Coach Bolts vs Coach Screws: What’s the Difference?

Coach bolts and coach screws are both widely used in timber construction, but they are designed for different types of fixing. Choosing the right one will affect strength, installation time and long-term performance.

This guide explains the difference between coach bolts, carriage bolts, lag bolts and coach screws, and when each should be used on site.

What is a coach bolt?

A coach bolt, also known as a carriage bolt or cup square hex bolt, is a through-bolt with a domed head and square neck.

The bolt passes through the material and is secured with a nut and washer on the opposite side.

• Requires access to both sides of the fixing
• Provides a strong clamped connection
• Prevents rotation with a square neck
• Used for structural and load-bearing connections

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What is a coach screw?

A coach screw is a heavy-duty timber screw with a hex head. It is driven directly into timber without the need for a nut.

Coach screws are sometimes referred to as lag screws or lag bolts, particularly in US terminology.

• Installed from one side only
• No nut required
• Faster to install than bolts
• Common for timber fixing and framing

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When to use coach bolts

Use coach bolts where strength and security are critical:

• Timber to timber structural connections
• Steel brackets to timber
• Fencing posts and rails
• Gate construction
• Load-bearing assemblies

When to use coach screws

Coach screws are more practical where access is limited:

• Fixing into timber where rear access is not available
• Decking and landscaping
• Timber frames and sleepers
• General construction work

Lag bolts and lag screws explained

The term “lag bolt” is often used in search but can refer to different products.

In most cases, it refers to a coach screw rather than a coach bolt. This is why checking the product type is important when ordering.

Which should you choose?

The choice comes down to access and required strength.

• Use coach bolts for maximum strength and through fixing
• Use coach screws for speed and single-sided installation