Making Treenails — Wooden Pegs for Timber Framing and Shipbuilding

Before iron nails were cheap, wood was fastened with wood. A treenail (pronounced 'trunnel') is a cylindrical wooden peg driven into a bored hole to lock a timber joint together. Treenails held together medieval roof trusses, Viking longships, and post-and-beam barns for centuries. In many applications they outperform iron nails — they do not rust, they swell when wet to tighten the joint, and they flex with the timber instead of working loose.

Treenails are always made from a wood harder or at least as hard as the timber they join. Oak treenails in oak frames, or locust treenails in softwood construction. They are riven from straight-grained stock, never sawn — riving preserves the grain continuity that gives the peg its shear strength. A sawn peg has cut fibres that snap under load.

The technique is simple: rive a billet into rough square sections, then round them by driving through a steel plate with a round hole (a rounding die) or by shaving with a drawknife. The finished treenail is slightly tapered so it can be started in the hole and driven home with a mallet. Some traditions split the protruding end and drive a small hardwood wedge into the split, locking the treenail permanently.