Structure and support · Frames & Load-Bearing Walls
Mass Timber Panel System
This entry describes how solid timber plates act as structure and enclosing surface at once, and why the connections and the information issued before manufacture carry the behaviour of the finished building.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What mass timber structure is
A mass timber panel is a plate rather than a member. It carries load down its own height, spans across a room, and resists being pushed out of square in its own plane, so a single element does work that a light frame divides between studs, sheathing and joists. A wall panel is a wall, a bracing element, and frequently the finished internal surface at the same time.
Because the panels are made in a factory, everything that will pass through them is settled before they arrive. Openings, service holes, recesses, rebates and connection preparation are part of the manufacturing information, so the drawing set closes earlier than for site-built construction and the coordination of services becomes a design activity rather than an installation one.
The nearest relative is loadbearing-precast-panel-system, and both are large factory plates whose behaviour lives in the joints between them. The test is what the connection is made of and what the panel needs from its environment: timber panels are screwed, plated and splined together and must be kept dry from delivery onward, while precast units are grouted or welded, are indifferent to wetting, and are unforgiving of levelling error.
The other consequence of the panel being the finish is that damage is permanent. Marking, impact and water staining during construction stay in the completed building, and anything that would ordinarily be concealed in a joist void - services, acoustic layers, levelling - has to be added deliberately as a layer above or below the plate.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- cross-laminated timber structure
- solid timber panel construction
- engineered timber panel structure
- mass timber construction
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Carry vertical load through solid timber plates that also enclose the spaces they surround.
- Provide in-plane stiffness through the panels themselves, so the building resists sideways force without a separate braced frame.
- Connect wall and floor plates so that separate manufactured elements behave as a single structure.
- Present a structural surface that can remain visible, where the design accepts it as a finished face.
- Fix the position of every opening, hole and connection before manufacture, so nothing structural is decided by cutting on site.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportStructural wall panels
Full-storey plates that carry load from above, stiffen the building in their own plane and define the room they enclose. Because one element does all three jobs, moving a wall to suit a layout removes bracing and enclosure along with the load path.
- Primary supportFloor and roof plates
Horizontal panels spanning between walls and beams, which also form the plane that ties the walls of a storey together. They are the surface the storey above is built from and, where left exposed, the ceiling of the storey below.
- AttachmentPanel-to-panel and panel-to-support connections
Screws, plates, brackets, splines and hold-downs that join separate plates into a structure. They transfer force between panels, resist uplift and locate each element, so the joint arrangement is as much the structure as the panels are.
- Primary supportBearing and compression zones
The positions where a wall panel lands on a floor panel and squeezes it across its layers. Timber resists load across the grain less readily than along it, so what happens at these zones is a deliberate part of the design rather than an incidental contact.
- SubstrateFloor topping and isolation layer
The build-up laid over the structural plate, which does the levelling, receives the finish and carries the separation between the floor and the structure. Its edges decide whether that separation is continuous or quietly bypassed at the perimeter.
- Service zonePre-planned routes, battens and voids
The holes cut in manufacture and the battened or suspended zones added afterwards to carry services past a solid plate. Where a route is not drawn before manufacture, it becomes a hole cut into a load-carrying element after the fact.
- ProtectionMoisture protection during erection and in use
Coverings, coatings and detailing that keep water off horizontal panel faces, exposed end grain and joints. Timber that has been repeatedly wetted while the building was open is not restored by drying, because the marking and any raised grain remain.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
A panel is stiff in its own plane, but a floor made of several panels is only as continuous as the joints between them. Until the panel-to-panel connection is able to pass force across the seam, the floor is a set of adjacent plates rather than one plane, and the wall panels below it are not yet being held together by anything.
Where a wall lands on a floor plate, load runs along the grain of the wall and then across the layers of the floor. That crossing repeats at every storey and accumulates down the building, so the number of horizontal panels in the path is a design consideration rather than a detail. It is one of the few places where the arrangement of storeys is a structural question in its own right.
Because the panel is often the ceiling below, the whole floor build-up above it has to do the levelling, the routing and the separation between structure and finish. An isolation layer bridged by a screed running to the wall, by a partition fixed through it, or by a fixing driven into the panel beneath is no longer separating anything, and the bypass is invisible once the finish is down.
Pre-manufacture information ties everything together. A hole that was drawn is a hole made in a controlled way within a plate designed for it; a hole that was not drawn is cut on site through a load-carrying element and through whatever surface was meant to remain visible. Late service coordination is therefore a structural change, not a variation of finish.
Protection and connection also interact. Joints, rebates and end grain are the positions that both take water and take fixings, so a connection zone that was allowed to stay wet during erection is a connection zone whose surroundings have moved and marked. Sequencing of the covering and the connections is settled together.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
These engineered products are commonly encountered as the plates and supporting members of mass timber structures. Which product belongs at any position, and in which orientation, is settled by the structural designer and the manufacturer's documentation.
Attachment
Connection hardware for panel structures is commonly encountered in these metals. Exposure, contact with damp timber and the fixings used all bear on selection, and that determination sits with the designer and the connection supplier.
Substrate
Toppings of this kind are commonly encountered over structural timber floor plates. Whether a wet or dry topping belongs over a given panel, and what it must not be allowed to touch, is a design determination rather than a property of the topping.
Control layer
These are commonly encountered at panel joints and where plates meet the external build-up. No individual layer makes an assembly resistant to air or water movement; that belongs to the completed and correctly detailed construction.
Thermal layer
Insulation is commonly encountered outboard of the structural plate in this system rather than within it. Nothing here states a thermal outcome; that is a property of the whole enclosure in its climate and is assessed by a qualified professional.
Protection
These are commonly encountered where timber is exposed to weather or to handling. Whether any treatment is appropriate for a structural panel, and what it does to later bonding or coating, is a question for the manufacturer's documentation.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Structural plate within an acoustic separation
Where a solid timber floor forms part of a separation between dwellings, what that separation achieves is a property of the complete tested assembly and of the flanking construction around it, and it belongs with a qualified acoustic professional. What this entry can say is that an isolation layer bridged at the perimeter, or by a fixing driven into the plate, is bypassed at that line.
Read about Separating Floor →Routing services past a solid plate
Nothing passes through a mass timber panel that was not planned, so services either run in a formed void or in a battened zone applied to the face. Deciding which, and where, is what determines how much of the timber can remain visible.
Read about Service Void →Cladding support fixed back to the panel
Brackets and rails fix directly into the structural plate, so every fixing is a penetration of a load-carrying element and of whatever controls water behind the cladding. The fixing pattern is settled with the panel design rather than after it.
Read about Rainscreen Facade →Timber meeting the substructure
The base of a panel structure is where dry engineered timber meets construction that holds moisture. Separation at that line, and the ability to keep it clear of standing water and splashing, is one of the defining details of the whole system.
Read about Damp-Proof Continuity →Continuity across panel joints
The joints between plates are numerous, repetitive and covered early, which makes them the natural line for the barrier and the easiest place to lose it. Sealing is applied as the structure goes up rather than revisited once the building is enclosed.
Read about Air Barrier System →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- The exposure that matters most happens before the building is closed, when horizontal plates collect water and joints hold it. Sequencing, covering and drainage of the open structure are planning questions with permanent consequences for a visible surface.
- Acoustic
- Acoustic behaviour is a property of the complete tested assembly and of the flanking construction around it, and continuous timber plates offer many flanking routes. That work belongs with a qualified acoustic professional rather than with the structural design.
- Fire
- Fire performance is a property of a tested assembly and of the way it is protected and detailed. This entry states none, and the tested system together with the relevant authority governs what is permitted where exposed timber is proposed.
- Buildability
- Panels arrive in sequence, are craned into position and cannot be trimmed to fit. Access, delivery order and lifting are design constraints, and a change agreed on site may have already been manufactured somewhere else.
- Movement
- Engineered plates move less than sawn sections but still respond to changes in moisture, most of it across the panel. Where a structure stacks many horizontal plates, the accumulated effect at the top of a building is worth designing for explicitly.
- Documentation
- The manufacturing model holds the position of every hole, recess and connection, and it is the most useful record a later owner can be given. Without it, altering the building means investigating a plate whose internal layout is not visible.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Which panels are providing in-plane stiffness, and does the layout keep them where the design needs them?
- Where does a wall panel bear on a floor panel, and how many of those crossings occur down the height of the building?
- What is the deadline for service routes, openings and fixings to be fixed before manufacture begins?
- Which timber surfaces are intended to remain visible, and what protects them through the construction period?
- How does the floor build-up separate the finish from the structure, and what stops that separation being bridged at the edges?
- How is the structure to be kept dry between erection and enclosure, and who holds that responsibility?
- What does the base detail do where the panels meet the substructure and the external ground level?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Mass timber is described as a frame, when the panel is a plate doing the work that studs, sheathing and joists would share.
- Exposed timber is treated as a finish decision, when it decides where services can run and how the floor build-up has to work.
- A hole is assumed to be addable later, when the position of every penetration was part of the structural information.
- Panels are thought of as interchangeable with precast units because both arrive complete, though they need entirely different care on site.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which panels are carrying and which are providing stability in each direction?
- How is bearing across the grain being handled where walls land on floor plates?
- When does the information for manufacture have to be complete, and what is still open?
- What is the protection strategy for the structure between erection and enclosure?
- How are services routed without cutting into panels after delivery?
- What separates the floor finish from the structural plate, and where could that separation be bridged?
- Will the manufacturing model be handed over as part of the record for the building?
What this page does not do
- Nothing here establishes whether a panel may be cut, penetrated or altered; that is a determination for the structural designer and the panel manufacturer.
- No panel thickness, connection arrangement or fixing pattern is stated in this entry, and none should be inferred.
- Fire and acoustic performance are properties of tested assemblies with their protective layers in place, and neither is stated here.
- Water damage to a visible structural surface is generally permanent in appearance, so protection is planned rather than remedied.
- Where exposed timber is proposed, what is permitted is decided by the relevant authority for the building and its use.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Primary Structural Frames and Load-Bearing Wall Systems
Complete primary load paths from roof to foundation in masonry, timber, steel and concrete, together with the stability system without which none of them stands up.
Browse all frames & load-bearing walls entries →