stop-splayed scarf joint

The stop-splayed scarf joint represents an evolution of the classical lap joint, where the lap surfaces and corresponding recesses are not executed perpendicular to the timber member but at an oblique angle. This modification of the traditional lap joint improves material distribution and reduces vulnerability to failure.
The stop-splayed scarf joint belongs to the family of lap and butt joints. As an I-Joint, it demonstrates structural kinship with the straight lap joint and other lap joint variations. Similar construction principles can be found in the Gerber joint and other oblique connections in traditional timber construction.
The connection is based on classical lap joint geometry along one axis, however the laps and their corresponding recesses are arranged obliquely to the member axis. This oblique execution results in narrower ends of both lap surfaces, while more material remains at the beginning of the laps. The material distribution follows the principle of positive interlocking engagement.
The decisive advantage of the stop-splayed scarf joint lies in its optimized material distribution. Through the oblique geometry, the narrower ends occur in less stressed regions, while the material-rich beginnings carry the main loads. This leads to reduced vulnerability to failure compared to the perpendicular lap joint and improves stress distribution in the timber.
The oblique execution requires precise craftsmanship and careful angle determination. Poor workmanship can result in stress concentrations at the transitions.
The stop-splayed scarf joint transfers compression forces evenly across the enlarged contact surface. The oblique geometry provides more favorable force introduction and reduces local stress peaks, making the connection less prone to failure than the perpendicular variant.
The stop-splayed scarf joint is traditionally used in sills for wall construction, plates in wall frames, and purlins in roof constructions. The connection has particularly proven itself under moderate loads where better force distribution is required compared to simple lap joints.
Manufacturing follows traditional carpentry techniques with precise angle determination. After marking the oblique geometry, the connection is sawn and chiseled out. The positive-locking connection functions without additional timber connectors but can be secured with wooden pegs or other fasteners when required.
The stop-splayed scarf joint can be executed in various angles and proportions, with the optimal inclination angle depending on timber species and expected loading.