 weeks |
Procurement Route
CLT projects work best with early contractor involvement (ECI) or design-and-build procurement:
- RIBA Stage 2: Engage CLT manufacturer for feasibility and budget pricing
- RIBA Stage 3: Manufacturer develops structural scheme with engineer
- RIBA Stage 4: Detailed panel design, connection design, production drawings
- Manufacturing: 8–12 weeks in factory
- Delivery and erection: Coordinated with site readiness
Related Resources
- Structural Timber Guide — comprehensive reference for timber construction
- C24 Structural Construction Timber — lamination material for CLT panels
- BSH Glued Laminated Beams — complementary beam solutions for CLT structures
- Timber Frame vs CLT — detailed comparison of timber structural systems