Cross-Laminated Timber (CLT) Guide

CLT’s strength, stability, and off-site fabrication make it suitable for a wide range of building types. Its planar nature supports rapid construction, consistent performance, and integration with other materials in hybrid systems.

This section summarises common applications of CLT across the building industry.

2.1 Residential Buildings

CLT is widely used in:

• Low- to mid-rise apartments (typically 3-8 storeys)
• Terrace housing and townhouses
• Volumetric modular housing systems

Its loadbearing capacity and two-way spanning performance allow for efficient floor layouts with fewer beams or supports. Lightweight superstructures enable vertical extensions over existing buildings or reduced foundations on poor soil.

Figure 2: CLT use in residential construction (WS TDG 16 p18)

2.2 Commercial and Institutional Buildings

CLT is increasingly adopted in:

• Office buildings
• Schools, childcare, and education centres
• Aged care and health facilities

CLT’s thermal comfort, acoustic performance, and fast erection timelines make it ideal for occupied sites and environments requiring minimal disruption. Visual-grade finishes also provide biophilic and wellness design benefits.

Figure 3: CLT use in commercial and institutional buildings (WS TDG 16 p21, Lendlease)

2.3 Hybrid and Composite Systems

CLT is often combined with:

• Concrete cores or shear walls for vertical stability
• Steel or Glulam post-and-beam systems for open-plan layouts
• Timber-concrete composite floors (CLT with concrete topping) to improve acoustic and vibration performance

These hybrid systems expand CLT’s flexibility for use in complex or multi-use buildings.

2.4 Modular and Prefabricated Construction

CLT’s format and CNC precision make it ideal for prefabrication. Systems include:

• 2D panelised construction - CLT walls, floors, and roofs assembled on site
• 3D volumetric modules - factory-built rooms craned into place with integrated services and finishes

This approach reduces build time dramatically and suits housing, hotels, and temporary infrastructure.

Figure 4: CLT project under construction (WS TDG 16 p28, Lendlease)

2.5 Early Design Considerations

To realise the benefits of CLT, the following issues should be addressed early in the design phase:

• Panel orientation and grain direction
• Service integration and opening coordination
• Crane access, transport limits, and panel weight
• Moisture control during install
• Fire and acoustic detailing strategy
• Tolerance and erection sequencing

These factors are covered in detail throughout this page and supporting subpages on structural design, fabrication, and compliance.

CLT is a solid engineered timber panel designed for structural performance, prefabrication, and two-way spanning action. Its strength, stiffness, and dimensional stability are the result of controlled manufacturing and lamella configuration.

This section describes how CLT is made and the material properties that influence its performance.

3.1 Composition and Format

CLT is composed of 3 to 7 layers of timber boards (called lamellae), arranged in alternating grain directions and bonded with structural adhesive under pressure.

Typical characteristics:

• Odd number of layers (e.g. 3, 5, 7) for symmetry
• Softwood species (e.g. Spruce, Pine) from plantation sources
• Panel thickness: 60 mm to 300+ mm
• Panel dimensions:
  • Width: up to 3.5 m
  • Length: up to 16-20 m depending on supplier
• CNC fabrication allows for:
  • Window and door openings
  • Service routing
  • Connection slots and bearing rebates

Note: Panel sizes, adhesive systems, and testing compliance vary between suppliers. Panels are not interchangeable.

Figure 5: CLT sections being placed on site (WS TDG 16 p25, Rothoblaas)

3.2 Mechanical and Structural Properties

CLT acts as a plate rather than a beam, and thereby distributes loads across both directions. While design often simplifies panels to one-way spans, two-way behaviour can be harnessed for efficiency.

Key performance traits:

• High in-plane shear and out-of-plane stiffness
• Good dimensional stability due to cross-lamination
• Strength and stiffness influenced by:
  • Layer count and thickness
  • Timber grade and species
  • Adhesive type and panel layup
  • Support conditions and joint detailing

3.3 Acoustic and Fire Performance

CLT supports high acoustic and fire performance when paired with appropriate detailing:

Acoustic

• CLT’s mass contributes to airborne sound insulation
• Additional linings, battens, and underlays control flanking and impact noise
• NCC acoustic criteria can be met using tested systems

Source: Atelier Crescendo

Fire

• CLT chars predictably, forming an insulating layer
• Performance depends on thickness, encapsulation, and joint protection
• Can achieve FRLs of 60-90+ minutes with tested or engineered solutions

Source: Observations and impact of char layer formation and loss for engineered timber

Figure 6: Cross section model of floor construction used in Forte apartments (WS TDG 16 p32, TDA)

3.4 Environmental Performance

CLT contributes to sustainability through:

• Carbon storage - timber sequesters carbon during growth
• Low embodied carbon- compared to steel and concrete
• Efficient material use - made from plantation timber
• Reduced construction waste - due to off-site fabrication
• Compatibility with passive design - good thermal mass and airtightness

Environmental credentials should be confirmed using:

• Product-specific EPDs (Environmental Product Declarations)
• FSC or PEFC chain-of-custody certification
• Verification of adhesive VOC content (e.g. PUR, MUF)

Designing with CLT requires a shift in approach from traditional framing or steel/concrete construction. CLT performs differently as a structural system, and its behaviour under moisture, sound, and fire exposure depends on correct detailing from the outset.

This section outlines the key design considerations that should be addressed during early documentation and coordination.

4.1 Structural Design

CLT acts as a stiff, plate-like element-carrying loads in both directions when designed appropriately. However, layout, support conditions, and connection detailing significantly affect performance.

Design inputs:

• Panel orientation: Grain direction drives span capacity
• Openings and notches: Require local reinforcement or redistribution of forces
• Joints: Shear transfer between panels relies on connection design (splines, screws, brackets)
• Supports: Continuous support reduces deflection and simplifies fire detailing
• Floor vibration: May require additional stiffness or hybrid topping layers

4.2 Acoustic Design

While CLT has mass, it requires additional treatment to meet NCC sound insulation requirements, especially in Class 2-3 buildings.

Design strategies:

• Walls: Use furring channels, cavity insulation, or separated double linings
• Floors: Incorporate resilient ceiling systems, underlays, or screeds
• Services: Avoid rigid paths through separating walls or floors
• Penetrations: Detail seals to prevent flanking noise paths

4.3 Fire Resistance and Detailing

CLT can meet fire resistance requirements through a combination of:

• Charring: Predictable burn behaviour allows design for sacrificial layers
• Encapsulation: Fire-rated linings (e.g. FR plasterboard) may be required to delay ignition
• Joint detailing: Prevents early flame spread or thermal bridging
• Performance Solutions: Required where exposed timber is used or linings are omitted

4.4 Moisture Management

Moisture is one of the most critical risk factors in CLT construction. Prolonged exposure-whether from site wetting or trapped vapour-can result in swelling, delamination, adhesive breakdown, or biological attack.

Moisture management for CLT must be addressed at three levels:

1. During transport and installation - Protect panels with breathable wrap, cover stacked panels off the ground, and minimise exposure time.
2. Through detailing - Avoid water traps, seal all cut edges (especially end grain), and provide drainage and drying paths at every interface.
3. Within the building envelope - Use vapour-permeable membranes and avoid detailing that traps condensation (e.g. dabbed linings or unventilated cavities).

These principles are detailed in the [Moisture] main guide, which provides comprehensive detailing, construction sequencing, and envelope strategies for CLT and other timber systems.

4.5 Services Integration

CLT requires early coordination of all building services, due to its solid-panel nature.

Design guidance:

• Plan all service routes and penetrations in advance so that they can be pre-cut during fabrication
• Use service cavities or furring channels to avoid cutting into panels
• Coordinate routes to avoid fire-rated zones, structural spans, or acoustic barriers
• Label, document, and track all penetrations, especially in floors and walls between units

CLT enables rapid, precise construction when properly detailed and sequenced. Because panels are prefabricated off-site, detailing must be resolved early, including fixings, penetrations, tolerances, and lifting logistics.

This section summarises key construction-phase considerations unique to CLT systems.

5.1 Prefabrication and Panel Logistics

CLT panels are CNC-machined to precise dimensions prior to delivery. Early coordination between designers, engineers, and suppliers is essential to avoid costly on-site modifications.

Design and fabrication checklist:

• Confirm panel dimensions, grain orientation, and lift point locations
• Include all penetrations, rebates, and service routes in shop drawings
• Account for transport limitations (panel length, road clearances, crane access)
• Plan just-in-time delivery to match erection sequence and minimise site storage

5.2 Panel-to-Panel Connections

CLT panels are typically connected using:

• Screws (e.g. long self-tapping screws for diaphragms)
• Steel brackets or plates (used at floor-wall, corner, and diaphragm junctions)
• Cover boards or splines (to bridge adjacent panels)

Connection priorities:

• Ensure fire and acoustic seals are not breached by connector layout
• Maintain tolerances to prevent cumulative misalignment
• Seal joints against moisture and air leakage (e.g. tapes, gaskets, fire putty)

5.3 Fixings and Fasteners

CLT construction often relies on engineered fasteners:

• Long screws inserted at angles for load transfer
• Concealed fixings in exposed surfaces
• Screw spacing aligned with acoustic or fire system test data
• Fire-exposed connectors may require embedding or protection via charring allowance

Fixing systems should be specified and confirmed with suppliers during early coordination.

5.4 Tolerances and Movement

CLT’s prefabricated nature demands tight control over tolerances across structure and finishes.

Best-practice detailing includes:

• Movement joints at regular intervals in long walls or floors
• Shimming at supports to accommodate slight height differences
• Vertical movement allowances at interfaces with concrete, steel, or lightweight walls
• Adjusted openings or packing zones to accommodate installation tolerances

CLT fabricators are very familiar with tolerance allowances. It is good practice to include your CLT fabricator in the conversation detailing the gaps, tolerances, and compliance requirements related to gaps and penetrations.

Figure 7: Wall to floor connection example (WS TDG 16 p19, TDA)

Figure 8: Proprietary connector (WS TDG 16 p19, TDA)

CLT panels are solid structural elements, which means services cannot be routed through hollow wall cavities or trusses as in conventional construction. All electrical, hydraulic, mechanical, and fire services must be carefully coordinated during early design.

Improper routing can undermine fire resistance, acoustic separation, and structural performance. Pre-cut openings should be CNC-machined during fabrication wherever possible.

6.1 General Approach

Plan early. Cut once. All major service routing decisions must be resolved prior to fabrication.

Coordination priorities:

• Identify service zones and routes before panel production
• Limit field drilling, especially in structural or fire-rated zones
• Avoid penetrating primary load paths, diaphragms, and boundary elements
• Establish labelling and documentation protocols for all penetrations

6.2 Routing Services in CLT

Service routing strategies vary depending on panel location:

Wall Panels

• Use furring channels or service cavities to conceal electrical and plumbing services
• Avoid horizontal chases in loadbearing walls
• Leave 10-15 mm clearance between panel face and finishes where required

Floor Panels

• Route services within ceiling build-ups, not through panel thickness
• Use pre-cut vertical risers for stacked plumbing or ductwork
• Avoid large penetrations near spans, supports, or cantilevered zones

Roof Panels

• Route services above the panel, within roof build-up zones (e.g. between insulation or battens)
• Avoid cuts in structural members or unsupported soffits

Note: Never route or remove material to create channels in structural CLT in the field unless explicitly approved.

6.3 Coordination with Other Disciplines

CLT requires direct collaboration between:

• Architects - plan spatial layout and finishes compatibility
• Services engineers - define clearances, penetrations, and tolerances
• Structural engineers - protect load paths and fire-rated zones
• CLT suppliers - confirm feasible cutouts and lead times

Critical milestones:

• Finalise routing before shop drawing sign-off
• Confirm tolerance stacking between trades and panel set-out
• Pre-coordinate bracing, ductwork, sprinklers, and risers

6.4 Penetration Detailing and Compliance

Service penetrations can compromise performance if not properly sealed and detailed.

Best-practice detailing includes:

• Intumescent collars or wraps for fire-rated pipe and duct penetrations
• Acoustic putty pads or foam grommets at recessed outlets
• Gaskets, tapes, or caulks to preserve air- and vapour-tightness
• Waterproof flashings or flexible boots for external penetrations

Fire, acoustic, and moisture performance all rely on joint and interface continuity. Services must not bypass these systems.

CLT buildings must comply with the NCC across multiple performance domains, including structure, fire, acoustics, moisture, and durability. Compliance may be achieved via:

• Deemed-to-Satisfy (DtS) solutions, where available; or
• Performance Solutions, supported by testing, modelling, or expert judgement.

Each CLT system must be assessed against the relevant criteria for its class, building height, and application.

7.1 Structural Performance

CLT is designed in accordance with:

• AS/NZS 1170 series (Structural actions - wind, earthquake, snow, etc.)
• AS 1720.1 (Timber structures - general design rules)
• AS 4055 (Wind loads - low-rise housing, where applicable)

Verification methods:

• Bending, shear, and bearing capacity based on test data or manufacturer design tables
• Span tables and finite element modelling for complex load paths
• Deflection and vibration limits checked under live load

7.2 Acoustic Performance

CLT buildings must meet NCC Volume 1 Section F5 acoustic requirements:

• Rw + Ctr ≥ 50 (walls/floors between dwellings)
• Ln,w ≤ 62 (impact sound insulation for floors)

Many CLT acoustic systems have been tested under TDG 44. These typically involve:

• Double-layer linings
• Cavity insulation
• Resilient mounts or ceiling systems
• Floor underlays or toppings

Test results are valid only for the specific product and configuration tested. Do not substitute panel types without verification.

7.3 Fire Resistance and NCC Pathways

CLT buildings must satisfy NCC Volume 1 Section C - Fire Resistance, including:

• FRLs (Fire Resistance Levels) for loadbearing walls and floors
• Non-combustibility provisions or use of fire-protected timber under DtS solutions
• Encapsulation of CLT with fire-rated linings to meet DtS criteria

Performance Solutions may be used where:

• Exposed timber is desired
• Charring is used as a fire protection strategy
• Systems fall outside DtS criteria

7.4 Moisture and Durability Compliance

The NCC references AS 3959, AS/NZS 1604, and other standards for managing:

• Moisture ingress in external walls/roofs
• Durability against decay, termite attack, and corrosion

Design requirements include:

• Vapour-permeable membranes
• Drainage and cavity systems
• Edge sealing and joint protection
• Documentation of durability classes or treatment certification (where applicable)

7.5 Verification Methods

In some cases, Verification Methods (VMs) are required to demonstrate compliance, especially where DtS provisions are not directly applicable.

Examples:

• Acoustic field testing under NCC VMs FV5.1 (floors) and FV5.2 (walls)
• Structural modelling using advanced methods (e.g. finite element analysis)
• Fire testing or performance-based design assessments

These methods often require:

• Independent testing or assessment reports
• Coordination with building certifiers
• Supporting documentation from suppliers and fabricators

The successful use of CLT in building projects relies heavily on early engagement with experienced manufacturers and suppliers. Because CLT is produced to order, its performance depends not just on design, but also on fabrication quality, lead time management, and coordination between disciplines.

8.1 Fabrication Process

CLT panels are manufactured in controlled factory conditions using:

• Layered lamellae of kiln-dried timber
• High-strength structural adhesives (typically polyurethane or melamine-based)
• Hydraulic or vacuum presses to achieve panel consolidation
• CNC cutting equipment to produce precise openings and panel shapes

Panels are factory-detailed to include:

• Door and window openings
• Electrical and plumbing service cut-outs
• Connection slots and bearing rebates

This high level of prefabrication reduces on-site adjustments and enables rapid construction.

8.2 Local and International Suppliers

CLT is supplied in Australia by both international manufacturers and local distributors. Fabrication capabilities have been established in regions including New South Wales and Victoria, with supplier and distributor networks expanding across the eastern states. Projects may also source panels from New Zealand or Europe, depending on design requirements and lead times. Suppliers often also include hybrid services offering panel fabrication, design assistance, and installation services.

⚠️ CLT products are rarely interchangeable - acoustic, structural, fire compliance tests are valid only for the specific panel configurations and products tested. Refer to supplier technical data and coordinate compliance documentation accordingly.

8.3 Lead Times and Logistics

CLT must be scheduled and sequenced into the project from an early stage:

• Panel manufacture typically requires 6-12 weeks, depending on supplier workload and panel complexity
• Shipping (for imports) adds additional time and coordination
• On-site delivery must be aligned with crane access and erection sequences

Best practices include:

• Finalising cutting lists and shop drawings early
• Holding coordination workshops between designers, fabricators, and installers
• Ensuring temporary weather protection during transit and on-site storage

8.4 Procurement Considerations

When specifying and procuring CLT:

• Confirm compliance with relevant structural, fire, and acoustic test data
• Request detailed panel properties (e.g. density, thickness, adhesive type)
• Ensure supplier supports installation guidance and compliance documentation
• Include provisions for tolerances, storage, lifting, and moisture protection