cog

A cog is a rectangular or square projection cut at the end of a timber beam, designed to slot into a matching recess (called a cog hole or mortise) in another beam. The cog bears directly on the inner edge of the receiving beam, transferring load by bearing rather than by fastener alone. It is one of the oldest timber joinery methods in heavy framing, still used in barn construction, bridge timbers, and structural carpentry where traditional methods are preferred or required.

The cog extends from one face of the beam end, typically cut to a depth of one-quarter to one-third the beam's thickness. The length of the cog varies with load requirements but commonly ranges from 4 to 8 inches on structural timbers. When the beam bearing the cog sits on the receiving timber, the cog shoulder transfers vertical loads to the bearing surface below, while nails or bolts through the beam may carry additional shear forces. The fit must be tight and level; a gap at the bearing point creates a hinge and negates the joint's strength.

Variants and limitations

A full cog occupies the entire depth of the beam end. A half cog extends from only one side, used when the receiving timber has limited depth or when the joint must sit flush. Double cogs (one on each face) appear in wide beams where a single cog would remove too much timber from the end. The cog is distinct from a mortise and tenon joint in that it relies on bearing area rather than enclosure; the receiving timber is not hollowed to fully contain the cog. This makes the joint simpler to cut but more sensitive to settlement and rot at the exposed bearing surface.

The main weakness of a cog is that it depends on continuous contact under load. Differential movement, foundation settlement, or wood shrinkage can lift the beam slightly off its bearing, converting the joint into a lever that opens under load. For this reason, cogged joints are typically secured with bolts or heavy lag screws driven through the beam. The top of the cog may also be cut at an angle (a sloped cog) to shed water and reduce rot risk where the joint is exposed to weather.

Cogging remains common in timber frame restoration and in regions where hand tool skills are preserved, but it has largely been displaced by bolted bearing plates and modern concealed connections in new construction. However, understanding cog joinery is essential for anyone repairing old buildings or working with hand-hewn timbers, where the joint may be the only fastening originally present.