Coach bolts vs coach screws: what's the difference

Coach bolts and coach screws get muddled more than almost any other fixing - the names are nearly the same, they're both big, both heavy-duty, and both live in the timber and structural end of the toolbox. But they fix in completely different ways, and reaching for the wrong one leaves you either with a bolt you can't tighten because there's no way to the back, or a screw where you actually needed to clamp two pieces together.

The difference is simple once it clicks. A coach bolt passes all the way through and is held by a nut on the far side, so it clamps two things together where you can reach both faces. A coach screw threads into the material and holds by grip alone, so it fixes something down from one side, no nut and no rear access needed. That one distinction settles most jobs.

What a coach bolt is

A coach bolt - you'll also hear it called a carriage bolt - has a smooth, domed head with a square shoulder tucked underneath it. Push it through a pre-drilled hole and that square shoulder bites into the timber, so the bolt can't spin as you tighten the nut on the other side. That's the whole idea: you tighten from the back with a spanner while the head stays put at the front, clamping the two pieces firmly together. The rounded head also sits neat and tamper-resistant on the show face, with nothing for a spanner to grip. It's the fixing for joining timber to timber - gates, fence rails, decking frames, structural joints - anywhere you can get to both sides.

What a coach screw is

A coach screw - also known as a lag screw or lag bolt - is essentially a very large, coarse-threaded wood screw with a hex head. You drive it with a spanner or socket rather than a screwdriver, and it cuts its own grip into the timber as it goes, so it holds without any nut on the far side. That makes it the go-to when you can only reach one face: fixing a heavy bracket, a ledger board, a joist hanger or a wall-mounted fitting into solid timber, or into masonry with the right plug. One side, one tool, done.

Side by side

So which should you use?

Ask two quick questions. Can you reach both sides, and are you clamping two pieces together? If yes, a coach bolt gives you the strongest, most positive joint - the nut lets you pull the pieces tight and re-tighten later if timber shrinks. If you can only get to one side, or you're fixing something onto a single timber or a wall, a coach screw is the answer - no nut to reach, quicker to place, and plenty strong for the job. When both would physically work, the bolt-and-nut usually wins on ultimate strength and serviceability, while the screw wins on speed and one-sided access.

Fixing either one properly

Both fixings want a pilot or clearance hole - timber this size will split if you drive a big fixing into it dry. For a coach bolt, drill a clearance hole the diameter of the shank through both pieces so it slides through and the square shoulder can pull into the head-side timber. For a coach screw, drill a pilot into the timber sized to the screw's core, so the thread bites without splitting the wood or shearing the screw; in masonry, use the correct plug. Fit a washer under the nut of a coach bolt (and under a coach screw head) to spread the load and stop it pulling into the timber, and tighten firmly but stop before you crush the fibres - overtightening a heavy fixing does more harm than good. Drive coach screws with a spanner or socket rather than an impact-happy trigger finger, and outdoors, choose a galvanised or stainless finish so the fixing outlasts the timber.

The short version

Coach bolt for clamping two pieces together with a nut where you can reach both sides; coach screw for fixing down from one side into timber or a plugged wall. Pilot-drill both, add a washer, don't overtighten, and match the finish to where it lives.

Browse our coach bolts and coach screws, or the wider screws and bolts range, and request a quote for the sizes and finishes you use most.

Explore the range

Frequently asked questions

What is the difference between a coach bolt and a coach screw?

A coach bolt passes all the way through the material and is secured with a nut and washer on the far side, so it clamps two pieces together where you can reach both faces. A coach screw threads directly into the timber (or a plugged wall) and holds by grip, so it fixes something from one side with no nut needed. Access is the deciding factor: both sides means a bolt, one side means a screw.

Is a coach screw the same as a lag screw?

Yes. "Lag screw" and "lag bolt" are other names for the same fixing - a large, coarse-threaded, hex-headed screw driven into timber with a spanner or socket. Coach screw is simply the more common UK term.

Do coach screws need a pilot hole?

Yes. Timber will split if you drive a fixing this size in dry. Drill a pilot hole sized to the screw's core diameter so the thread bites without splitting the wood or shearing the screw, and use the correct plug when fixing into masonry.

Can you use coach bolts in masonry?

Coach bolts are designed for through-fixing timber, where a nut secures them on the far side. For masonry you'd normally use a coach screw with a suitable wall plug, or a dedicated masonry anchor, rather than a coach bolt.

Which is stronger, a coach bolt or a coach screw?

Where both are an option and you can reach both sides, a coach bolt generally gives the stronger, more positive joint because the nut clamps the pieces together and can be re-tightened. A coach screw is still very strong and is the right choice when you only have access to one side.