Coach screws, also known as lag screws or lag bolts, are a type of fastener used in construction, furniture making, and other applications requiring a strong and secure hold. Characterized by their hexagonal head and coarse external thread tapering to a point, coach screws bite into materials and hold fast.
Coach screws feature an external thread running along a tapered shank terminating in a sharp point. The taper and point allow the screw to bite into materials like wood without the need to drill a pilot hole.
The Difference Between Coach Screws and Lag Screws
Coach screws are often confused with lag screws, leading many people to use the terms interchangeably. However, while similar, the two fasteners have distinct features and uses:
- Coach screws taper to a point, allowing them to bore into wood and other materials without a pilot hole. The coarse thread on coach screws also gives them high holding power.
- Lag screws have a thicker, finer thread that requires tapping a pilot hole for installation. While the finer thread doesn’t grip as tightly, it allows lag screws to withstand shear loads better than coach screws.
So in summary:
- Coach screws are better for biting into materials and resisting pull-out forces. Their tapered tip and coarse thread provide excellent holding power.
- Lag screws have finer threads optimized for resisting shear loads. They need a pilot hole but can handle lateral forces better.
Many timber construction projects rely more on pull-out resistance than shear strength. So despite the name confusion, coach screws are much more common in structural applications than lag screws.
Coach Screw Sizes, Dimensions, and Standards
Coach screws are manufactured in many standard sizes but are most commonly found in diameters from 5mm to 12mm. Lengths for a given diameter usually start around 25mm up to around 200mm.
Some of the common coach screw sizes used in construction and woodworking include:
- M6 x 50mm
- M8 x 80mm
- M10 x 100mm
- M12 x 150mm
The designation gives the nominal diameter in mm followed by the length in mm. The coarse metric thread standard on coach screws is compatible with nuts and threaded holes.
In some countries coach screws may use inch sizes like 1/4″, 3/8″, and 1/2″ diameter. Lengths from 1″ to 6″ cover many applications.
Most coach screws conform to international standards like DIN 571. This defines dimensions like minimum and maximum shank diameter, head thickness, and width across the flats. Standardized sizing allows interchangeability across brands.
Materials Used for Coach Screws
Steel is the most common material used to manufacture coach screws. Steel provides high strength and durability at a reasonable cost. However, plain steel screws corrode when exposed to outdoor conditions over time.
Various coatings and treatments help protect steel coach screws:
- Zinc plating provides corrosion resistance for indoor applications. The shiny silver finish has a mild cosmetic appeal as well.
- Hot-dip galvanizing coats the steel with a thick layer of zinc. The dull gray finish is maintenance-free for up to 20 years in outdoor applications.
- Stainless steel contains chromium to provide excellent corrosion resistance, even in marine environments. The 300 series grades used for coach screws maintain their smooth silver appearance indefinitely.
Brass coach screws are also available for applications where non-magnetic, non-sparking properties are needed. However, brass is weaker than steel and costs substantially more.
Uses, Applications, and Advantages of Coach Screws
The holding strength of coach screws makes them well-suited for a variety of demanding applications:
Structural Framing Joints
Timber construction relies on coach screws to resist withdrawal forces in critical joints:
- Joist hangers
- Beam hangers
- Truss plates
- Bracket connectors
For example, attaching a deck ledger to the house calls for multiple coach screws set into the band board. The tapered tip and coarse thread resist the pull-out loads better than smooth shank nails or bolts.
Coach screws are the preferred fastener for:
- Attaching deck boards to joists
- Securing stair stringers
- Anchoring decking to ledgers
The resistance to pull out prevents deck boards from popping loose over time. Stainless steel resists corrosion from outdoor exposure.
Fences and Posts
For constructing wooden fences and posts, coach screws provide strength to resist:
- Fence boards pulling away from posts
- Gate hinges ripping out of posts
- Posts pulling out of the ground
Repeated lateral loads from wind, animals, and people eventually weaken nails and cause fences to fail. Coach screws last much longer.
Furniture and Cabinetry
Coach screws excel at providing clamping force to hold furniture parts together. Common applications include:
- Securing table legs
- Joining cabinet frames
- Attaching shelving standards
- Assembling outdoor furniture
Regular wood screws may loosen over time as wood shrinks and shifts. The holding power of coach screws prevents joints from separating.
Railings, Steps, and Landings
Coach screws anchor critical structural parts including:
- Balusters to handrails
- Stringers to deck steps
- Rail posts to landings
Vibration and loads imposed by people can work conventional nails loose over the years. Coach screws maintain a tight grip.
Securing to Masonry and Concrete
While lag shields and masonry bolts are designed to anchor into concrete and stone, coach screws also work when paired with lead anchors sized to match the screw diameter.
- Attaching framing to foundations and slabs
- Securing ledgers to brick and concrete
- Anchoring handrails to mortared block walls
The holding power resists dynamic loads that could eventually compromise friction fit anchor sleeves.
Key Takeaway: Coach screws are the preferred fastener for critical structural joints in wood construction exposed to withdrawal and pull-out forces.
Proper Installation Techniques
While the tapered tip allows coach screws to self-drill into wood, taking some care during installation enhances performance:
Drilling Pilot Holes
- Softwoods like pine only require a pilot hole slightly smaller than the root diameter.
- Hardwoods like oak demand a larger pilot hole up to 75% of the screw’s outer diameter to prevent splitting.
- Sheet goods like plywood and OSB need undersized holes to keep from stripping out.
Setting Embedment Depth
- At least 1.5x the screw diameter provides optimal holding power.
- Keeping screw heads flush or just below the surface allows for concealment.
- Leaving 1/4″ of the shank exposed gives room to apply a locking nut against the underside.
- Washers protect wood surfaces and prevent the heads from counter-sinking.
- On softwoods, a washer spreads the load to help avoid stripping.
- Stacked washers space the coach screw head off the surface when needed.
Applying Anti-Seize Lubricant
- Lubricating the screw shank eases driving and prevents seizing in the hole.
- Anti-seize lubricants like molybdenum grease provide wetting and leave a protective film.
Choosing Compatible Driver Bits
- Power drivers with hex shank bits offer high torque to seat coach screws.
- Hand drivers should have a squared end to fully engage the recess.
- Snug the fit to prevent cam-out damage to screw heads or driver tips.
Coach Screw Strengths Compared to Other Fasteners
Coach screws provide a set of advantages over other methods used to join materials:
Vs. Lag Screws
- Better grip in side grain wood thanks to deeper, coarser threads
- Bore own starter holes for easier installation
- Withstand greater pull-out loads
Vs. Wood Screws
- Stand up to shear loads better with thicker shanks
- Resist loosening from wood shrinkage and settling
- Provide clamping force comparable to bolts
- Hold against withdrawal forces without pulling out
- Won’t work loose over time from wood movement
- Install flush without splitting surface veneers
Vs. Carriage Bolts
- Bore their own pilot holes
- Hold even if the nut loosens
- Install quickly without need for access behind
Vs. Concrete Screws
- Provide comparable pull-out strength in block and stone
- Useful for heavy attachments to mortared materials
- Often less expensive than specialized masonry screws
So whether building a fence, anchoring stairs, or assembling furniture, coach screws have clear fastening advantages over screws, nails, and bolts for critical structural joints.
Table summarizing differences between coach screws and carriage bolts:
|Feature||Coach Screw||Carriage Bolt|
|Head Shape||Hex||Rounded Dome|
|Shank Finish||Threaded||Partially Threaded|
|Used With||None||Nut Required|
|Installation Method||Self-tapping||Requires Drilled Hole|
What types of materials can I use coach screws on?
Coach screws work on most timber species. They also anchor into materials like OSB, plywood, particle board, plastic composites, brick, concrete block, and stone. Stainless steel models resist corrosion in treated and acidic woods like oak, cedar, and redwood.
Do coach screws corrode?
Plain steel coach screws eventually corrode when exposed to moisture. Adding a zinc plating provides moderate outdoor corrosion resistance. For a maintenance-free finish, opt for stainless steel coach screws. Grade 18-8 works for most climates while grade 316 has extra resistance for ocean environments.
What size pilot hole do I drill for coach screws?
For softwoods, drill a pilot hole around 75% the diameter of the screw’s shank. For hardwoods, go with a 50% diameter hole. These sizes prevent splitting while allowing the threads to grip. Reduce pilot holes slightly in manmade boards to keep them from stripping out.
How do I stop coach screws from pulling out?
Getting full embedment provides optimal resistance to pull-out. For extra holding power, consider adding a nut tightened against the insertion surface or using extra washers to span the joint. For fences and decks, installing screws at opposing angles rather than parallel increases lateral stiffness.
What is the best way to remove a stuck coach screw?
First, spray penetrating oil around the screw head and let it soak in overnight. Then, try an impact driver to loosen the screw. Hold it flush to the head to prevent cam-out stripping. As a last resort, cut the head off with an angle grinder and push the shaft through.
The unique design of coach screws makes them a versatile fastening option for construction, woodworking, and other demanding applications.
Understanding the features and advantages of coach screws allows selecting the optimal type and size for the job. Following best practices for creating pilot holes and setting proper embedment depth ensures installed coach screws perform as designed. The result is secure, long-lasting fastening in wood construction, furniture joints, and other key applications.