hook pin

A hook pin is a tapered steel pin, typically 3/8 inch to 1/2 inch in diameter and 12 to 24 inches long, with a forged hook or eye bent at one end. The opposite end comes to a blunt or slightly rounded point. It drives vertically through overlapping timber members, where the hook catches on the top surface and prevents the pin from being driven further, locking the joint under compression.

Hook pins are standard in post-and-beam and timber-frame construction for assembling floor girders, roof trusses, and wall frames. Two or more timbers are aligned, the pin is driven through a drilled hole in the top member, and the hook wedges under the rim of that timber. The weight of the structure itself, combined with the mechanical lock, holds the joint tight without bolts, nuts, or tools on site. This made them invaluable before modern bolting became widespread, and they remain common in renovation work and traditional heavy-timber building.

Variants and installation

Pins vary in length and gauge depending on how many timbers must be pinned and how deep the penetration must be. Some have a ring or clevis eye instead of a hook, serving the same locking function. The pin hole is drilled slightly undersize, typically 1/16 inch smaller than the pin diameter, so the pin must be driven with a sledge. Once the hook is seated, removal requires striking the pin upward or boring it out, which is why they are essentially permanent.

The hook pin falls between a simple drift pin, which has no head or lock, and a modern through-bolt with a nut. It requires no torque wrench, no power tools, and no hardware beyond the pin itself. The downside is that joints cannot be easily adjusted or disassembled, and the force required to drive the pin must be controlled by the worker to avoid overshooting and damaging timber. Rust and corrosion can seize the pin over decades, making removal difficult even for inspection or repair.