Structural and base materials · Timber & Wood
Cross-Laminated Timber (CLT)
Explains cross-laminated timber as a plate-forming panel for walls, floors and roof decks, and separates it clearly from glued-laminated timber and laminated veneer lumber, which are shaped as linear members.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What cross-laminated timber is
Cross-laminated timber is a large-format panel built up from layers of sawn timber boards, bonded face to face with the grain of each layer running perpendicular to the layers either side of it. That crosswise arrangement is the defining feature: it gives the finished panel usable stiffness in two directions, so the product behaves as a plate rather than as a beam.
Panels are produced under factory conditions and are normally cut to a project-specific shape before delivery, with window and door openings, service routes and connection details machined in the works. The result is a prefabricated element intended to arrive ready to lift into position, which is why the design and fabrication information matters as much as the material itself.
The distinction from its closest relatives is one of geometry and purpose. Glued-laminated timber stacks its laminations all in the same direction to make beams, columns and arches. Laminated veneer lumber bonds thin peeled veneers, again mainly running one way, to make highly consistent linear members. Cross-laminated timber alternates direction so it can form walls, floors and roof decks.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- CLT
- Cross laminated timber
- X-lam
- Crosslam panel
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Structural wall panels in timber buildings, where the panel forms both the loadbearing element and the substrate for internal linings.
- Floor and roof decks spanning between supporting walls or beams, formed from panels rather than from rows of individual joists.
- Lift shafts, stair cores and other vertical elements assembled as prefabricated boxes or as sets of connected panels.
- Exposed soffits and ceilings where the underside of the panel is intended to remain visible as the finished internal surface.
- Extensions and upper-storey additions where prefabrication and a shorter period of work on site form part of the reasoning.
- Terrace and balcony decks forming part of the structure, with the waterproofing above them considered as a separate matter.
Consideration
Where it is commonly considered
- Projects where a repeating plate element, with walls and floors of similar build-up, can be rationalised into a small number of panel types.
- Situations where the timber surface is intended to be seen, so that the structure and the internal finish are deliberately the same thing.
- Sites with restricted access or limited working space, where craning prefabricated panels is being weighed against building in place.
- Buildings where a lighter superstructure is being discussed in relation to existing foundations or ground conditions, subject to engineering assessment.
- Schemes where openings and service routes can be settled early enough for them to be machined into the panels before delivery.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Any location where panels would be persistently wet or subject to standing water, since moisture management sits at the centre of how timber panels behave.
- Highly irregular geometry that would leave little repetition and a large proportion of bespoke panels and off-cuts.
- Projects where the design cannot be frozen early, because late changes to openings and service routes affect what has already been fabricated.
- Buildings where the fire strategy, acoustic separation and compartmentation have not yet been settled by the relevant professionals.
- Situations where crane access, delivery vehicle routes or lifting positions cannot be established before the panels are ordered.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- Timber panels take up and give off moisture with the surrounding air, and they are vulnerable to prolonged wetting during construction as well as in service. Protection during transport, storage and the period before the building is closed in is a normal subject for discussion.
- Thermal behaviour
- The panel is a structural layer rather than an insulating one. Insulation is added to it, and where that insulation sits in relation to the panel changes how the whole assembly manages vapour and drying.
- Acoustic behaviour
- A panel is a relatively thin, stiff and light plate, so sound transfer between floors and between rooms is normally handled by build-ups added above and below it rather than by the panel alone. What those build-ups need to achieve is a matter for a qualified professional.
- Fire-related questions
- Fire behaviour of timber panels, including the treatment of exposed surfaces and of the bond lines between layers, varies by product and by the assembly around it, and is a matter for the manufacturer's documentation and the project's fire strategy.
- Appearance
- The visible face is graded separately from the hidden layers, so a panel intended to be seen is specified differently from one that will be covered. Board joints, finger joints and colour variation are part of the character rather than defects.
- Installation dependency
- Panels are craned into position and joined with designed connections, so lifting points, erection sequence, temporary propping and tolerances are settled in the design rather than resolved by whoever is on site that day.
- Documentation
- The fabrication drawings and cutting files effectively become the building, so agreeing openings, chases and connection positions before manufacture matters more here than it does with framing that is cut on site.
Exposure
Exposure and environmental conditions
- How will the panels be protected from rain and standing water between delivery and the point at which the building becomes weathertight?
- Is any part of a panel in a position where it could be wetted repeatedly in service, such as near a threshold or a wet room?
- What internal humidity is expected in the finished building, and does that environment suit an exposed timber surface?
- Where a panel forms part of an external wall, what layers sit outside it and how do those layers manage water and vapour?
- Is the site exposed enough that lifting and temporary stability during erection need particular attention from the designer?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- Connections to foundations, steelwork or existing structure are designed elements rather than generic details, and they are settled before fabrication.
- Insulation, cladding, membranes and internal linings all bear on or fix back to the panel, so fixing methods and layer build-ups interact.
- Panel-to-panel joints carry a structural role and an airtightness role at the same time, and how each is achieved is agreed as part of the design.
- Service routes are machined or planned in advance, because drilling through a structural plate afterwards is an engineering question.
- Where a floor panel meets a wall panel, that junction affects acoustic separation, air leakage and the structural connection all at once.
Appearance
Appearance and finish considerations
- Visual grades govern knots, colour variation and surface repairs on the face intended to be seen, and are specified deliberately rather than assumed.
- Timber left exposed indoors changes colour over time, particularly where daylight falls unevenly across a wall or a soffit.
- Panel joints, fixings and connection plates remain visible unless the design conceals them, so they become part of the aesthetic result.
- Clear and lightly pigmented finishes affect how the timber ages, and sample panels are the usual way to settle the starting appearance.
- Marks made during construction are difficult to remove from an exposed face, which is why protection during the build is discussed early.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What moisture conditions is this panel expected to see in service, and how does the design keep it away from persistent wetting?
- How are the panel edges and the exposed end grain protected at the points where they are most likely to get wet?
- What would indicate that a panel had been wetted during construction, and what does the manufacturer's guidance cover in that case?
- Where a panel is part of an external wall, how is any water that gets past the outer layers drained away and allowed to dry?
- How is movement between the timber structure and the adjoining rigid materials accommodated at junctions?
Upkeep
Maintenance and repair considerations
- What routine inspection does the exposed timber surface need, and who is expected to carry that out?
- If a panel is damaged locally, what does the manufacturer say about repair as opposed to replacement of the panel?
- How would a service alteration be handled later, given that the panel is a structural element rather than a lining?
- What finish has been applied to the exposed faces, and what does refreshing that finish actually involve?
- Is there a record identifying which panel is which, so that any later work can refer back to the fabrication information?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Crane capacity, standing position and reach determine the practical panel sizes as much as the structural design does.
- Delivery sequence matters because panels are erected in a set order and there is rarely room on site to store them beforehand.
- Temporary propping and bracing during erection are designed rather than improvised, and form part of the information issued for the works.
- Weather protection during erection is planned in advance, since panels can be wetted repeatedly before the roof goes on.
- Setting out and tolerance at the interface with foundations or existing structure are checked before the panels are manufactured.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- Manufacturer's product documentation covering the panel build-up, the adhesive used and the declared characteristics of the product.
- Fabrication and setting-out drawings showing openings, machined service routes and the position of every connection.
- Handling, storage and site protection guidance covering the period between delivery and the building being closed in.
- Information on the visual grade applied to any panel face that is intended to remain exposed in the finished building.
- Details for panel-to-panel and panel-to-substructure connections as designed for this particular project.
- Sourcing information for the timber, where the project has requirements about origin or chain of custody.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Is a panel structure appropriate for this building, or would a beam-and-column frame suit the geometry better?
- Which elements are cross-laminated panels and which are other timber products, and what is the reasoning behind that split?
- How early does the design need to be fixed for fabrication, and which changes become difficult after that point?
- What is the fire strategy for this building, and how does it treat the exposed timber surfaces?
- How is acoustic separation between floors, and between dwellings, achieved above and below the panels?
- What protection from weather is planned between erection and the building becoming weathertight?
- How are services routed, and what is the process if a route has to change after the panels are made?
- Who is responsible for the connection design between the panels and the supporting structure below?
Blind spots
Commonly overlooked points
- The panel is a structural plate, so cutting or drilling it later is an engineering question rather than a decision to be taken on site.
- An exposed panel is both structure and finish at once, which means construction damage cannot simply be covered up afterwards.
- Airtightness at panel joints is a separate design task from the structural connection made at the very same joint.
- Fabrication lead times can shape the programme considerably more than the short period of erection on site.
- The timber moves a little with humidity, and junctions with rigid materials are detailed to allow for that movement.
What this page does not do
- Whether cross-laminated timber may be used in a particular building depends on local requirements, which must be confirmed with the relevant authority.
- Structural sizing, connection design and overall stability are engineering decisions for a qualified professional, and nothing here substitutes for that.
- Fire and acoustic behaviour depend on the whole assembly and on the specific product, and belong to the project's professionals and the manufacturer's documentation.
- This page is a general reference published for owners; Build Design Hub does not test, approve or recommend any product.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Timber & Wood-Based Materials
Reference entries for solid timber and wood-based panels and engineered products — from structural softwood and plywood to cross-laminated timber, OSB, MDF and veneers.
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