Coach Bolts / Carriage Bolts / Cup Square Bolts inc Nuts – Secure Timber & Steel Fixings

Coach Bolts / Carriage Bolts / Cup Square Bolts inc Nuts provide a strong, reliable fixing solution for timber-to-timber and timber-to-steel connections. Supplied complete with matching nuts, these cup square bolts are ideal for structural timber such as C24 graded joists and posts, outdoor projects, garden structures, decking, sleepers, fencing and general construction.

Coach Bolts / Carriage Bolts / Cup Square Bolts inc Nuts are classic, heavy‑duty fixings used throughout the building and landscaping trades wherever a strong, through‑bolted connection is needed. Featuring a rounded (cup) head with a square shoulder under the head, these bolts lock into the timber and prevent rotation as the nut is tightened, making installation quick and straightforward from one side only. In addition to these fasteners, cup washers with brass plated finish provide an elegant and durable solution for securing nuts in place while enhancing the overall aesthetic of the installation. Their resistance to corrosion ensures longevity, making them an ideal choice for both indoor and outdoor applications. This combination of functionality and style ensures that every project meets both structural integrity and visual appeal.

Commonly referred to as coach bolts or carriage bolts, these fixings are ideal for joining structural timber such as C24 graded joists, beams, posts and sleepers. When sized and spaced in line with a structural engineer’s design, they provide excellent clamping force and long‑term stability in decks, balconies, stair strings, pergolas, garden structures and heavy fencing. These fixings are also compatible with horseshoe packers for window frames, ensuring a seamless installation process. By incorporating horseshoe packers, builders can achieve a precise alignment and enhanced durability for their window structures. This combination of robust fixings and innovative design elements significantly contributes to the overall integrity and aesthetic appeal of the construction.

Each bolt is supplied complete with a matching hex nut, ensuring you have everything you need for a secure through‑fix. For best practice, use with suitable flat washers (and penny washers for softer timbers) under the nut and/or head to spread the load and protect the timber surface, especially on external work. Understanding bolt types and their applications can help you choose the right hardware for various projects. For instance, a lag bolt is ideal for heavy timber connections, while a machine bolt is often used in metalwork. Selecting the appropriate bolt ensures optimal performance and durability in your assembly. When installing materials like fibre cement cladding, the choice of fixings is crucial for long-lasting results. Browse our extensive selection of fibre cement cladding fixings online to find products that meet your specific requirements, ensuring a perfect match for your project. With durable options available, you can be confident in the integrity of your installation, providing both protection and aesthetic appeal.

The cup square design offers a clean, low‑profile appearance on the main visible face of your work. Once the square section bites into the timber, you can tighten the nut from the reverse side without needing to hold the head, which is particularly useful for pre‑assembled frames, tight spaces and one‑person installation.

These bolts are widely used in applications designed to the principles of Eurocode 5 / BS standards for timber construction when correctly specified. For structural or safety‑critical projects, always follow the guidance of a competent designer or structural engineer and refer to relevant codes of practice.

Typical Use Cases

• Fixing C24 timber joists and beams in decking and balcony structures.
• Through‑bolting fence posts and gate posts to rails, cleats and straps.
• Building garden structures such as pergolas, arbours, pergola rafters and heavy trellis frames.
• Securing railway sleepers in retaining walls, raised beds and landscaping projects.
• Joining timber to steel plates, brackets or shoes in structural frames and heavy carpentry.

Installation & Fixing Tips

1. Check length and diameter: Select a bolt length that allows full penetration through all materials plus enough thread for the nut and washers. Common diameters include M6, M8, M10 and M12 – follow your design or engineer’s specification.
2. Drill clearance holes: Drill a straight clearance hole through the first timber member (and any attached plates) to the recommended size – usually slightly larger than the bolt diameter (e.g. 10 mm hole for an M10 bolt) to allow easy insertion.
3. Drill through the second member: Use the first hole as a guide to continue drilling through the second timber or steel plate, ensuring good alignment.
4. Seat the square neck: Insert the bolt from the visible face so that the square section under the head bites into the timber. Gently tap the head with a hammer if required to set it fully.
5. Add washers and nut: Fit a suitable flat washer (or penny washer) over the projecting thread, then screw on the nut by hand.
6. Tighten progressively: Using a spanner or socket, tighten the nut until the joint is secure and the timber is firmly clamped, but avoid crushing the fibres excessively. For structural work, follow any specified torque settings or tightening sequence.
7. Periodic checks (external work): For outdoor structures exposed to weather and movement, re‑check and re‑tighten nuts after initial seasoning and periodically thereafter.

Note: For exposed external or corrosive environments, select bolts with an appropriate corrosion‑resistant finish (e.g. hot‑dip galvanised or stainless steel). Check product details and building regulations for your specific project. the advantages of stainless steel screws include their high resistance to rust and corrosion, making them ideal for outdoor and marine applications. Additionally, they offer superior strength and durability compared to other materials, ensuring long-lasting performance even under challenging conditions. This makes them a reliable choice for both construction and DIY projects where longevity is a priority.

Product Features

• Cup square head design – smooth, rounded head with square neck prevents rotation and gives a neat, tamper‑resistant finish.
• Complete with hex nuts – every bolt is supplied with a matching nut for a secure through‑fixing.
• Ideal for structural timber such as C24 – widely used for joists, beams, posts and sleepers in structural and landscaping work.
• Excellent clamping action – provides strong compression across connected members, minimising movement when correctly detailed.
• Versatile sizes – typically available in popular diameters (e.g. M6–M12) and lengths to suit a wide range of build‑ups.
• Simple installation – only requires drilled holes and basic hand or power tools; ideal for trade and competent DIY users.
• Suitable for timber-to-timber and timber-to-steel – perfect for connecting timber frames to steel brackets, plates and hangers.

Technical Information

• Type: Cup square coach / carriage bolt with hex nut.
• Head style: Domed (cup) head with square neck under head to resist turning.
• Typical diameters: Commonly supplied in M6, M8, M10, M12 (check product variant for exact sizes).
• Lengths: Available in a range of lengths to suit post, beam and sleeper thicknesses.
• Material: Steel (finish typically zinc‑plated for internal use; galvanised or stainless options recommended for external or high‑moisture conditions – check product specification).
• Substrates: Suitable for C16 / C24 softwood, hardwood, treated timber, sleepers and timber-to-steel connections when correctly designed.
• Standards & design: Commonly used within designs based on Eurocode 5 principles for timber structures. For structural applications, follow a qualified engineer’s specification and refer to relevant British / European standards.

For safety‑critical or load‑bearing structures (e.g. balconies, raised decks, guardrails), always consult a structural engineer and use bolts, washers and finishes that meet the project’s technical and regulatory requirements.

Frequently Asked Questions – Coach Bolts / Carriage Bolts / Cup Square Bolts inc Nuts

1. What is the difference between a coach bolt and a standard hex bolt?

A coach bolt (carriage bolt / cup square bolt) has a smooth domed head and a square section under the head that bites into the timber, stopping the bolt from rotating as you tighten the nut from the other side. A standard hex bolt has a hexagonal head and usually requires a spanner or socket on the head as well as the nut when tightening.

2. Are these bolts suitable for structural C24 timber?

Yes, coach bolts are routinely used with C24 graded structural timber for decks, frames, posts and beams. However, the diameter, length, spacing and quantity of bolts must be designed correctly. For any structural or safety‑critical work, follow a structural engineer’s details or recognised design guidance rather than guessing sizes.

3. Should I use washers with coach bolts?

In most cases, yes. It is good practice to use a flat washer under the nut and, where appropriate, under the head (especially in softer timbers) to spread the load and protect the timber surface. For external or heavily loaded joints, large penny washers are often recommended.

4. Can I use these bolts outdoors for decking and garden structures?

Yes, coach bolts are commonly used for decking, pergolas, fencing and garden structures. For external work, always select a bolt with an appropriate corrosion‑resistant finish (e.g. hot‑dip galvanised or stainless steel) and use compatible washers and nuts. Check the product specification and local building regulations for outdoor structural applications.

5. How tight should I make coach bolts in timber?

Bolt joints should be tight enough to clamp the members firmly without crushing the timber fibres or deforming washers. As a guide, tighten until the joint closes and the components no longer move, then stop. For engineered or safety‑critical connections, follow any specified torque values or tightening sequence provided by the designer or manufacturer.

Common Uses / How to Use

Common Uses

• Building timber decks by fixing beams, ledgers, posts and stair strings.
• Constructing pergolas, gazebos and heavy garden frames.
• Securing fence posts and gate posts to rails, hinges and brackets.
• Stacking and fixing railway sleepers in retaining walls and raised beds.
• Joining timber beams to steel plates or cleats in mixed timber/steel structures.

How to Use – Step‑by‑Step Summary

1. Plan the connection: Confirm bolt size, spacing and number based on design or engineer’s details.
2. Mark and drill: Mark hole positions accurately, then drill clearance holes straight through the members using an appropriate wood or metal bit.
3. Insert the bolt: Push the bolt through from the visible face so the cup head sits neatly against the timber.
4. Engage the square neck: Tap the head gently with a hammer until the square section bites into the timber and stops turning.
5. Fit washer and nut: Place a washer over the protruding thread, then screw on the nut by hand.
6. Tighten: Use a spanner or socket to tighten the nut until the joint is secure and fully clamped.
7. Inspect: Check that all bolts are properly seated, nuts are secure and no components are distorted or over‑compressed.