Coach Screws

About Coach Screws

Coach screws, also known as lag screws or coach bolts in some contexts, are heavy-duty fasteners designed for applications requiring exceptional holding power in timber and masonry materials. Distinguished by their hexagonal heads and robust threading, these versatile fixings are engineered to handle substantial loads where standard wood screws simply won't suffice. The hexagonal head design allows for easy installation using a spanner or socket, providing superior torque transfer compared to traditional slotted or Phillips head fixings.

Typically manufactured from high-grade steel or stainless steel, coach screws are available in various lengths and diameters to suit different structural applications. The coarse threading along the shaft provides excellent grip in timber, whilst the unthreaded shank portion ensures the fastener can pull materials tightly together without interference. At TooledUp.com, we stock quality coach screws from trusted manufacturers including SIRIUS, offering corrosion-resistant stainless steel options such as the A2 304 grade that delivers outstanding durability in both interior and exterior applications.

These essential fixings are indispensable for heavy construction work, timber framing, decking installations, and numerous structural carpentry projects where reliability and strength are paramount. Whether you're securing joists to masonry walls, installing outdoor structures, or undertaking renovation work, coach screws provide the robust connection your project demands.

Jargon Buster

A2 304 Stainless Steel

A corrosion-resistant grade of stainless steel containing 18% chromium and 8% nickel, suitable for both internal and external applications where rust prevention is essential.

Hexagon Head

The six-sided head design that requires a spanner or socket for installation, providing superior torque application compared to screwdriver-operated fixings.

Pilot Hole

A pre-drilled hole that guides the coach screw during installation, preventing timber splitting and ensuring accurate placement.

Thread Diameter

The measurement across the threaded portion of the screw, typically ranging from 6mm to 12mm for standard applications.

Shank

The smooth, unthreaded portion of the coach screw between the head and threaded section, which allows materials to be drawn tightly together.

Gauge

A sizing measurement indicating the diameter of the screw shaft, with higher numbers representing larger diameters.

Who Uses Coach Screws?

Coach screws are essential fixings for a wide range of trades and applications:

• Carpenters and Joiners – for structural timber connections and heavy-duty framework
• Builders and General Contractors – securing timber to masonry, steel, and concrete substrates
• Decking Specialists – fixing joists to supports and installing structural components
• Roofers – securing rafters, purlins, and heavy roofing timbers
• Shopfitters – installing heavy shelving, display units, and commercial fixtures
• Fencing Contractors – assembling robust fence posts and structural garden features
• Landscape Gardeners – constructing pergolas, raised beds, and outdoor timber structures
• DIY Enthusiasts – tackling home improvement projects requiring strong, reliable fixings
• Maintenance Engineers – repair and installation work across commercial and industrial premises

How to Choose the Right Coach Screws

Selecting the appropriate coach screws for your application requires consideration of several key factors:

Material and Coating: For exterior applications or damp environments, specify stainless steel coach screws such as A2 304 grade to prevent corrosion. For internal, dry conditions, zinc-plated steel options may be sufficient and more economical.

Length and Diameter: Choose a length that penetrates at least 50mm into the substrate material for adequate holding power. The diameter should be proportionate to the load – heavier applications require larger gauge screws. As a rule of thumb, the threaded portion should extend at least halfway into the receiving material.

Thread Type: Coarse threads provide superior grip in softwoods and timber, whilst finer threads may be preferable for hardwood applications. Ensure the thread pattern matches your specific material requirements.

Head Type: Standard hexagonal heads are most common, but verify you have the correct spanner or socket size available before purchasing. Some applications may benefit from washers to distribute load across the timber surface.

Quantity Requirements: Calculate your requirements carefully, accounting for spacing recommendations typically between 300-600mm centres depending on load calculations and building regulations.

Popular Accessories

To ensure successful installation and optimal performance of coach screws, consider these complementary products:

• Washers – stainless steel or zinc-plated washers distribute load and prevent the head from pulling through timber
• Socket Sets and Spanners – quality tools from brands like Bahco, Wera, or Stanley for proper installation
• Drill Bits – HSS or brad point bits for creating accurate pilot holes
• Rawlplugs or Fischer Fixings – when securing to masonry substrates
• Ratchet Spanners – speed up installation in repetitive applications
• Spirit Levels – ensuring accurate alignment during installation
• Wood Preservative – protecting exposed timber in exterior applications
• Torque Wrenches – for applications requiring specific tightening specifications

Safety Information

When working with coach screws, observe these important safety considerations:

Eye Protection: Always wear safety glasses when drilling pilot holes or driving coach screws, as wood chips and metal fragments can cause serious eye injuries.

Pilot Holes: Never attempt to drive coach screws without pre-drilling appropriate pilot holes. This prevents timber splitting, reduces the risk of the screw shearing, and minimises the physical strain of installation.

Tool Selection: Use properly sized spanners or sockets that fit the hexagon head precisely. Ill-fitting tools can slip, potentially causing hand injuries or damaging the fastener head.

Structural Applications: When using coach screws for structural purposes, ensure your installation complies with relevant building regulations and HSE guidelines. Consult a structural engineer for load-bearing applications if you're uncertain about specifications.

Material Compatibility: Verify that the coach screw material is compatible with the substrate and won't cause galvanic corrosion when different metals are in contact, particularly in exterior or marine environments.

Frequently Asked Questions

Q: Do I need to drill a pilot hole for coach screws?
A: Yes, always drill a pilot hole before installing coach screws. The pilot hole should be slightly smaller than the thread diameter – typically around 60-70% of the screw's core diameter. This prevents timber splitting and makes installation significantly easier whilst maintaining holding power.

Q: What's the difference between A2 and A4 stainless steel coach screws?
A: A2 304 stainless steel (also called 18/8) offers excellent corrosion resistance for most interior and general exterior applications. A4 316 stainless steel contains molybdenum for enhanced resistance to chlorides and acidic environments, making it ideal for marine or coastal installations where salt exposure is a concern.

Q: Can I use coach screws in masonry or brick?
A: Yes, but you must first drill a hole and insert a suitable wall plug, such as those from Fischer or Rawlplug. The plug diameter should match the coach screw's outer thread diameter. For heavy-duty masonry applications, resin anchors or through-bolts may provide superior holding power.

Q: How much weight can a coach screw hold?
A: Load capacity depends on numerous factors including screw size, timber species, grain direction, and installation quality. As a general guide, a 10mm x 100mm coach screw properly installed in structural timber can typically support 200-400kg in shear loading. However, always consult load tables and building regulations for structural applications, and consider engaging a structural engineer for critical installations.

Q: Why are they called coach screws?
A: The name derives from their historical use in coach building, where these robust fasteners secured the heavy timber frames and metal components of horse-drawn carriages. The hexagonal head design allowed coachbuilders to apply significant torque, creating strong, reliable joints capable of withstanding the stresses of road travel.