Lag Bolts: Heavy-Duty Wood Fasteners

Wood construction demands fasteners engineered for timber's unique properties. Unlike steel or concrete, wood compresses under load, grips threads through fiber interlock, and responds to moisture and age in ways that affect joint performance over time.

This guide covers the essential fasteners for structural wood connections—from lag bolts to carriage bolts to modern construction screws.

Lag Bolts: Heavy-Duty Wood Fasteners

Lag bolts (also called lag screws) are large-diameter fasteners with coarse threads designed to develop high withdrawal resistance in wood. They're the workhorse fastener for structural wood-to-wood and wood-to-steel connections.

When to Use Lag Bolts

• Ledger board attachment to framing
• Deck post-to-beam connections
• Heavy equipment mounting to wood structures
• Steel connector plates to wood
• Any application requiring high withdrawal or shear capacity

Pilot Holes Are Mandatory

Never drive lag bolts without pilot holes. This isn't optional—it's essential for proper performance:

Shank pilot hole: Drill the full diameter of the lag bolt shank (unthreaded portion) through the top member. This allows the bolt to pull members tightly together.

Thread pilot hole: Drill approximately 2/3 to 3/4 of the thread diameter into the receiving member. This reduces driving torque while maintaining thread engagement.

Why Pilot Holes Matter

Without proper pilot holes:

• Wood splits, reducing holding capacity
• Driving torque may exceed lag bolt strength
• Threads strip rather than engage
• Joint fails to pull tight
• Head breaks off during installation

Installation Best Practices

• Use a socket or wrench—do not impact without control
• Lubricate with wax or soap for easier driving (don't use oil—attracts dirt)
• Stop when head seats firmly—over-torquing crushes wood
• Install washers under the head to prevent pull-through
• Countersink heads only where specified (reduces capacity)

Carriage Bolts: Self-Securing Through-Bolts

Carriage bolts feature a smooth, dome head and a square shoulder beneath that pulls into the wood, preventing the bolt from spinning during nut tightening.

How Carriage Bolts Work

1. Insert bolt through a round hole (square shoulder seats against wood)

2. As you tighten the nut, the square shoulder digs into wood

3. The bolt cannot rotate—no need to hold the head

4. Result: a smooth, tamper-resistant head on the exposed side

Ideal Applications

• Playground equipment (no exposed hex heads to injure children)
• Deck railings and guard posts
• Exposed timber construction where appearance matters
• Connections accessible from one side only
• Applications requiring tamper resistance

Sizing and Installation

Hole size: Drill the same diameter as the bolt—the square shoulder needs a snug fit to prevent spinning.

Length selection: Through both members plus washer plus nut, with 1-3 threads protruding.

Washer requirement: Always use a flat washer under the nut to prevent wood crushing and nut embedment.

Timber Bolts: Designed for Large Lumber

Timber bolts are specialized fasteners for heavy timber connections, featuring:

• Dome or button head (low profile, no sharp edges)
• Integral washer under the head
• Coarse threads for wood engagement
• Often available in hot-dip galvanized for outdoor use

When to Choose Timber Bolts

• Post and beam construction
• Timber frame connections
• Heavy guardrails and bollards
• Bridge timbers and marine pilings
• Large landscape structures

Timber bolts provide the head profile advantages of carriage bolts with better performance in large-diameter timber applications.

Wood Screws vs. Construction Screws

Modern construction screws have largely replaced traditional wood screws for structural applications. Understanding the differences helps you select the right fastener.

Traditional Wood Screws

Design features:

• Tapered shank (thicker at head, narrower at tip)
• Threads cover about 2/3 of the shank
• Slotted or Phillips drive (easy cam-out)
• Typically available in smaller sizes

Best uses:

• Finish carpentry
• Hardware attachment
• Non-structural applications
• Where traditional appearance matters

Modern Construction Screws

Design features:

• Consistent shank diameter
• Threads often extend full length or have "thread cutting" design
• Star drive (T25, T30) or proprietary drives resist cam-out
• Self-drilling tips eliminate pilot holes in many applications
• Engineered coatings for treated lumber compatibility

Advantages:

• Much faster installation
• Higher withdrawal strength per diameter
• Better torque transfer (star drive)
• Load-rated designs available
• Variety of specialized types (structural, deck, cabinet)

Best uses:

• Deck construction
• Subfloor attachment
• Sheathing installation
• General framing where approved
• Any application valuing speed and holding power

Structural Screws

A subset of construction screws, structural screws are engineered and load-rated for specific connections:

• Tested and published withdrawal values
• Shear and tension capacities listed
• May substitute for lag bolts per manufacturer tables
• Require no pilot holes in most applications
• Examples: Simpson SDS, GRK RSS, SPAX PowerLag

Code acceptance: Structural screws with ICC-ES evaluation reports can substitute for lag bolts where load tables permit.

Withdrawal Strength: What Holds the Screw in Place

Withdrawal strength—resistance to pulling straight out—determines fastener capacity in many wood connections.

Factors Affecting Withdrawal

Wood species: Denser woods provide higher withdrawal resistance. Oak and maple grip better than pine and spruce.

Moisture content: Wet wood holds poorly. Fasteners installed in wet wood may loosen as it dries.

Grain direction: Fasteners perpendicular to grain (into face or edge) hold better than those parallel to grain (into end grain). End grain connections are inherently weak.

Thread engagement: More thread length in the receiving member increases capacity—up to a point.

Pilot hole size: Oversized pilot holes reduce withdrawal. Follow manufacturer recommendations.

Improving Withdrawal Performance

• Increase fastener diameter
• Increase embedment depth
• Use coarse-thread designs
• Avoid end grain connections when possible
• Apply adhesive in critical applications
• Use larger or more fasteners

End Grain Connections

Fasteners into end grain provide only 50-75% of side grain capacity. For structural end grain connections:

• Use larger fasteners than side grain calculations suggest
• Consider through-bolts instead of screws
• Use metal connectors designed for end grain
• Consult engineering tables for specific values

Ledger Connections: Where Decks Meet Houses

Ledger connections are among the most critical and failure-prone joints in residential construction. These fasteners attach deck ledgers to house framing and must transfer full deck loads.

Required Fastener Capacity

Ledger connections must resist:

• Dead load: Weight of the deck structure itself
• Live load: People, furniture, snow
• Lateral forces: Wind, seismic, impact

Approved Fastener Options

Lag bolts: Traditional method—1/2" diameter HDG or stainless, with specific patterns per IRC Table R507.9.1.3.

Structural screws: Approved products per manufacturer tables may substitute for lag bolts.

Through-bolts: When accessible from inside, through-bolts with washers and nuts provide excellent capacity.

Critical Installation Requirements

• Fasteners must penetrate solid rim joist or blocking—not just sheathing
• Stagger fasteners vertically to prevent splitting
• Maintain edge distance (typically 2" minimum)
• Flash above ledger to prevent water intrusion
• Use spacers if required by code for drainage

What Ledger Fasteners Must NOT Do

• Bear on band board or rim board alone (without solid backing)
• Penetrate only into sheathing or siding
• Rely on nails or small screws for structural capacity
• Skip required washers or use undersized diameters

Wood Fastener Selection Summary

Final Recommendations

1. Drill pilot holes for lag bolts—always, no exceptions

2. Match fastener coating to treated lumber—ACQ requires HDG or stainless

3. Never rely on end grain alone—use connectors or through-bolts for critical connections

4. Use structural screws where load-rated—they're faster and often stronger

5. Install washers under bolt heads and nuts—prevents pull-through and crushing

6. Follow code requirements for ledger connections—this is where decks fail

7. Consider moisture movement—allow for wood shrinkage and swelling in connection design

Wood construction with proper fasteners creates durable, long-lasting structures. Understanding each fastener type's strengths and limitations ensures your connections perform reliably throughout the building's life.