Pegged Oak Pan-de-Bois Stud and Diagonal Cross Carpentry: Heavy Timber Framing in Bresse

In the French architectural tradition of pan de bois, the choice of timber dictates the lifespan of the building. The Bresse region was blessed with vast communal forests of pedunculate oak (Quercus robur), celebrated for its tight growth rings, high tannin content, and superior mechanical bending strength. Felled during mid-winter dormancy when sap levels were lowest, the logs were seasoned slowly under bark before being squared with the broad axe (doloire).

Unlike softwood timber framing, which relies on light, closely spaced studs, Bressan carpentry is based on massive heavy framing. Sill plates (soles) and corner posts (poteaux corniers) regularly measure 200 to 250 mm square. This monumental mass provided structural stability, high fire resistance (as dense oak chars slowly at just 0.6 mm per minute, preserving structural integrity), and immense load-bearing capacity for the heavy tiled roofs above.

Long before computer-aided design or printed architectural blueprints, master carpenters executed full-scale geometric drafting directly on the workshop floor—a technique known in French guild tradition as the épure. The carpenter leveled a bed of sand or planed timber planks, scribing the exact dimensions of the wall using chalk lines, dividers, and square.

Timbers were laid out horizontally on the épure, overlapping at intersections. Using a plumb bob, the carpenter transferred cutting lines across intersecting members, cutting every joint to fit its specific matching partner. Before dismantling the frame for transport, the carpenter gouged paired marques d'assemblage (assembly marks) across joint faces using specialized curved chisels (rainettes). Consisting of Roman numerals, fish hooks, and crescent moons, these marks allowed rural builders to erect complex multi-ton frames in a single day with absolute precision.

The signature visual and structural element of Bressan half-timbering is the croix de Saint-André (St. Andrew's cross). While simple vertical studs support vertical dead loads, an unbraced post-and-beam frame has zero resistance against lateral forces; strong horizontal wind gusts push the vertical posts into a parallelogram, culminating in catastrophic racking collapse.

To prevent racking, Bressan carpenters infilled wall bays with intersecting diagonal oak struts crossing in an 'X' pattern. Where the two diagonal timbers intersect, they are halved together using an interlocking half-lap joint. At their outer ends, each diagonal is mortised and tenoned into the vertical studs and horizontal sills. This creates four rigid structural triangles within a single bay. Because triangles cannot deform without bending or crushing the solid oak timbers, the St. Andrew's cross imparts extraordinary shear rigidity to the farmstead.

The ultimate strength of historic French carpentry lies not in modern metal hardware, but in the cheville en chêne (oak trunnel peg). To lock a tenon into its mortise, Bressan carpenters utilized draw-boring (le chevillage à tire pour point).

The carpenter bored a 20 mm auger hole through the cheeks of the mortise. The tenon was then inserted into the mortise, and the center of the hole was marked onto the tenon tongue. The tenon was removed, and its hole was drilled deliberately 2 to 3 millimeters closer to the tenon shoulder. When the frame was erected, a tapered, hand-split octagonal oak peg was hammered through the misaligned holes. The wedge action of the peg forcibly drew the tenon shoulder dead-tight against the mortise face with tons of mechanical clamping force, permanently locking the joint under active spring tension across centuries of service.