Structure and support · Frames & Load-Bearing Walls
Post and Beam Timber Frame System
This entry explains why the joints rather than the members govern a heavy timber frame, and why the envelope around it has to be designed as a layer that moves independently of the structure it covers.
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 post and beam frame is
In a post and beam frame the members are large and few, so load is gathered at a small number of positions and passed through connections cut into the timber itself. A post commonly receives beams from more than one direction, which means material is being removed at exactly the point where most is arriving. The frame is therefore sized by what remains at its connections, not by the members considered alone.
A heavy frame is not arranged to rely on sheathing for its resistance to racking; that work is given to braced bays. Diagonal braces triangulate the bays, and the frame's resistance to leaning is a property of where those braces sit and of how firmly each end of a brace is held. Where an enclosure is wrapped across the frame, whether anything structural is expected of that enclosure is a design decision rather than a property of the frame. Bays are read across the whole frame rather than one at a time, because a brace missing in one place is answered by the others.
The nearest relative is structural-steel-frame-system, and the joint settles which entry applies. Look at a connection: a worked timber joint is cut and pegged, and it has to keep working while the section dries and moves, so the frame is stabilised by braces. A bolted or welded steel joint holds its dimension and can be made to resist rotation, so the frame itself can be the stabilising element.
It is equally not timber-platform-frame-system, where the walls are the structure. Here the walls are infill or wrapping. In a wrapped frame the timber sits inside the insulated envelope and the frame reads only from within; in an infilled frame the timber appears in the external face and every panel edge becomes a junction that has to be weathered and allowed to move.
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.
- heavy timber frame
- oak frame construction
- braced timber frame
- timber post and beam
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.
- Gather load at a small number of posts and carry it through worked connections into the base of the frame.
- Hold the frame square through braced bays rather than through any sheet or panel fixed across it.
- Arrange the structure so that infill and non-loadbearing enclosure need not carry vertical load, and walls can sit off the structural grid.
- Allow substantial timber sections to dry and move without the connections or the envelope being damaged by it.
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 supportPosts
The vertical members that collect load from the beams meeting them and carry it down to the base of the frame. Because several members often meet at one post, the useful section is what survives the cutting, and a post is rarely a plain rectangle where it matters most.
- Primary supportBeams, plates and tie members
The horizontal members that span between posts, carry floors and roof, and tie the tops of the posts together so the frame acts as a whole. Tie members are the ones that resist a roof pushing the frame outwards, and they are the easiest to mistake for a decorative feature.
- Primary supportDiagonal braces
Short members set across the corner of a bay so that the rectangle cannot deform into a parallelogram. A brace works only when both ends are firmly held, so its value depends entirely on the members it lands into rather than on its own size.
- AttachmentWorked joints, pegs and metalwork
The cut connections, their pegs, and any plates or concealed metalwork that supplement them. These are the parts that decide how force passes from one member to another, and they behave differently as the timber around them shrinks onto or away from them.
- SubstrateSill beam and base connection
The horizontal member at the foot of the frame and the fixing arrangement that anchors it to the substructure. It is where every post arrives, where the frame is located, and where timber sits closest to the wettest part of the building.
- Edge and terminationFrame-to-envelope junction
The line where the enclosing construction meets the frame, whether it is a panel edge in an infilled frame or a wrapping layer passing across it. Each junction has to resolve weather, air movement and differential movement at the same position.
- Jointing and sealingPerimeter seals and movement allowance
The compressible and sealing elements that keep a junction closed while the timber either side of it changes shape. They are working continuously rather than settling into a final position, which is why they are treated as a maintainable part of the building.
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.
The connection sets the member, not the other way round. Where a tenon, housing or lap removes material, what is left is what actually carries, so an apparently generous post can be at its most heavily worked precisely where beams and braces converge on it. Changing a joint arrangement to suit a room layout changes the structural behaviour of the whole bay it sits in.
A brace is only as good as its two landings. Cut away the foot of a brace to form a doorway, or land its head on a member free to rotate, and the triangle it was forming stops existing while the timber that formed it remains in view. The frame then redistributes that work through the other braced bays, which is a whole-frame consequence of a local alteration.
As the sections dry, the joints change. Shoulders close up, pegs bear differently, and the frame settles into a slightly different shape from the one it was raised in. Anything that assumes the frame is holding still - a rigid lining tight to a post, a window fixed hard into an opening formed by frame members, a stair bearing on a beam - inherits that movement.
This is why the envelope is so often wrapped past the frame rather than fitted between it. A wrapping layer crosses the structure in one continuous plane and can be detailed to let the frame move behind it, whereas an infill panel presents a junction at every post and beam, where the weather line, the air line and the movement allowance all have to be resolved together in the same joint.
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 are commonly encountered as the posts, beams and braces of heavy timber frames, and they behave differently as they dry. Which family belongs in a given frame is a matter for the designer and the supplier's documentation rather than appearance.
Attachment
Metalwork supplementing or replacing a cut joint is commonly encountered in these metals. Exposure, the fixings used and any contact with treated timber all bear on selection, which belongs with the designer and the product documentation.
Protection
These are commonly encountered where the sill and base of a frame sit nearest to moisture. Whether separation or treatment is called for at a particular position depends on the detail and the exposure, and is determined by the designer.
Finish surface
Infill faces in traditional and traditionally modelled frames are commonly encountered in these materials. Nothing here states how any of them behaves against weather; that is a property of the completed panel and its junctions with the frame.
Thermal layer
These are commonly encountered in the wrapping or infill around a heavy frame. No thermal outcome is stated here; how an enclosure performs depends on the whole build-up, its continuity and the climate, and is assessed by a qualified professional.
Jointing and sealing
Sealing products of this kind are commonly encountered at the junction between a moving frame and the enclosure around it. Their behaviour at a joint that keeps moving is a matter for the manufacturer's documentation and the detailer.
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.
Point loads at the base of each post
Load arrives at the substructure in a handful of concentrated positions rather than along a line, which is why the foundation arrangement follows the frame grid. Setting out has to be right before the frame is raised, because the frame cannot be adjusted to suit a base that has moved.
Read about Pad Foundation →Wrapping the frame with a panel envelope
A panel wrap crosses the frame in one plane so the timber can move behind it, but the panels then need their own support and their own connections back to the frame. Where the two systems fix to each other is where both movement and continuity have to be settled.
Read about Structural Insulated Panels →Openings formed within the frame itself
A window set directly between frame members is fixed into something that is expected to change shape. The junction has to accommodate that while still controlling water and air, which is a harder problem than an opening formed within a dimensionally stable wall.
Read about Window Opening Interface →Barrier continuity where the frame is exposed
Any member passing from inside to outside carries the barrier line across a permanently moving junction. An exposed frame therefore multiplies the number of these crossings, and each one is a detail rather than a repetition of the same detail.
Read about Air Barrier System →Roof structure carried on the frame
Purlins and rafters land on beams and plates, and the outward push of a roof is resolved by the tie members of the frame. Where a roof is opened up or altered, the tying arrangement it depended on is the first thing to establish.
Read about Roof Carcass →
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.
- Movement
- Large sections dry slowly and continue to change shape long after they are in place, mostly across the grain. The design question is not how to stop that but what is allowed to be rigid against it, and which junctions are expected to keep working while it happens.
- Moisture
- Timber in contact with wetter construction, or set where water can sit on an end grain surface, is where problems concentrate. The base of a post, an exposed beam end and a badly draining sill are the positions worth designing carefully and inspecting later.
- Durability
- An exposed frame is weathering as well as carrying, and the parts that show are the parts that get wet. Where a frame is wrapped, the timber is protected but concealed, which trades a maintenance question for an inspection question.
- Interfaces
- Every place the enclosure meets the frame is a junction between something that moves and something that is trying to stay closed. In an infilled frame these junctions are numerous and visible; in a wrapped frame they are few but hidden.
- Buildability
- The frame is cut before it is raised, which means the drawing set is closed early and changes made afterwards are made in an object that has already been fabricated. Coordination of services, openings and fixings happens ahead of that point.
- Documentation
- Frames are often significant in their own right and are frequently altered over long periods. A record of which members are structural, which braces are original and what supplementary metalwork exists is worth holding for whoever works on it next.
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 bays are braced, and does the intended room layout leave every brace with both ends properly landed?
- Where do beams and braces converge on a post, and what section of that post remains where they meet?
- Is the frame intended to be seen from inside, from outside, or from both, and what does that decide about the envelope?
- How does the enclosure cross the frame, and what is allowing for the frame changing shape behind or around it?
- Where does timber come closest to moisture at the base of the frame, and what separates the two?
- What has to be resolved before the frame is cut, given that it arrives fabricated rather than adjustable?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Braces are read as decorative because they are visible, when they are frequently the only thing keeping a bay square.
- Infill panels are assumed to be carrying load because they are solid and continuous between the frame members.
- Splitting along the grain of a large section is treated as failure, when it is a normal consequence of drying that is assessed rather than assumed.
- A wrapped frame is described as a timber building outside, when the timber sits within the envelope and only reads from inside.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Which members and braces are structural in this frame, and which have been added or altered later?
- How is the frame stabilised in each direction, and does the proposed layout affect that?
- What has been allowed for the sections drying and moving, and where does that show in the details?
- How is the envelope arranged relative to the frame, and where does it cross the structure?
- What protects the base of the frame from the construction and the ground it meets?
- If the roof is being altered, what is resolving its outward push at present?
What this page does not do
- Nothing here establishes whether a member or brace may be cut, moved or removed; that is a determination for a structural engineer inspecting the actual frame.
- No member size, joint dimension or fixing arrangement is stated in this entry, and none should be inferred from the descriptions.
- Historic frames may be protected in law, and any obligation attached to them sits outside the scope of this reference.
- Signs of decay at bases, beam ends and joints are assessed by opening up and probing, not by reading how the system is meant to work.
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 →