Coach screw or coach bolt — which one do I need?

The two look similar in a bin and are frequently sold side by side, but they carry load in completely different ways. A coach screw relies entirely on wood fibres gripping a thread. A coach bolt clamps two pieces between a head and a nut, and the timber only has to resist being crushed. Where a joint matters, that difference is the whole answer.

Safety

Step by step

1. Establish whether you can reach the back of the joint.If you can, use a coach bolt. Through-bolting removes the dependency on the timber holding a thread and gives you a joint you can re-tighten as the timber shrinks.
2. Identify a coach bolt by its head.Domed or mushroom head with no drive at all, and a square shoulder under it. The square pulls into the timber as the nut is tightened and stops the bolt turning, which is why it needs no spanner on the head.
3. Identify a coach screw by its head and thread.A hex head that takes a spanner or socket, and a coarse wood thread running most of its length with a point on the end.
4. Drill both holes for a coach screw, not one.A clearance hole through the piece being fixed at the screw's outer diameter, and a pilot in the receiving timber at roughly the core diameter. Skipping the pilot splits the timber and skipping the clearance hole means the screw never pulls the joint up.
5. Always use a washer under a coach screw head.The head is small relative to the load and will crush straight into softwood. A penny washer is better again where the timber is soft.
6. Drill a snug hole for a coach bolt.The hole should match the shank so the square collar has something to bite into. Tap the head down with a mallet as you start tightening so the square pulls fully in.
7. Use a washer under the nut, always; under the domed head, never.The domed head is meant to bear directly on the timber so the square can engage. The nut end needs a washer, and a penny washer on soft timber.
8. Drive a coach screw with a spanner or a socket, not a drill.It needs controlled torque, and the resistance you feel through a spanner is the only information you get about whether the timber is about to give up.
9. Re-tighten after the first season outdoors.Timber shrinks as it dries and the joint slackens. One re-tighten a few months in stops years of gradual working loose.

Tips

• Coach bolts are the correct fixing for gate hinges into a timber post, for pergola connections and for anything a child will hang off — the square collar means the head cannot be undone from the front, which is a small security benefit too.
• A coach screw into masonry needs a proper plug or a resin anchor sized for it, not an ordinary red wall plug.
• If the domed head of a coach bolt is spinning as you tighten, the square is not engaged. Undo it, tap the head home, and start again.

Common mistakes

• Driving a coach screw with an impact driver — It is very easy to shear the head or to strip the thread in the timber, and you get no feel for how close you are to either.
• No clearance hole through the top piece — The thread grips both pieces and jacks them apart, so the joint stays open however hard you pull on the spanner.

Questions people ask