A carriage bolt is a fastener with a smooth, domed head and a square section (the neck) just under the head. It is designed mainly for wood. When you tighten the nut, the square neck bites into the wood and stops the bolt from turning, so you only need a wrench on one side.
Why the domed head and square neck?
The smooth head has no slot or hex, so it cannot easily be loosened from outside. This makes carriage bolts slightly more tamper-resistant and safer to brush against, for example on gates, playsets and benches.
The square neck locks into the wood. That is handy when the head side is hard to reach with a tool.
The name comes from their historical use in building horse-drawn carriages.
Common uses
Decks, fences, gates and pergolas
Wooden playsets and outdoor furniture
Attaching wood to metal brackets with square holes
Workbenches and shelving
Carriage bolt vs other bolts
Hex bolt: has a hex head and needs a wrench on both sides; better for metal-to-metal joints and when you need to torque the head.
Lag bolt (lag screw): has a pointed thread and screws directly into wood without a nut, useful when you cannot reach the back side.
Carriage bolt: passes completely through the parts and is held by a nut; strong and neat for through-connections in wood.
Choosing the right one
Diameter and length: the bolt should pass through all the parts plus enough length for a washer and the full nut, with a couple of threads showing.
Finish: for outdoor projects and pressure-treated lumber, use hot-dip galvanized or stainless steel hardware. Modern treated wood can corrode plain or zinc-plated steel quickly. Check the lumber manufacturer’s recommendations.
Matching nuts and washers in the same material, to avoid galvanic corrosion.
How to install a carriage bolt
Clamp the parts together in their final position.
Drill a hole the same diameter as the bolt, straight through. Wear safety glasses.
Insert the bolt from the side where the head will show and tap it with a hammer until the head sits flush and the square neck is seated in the wood.
Add a washer and the nut on the other side.
Tighten with a wrench until snug. Do not overtighten, which can crush the wood fibers or pull the head into the wood.
If the bolt spins
When the square neck no longer grips, usually in soft or old wood, try pressing the head with a pry bar or a screwdriver while turning the nut, or grip the head with locking pliers. For a stuck nut, use penetrating oil. As a last resort, cut the bolt with a hacksaw or an oscillating tool.
