Through-Wedged Tenons: Decorative Joinery That Holds Forever

Wedges in through tenons are about more than looks. Yes, the contrasting wood creates eye-catching detail. But the real magic happens mechanically: a properly wedged tenon expands within the mortise, locking itself in place with enough force to outlast the glue that holds it. This is joinery that survives centuries.

How Wedging Works

When you drive wedges into saw kerfs cut in the tenon end, the tenon spreads outward. This expansion presses the tenon cheeks against the mortise walls with tremendous force, creating a friction lock independent of any adhesive.

The pressure also closes any minor gaps in the joint. A tenon that’s slightly undersized will still create a rock-solid connection once wedged. This forgiving nature made wedged tenons the choice of timber framers for generations before modern glues existed.

Kerf Placement

Cut the wedge kerfs perpendicular to the grain of the mortised piece. If you orient them parallel to the mortise grain, the expanding tenon acts as a splitting wedge, potentially cracking the mortised piece.

Position the kerfs about one-third of the tenon width from each edge. This provides substantial meat on both sides while allowing enough spread to engage the mortise walls. Kerfs too close to the edge weaken the tenon sections; too close together limits expansion.

Saw the kerfs to about two-thirds of the tenon length. Stop short of the shoulder by at least 1/8 inch to prevent the wedge from cracking into the visible shoulder line.

Mortise Modification

For maximum wedge effectiveness, flare the exit side of the mortise slightly. This dovetail-shaped opening gives the expanding tenon something to lock into. Without flare, the wedges simply force the tenon harder against parallel walls, relying on friction alone.

Widen each end of the mortise by about 1/16 inch on the exit face, creating a subtle funnel shape. The entry face stays at normal tenon size for a snug fit. This hidden flare provides mechanical lock without visible change to the joint appearance.

Making the Wedges

Wedge taper matters. Too steep, and the wedge bottoms out before spreading the tenon sufficiently. Too shallow, and excessive insertion depth weakens the tenon end.

Aim for a taper ratio of about 1:6 to 1:8, meaning the wedge gains 1/16 inch of width for every 3/8 to 1/2 inch of length. The thin end should fit the kerf snugly without forcing.

Cut wedges from straight-grained wood to prevent breaking during driving. Cross grain in a wedge guarantees failure. Match the species to your project needs: same species for subtle integration, contrasting species for visual emphasis.

Assembly Sequence

Apply glue to the mortise walls and tenon cheeks as usual. Don’t glue the wedges themselves; they should remain dry to prevent hydraulic lock during driving.

Insert the tenon fully before adding wedges. If you pre-wedge, the expanded tenon won’t enter the mortise. The joint must be tight at the shoulder before wedging begins.

Drive both wedges simultaneously, alternating light taps between them. This ensures even expansion. Keep checking that the tenon isn’t being driven back out as the wedges spread it.

Trimming and Finishing

Let the glue cure completely before flush-trimming the wedge and tenon ends. Cutting before cure risks shifting the joint or cracking the wedges.

Use a flush-cut saw for initial trimming, staying a hair proud of the surface. Then pare flush with a sharp chisel or block plane. Work from the edges toward the center to prevent tearout at the end grain.

Sand the exposed end to reveal the wedge pattern. The tenon end with its contrasting wedge inserts becomes a decorative element that announces quality craftsmanship to anyone who looks closely.