Oak Frame Connections: Joints and Assembly
Oak timber frame construction relies on a set of joinery connections that have been refined over several centuries. Each joint is sized and cut to transfer specific loads — bending, shear, and tension — between members, while accommodating the movement that occurs as the wood continues to dry after the frame is raised.
Mortise-and-Tenon
The mortise-and-tenon is the foundational connection in an oak frame. A rectangular projection (the tenon) is cut at the end of one member and fits into a corresponding cavity (the mortise) cut into the receiving member. Proportions vary by application, but a common rule holds that tenon thickness should be roughly one-third of the member's cross-section.
In Poland, oak used for structural tenons is typically dried to 18–22% moisture content before cutting, since green oak shrinks significantly across the grain as it dries. A tenon cut from green stock and pegged into a mortise will tighten over time as the wood loses moisture, creating a self-wedging effect used deliberately in traditional frames.
Green oak (świeże drewno dębowe) is used in traditional Polish frame construction precisely because its working properties — softness and predictable shrinkage — make it easier to cut and peg than kiln-dried material.
Scarf Joints
Scarfs extend a timber longitudinally where a single-piece member is not available in the required length. In load-bearing applications, a housed scarf with a face shoulder carries compression, while pegs resist tension along the splice. The splice location in a bent or plate should avoid positions of maximum bending moment — typically mid-span.
Common scarf types used in Polish oak frames
| Type | Load capacity | Typical use |
|---|---|---|
| Bladed scarf | Moderate compression | Wall plates, sill beams |
| Housed scarf with key | Compression + limited tension | Ridge purlins |
| Tabled and wedged scarf | Compression and tension | Long tie beams |
Lap Joints
Half-lap and notched-lap joints appear at intersections where two members cross rather than butt. In braced panels, diagonal braces are typically let into posts and rails using a housed or notched lap, keeping the faces flush and reducing eccentricity in the load path.
Peg Selection
Wooden pegs — traditionally riven oak or hornbeam (grab, Carpinus betulus) — are driven into a slightly offset pre-bored hole to create drawboring: the peg pulls the joint shoulder tight as it is driven. Peg diameter is commonly 22–28 mm for principal connections. Metal hardware is avoided in traditional frames because differential thermal movement between steel and oak accelerates weathering at the joint face.
Seasonal Movement and Tolerances
Oak moves tangentially (across the growth rings) at roughly twice the rate it moves radially. A 200 mm square post might move 4–6 mm across its width between summer and winter conditions in the Polish climate (continental, with average January humidity significantly higher than July). Joint geometry must leave room for this movement without racking the frame or splitting the tenon shoulder.
Assembly sequence
Traditional frame assembly in Poland follows the bent-by-bent sequence: each transverse bent (wiązar) is assembled flat on the ground, raised as a unit, and then connected by longitudinal plates and purlins. This approach allows joints to be fitted and pegged under controlled conditions before the frame is at height.
The Timber Research and Development Association (TRADA) publishes technical guidance on timber joinery proportions, including recommendations applicable to oak frame construction.