What is the difference between a lag bolt and a carriage bolt?

A lag bolt is a large wood screw with a hex head driven with a wrench - it grips wood fibers and only requires access from one side. A carriage bolt passes completely through both wood members and is secured with a washer and nut on the far side - requiring access to both sides but providing higher shear strength. Lag bolts are for one-sided access; carriage bolts are for through-connections.

What size lag bolts for deck ledger attachment?

IRC R507.9 requires minimum 1/2 inch diameter lag bolts for deck ledger connections with minimum 1.5 inch penetration into the band joist or rim joist. A typical installation through a 2x ledger into a 2x rim joist requires a minimum 3 inch lag bolt - most installers use 3.5 to 4 inches for additional margin.

What coating do I need for outdoor lag bolts and carriage bolts?

For outdoor and pressure-treated lumber (ACQ, CA) use hot-dipped galvanized (HDG) or stainless steel. ACQ treatment is highly corrosive to standard zinc-plated fasteners which will fail within 1-3 years outdoors. For coastal and marine environments within 1 mile of saltwater use Type 316 stainless steel.

Lag Bolts vs Carriage Bolts: Which One Do You Need?

Q: Can I use carriage bolts instead of lag bolts for a deck ledger?

Only if you have access to both sides of the rim joist which is rarely possible on an existing house. IRC R507.9 permits both lag screws and bolts for ledger connections. Lag bolts are standard because they only require access from the deck side. If the rim joist is accessible from a crawl space or basement through-bolts are structurally superior.

Lag bolts and carriage bolts are both large-diameter fasteners used in wood construction — but they work completely differently and are not interchangeable. Using the wrong one costs you holding strength, code compliance, or both. Here is exactly which one to use and when.

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The Screw Size Selector gives you the exact gauge, length, pilot hole, and coating for any fastening application — free, instant.

What Is a Lag Bolt?

A lag bolt (also called a lag screw) is a large wood screw with a hex head driven by a wrench rather than a screwdriver. It has coarse wood screw threads that cut into and grip wood fibers — the same mechanism as a standard wood screw, just much larger. Lag bolts are self-threading: they create their own threads as they're driven into a pre-drilled pilot hole.

Typical sizes: 1/4" to 3/4" diameter, 1" to 16" length. The hex head is driven with a socket wrench or impact driver with a hex socket.

What Is a Carriage Bolt?

A carriage bolt has a round, smooth head with a small square section beneath it. The square section embeds into the wood as the nut is tightened on the opposite side, preventing the bolt from spinning. Unlike lag bolts, carriage bolts pass completely through the wood members and are secured with a washer and nut on the far side.

Typical sizes: 3/16" to 3/4" diameter, 3/4" to 20" length. Installation requires access to both sides of the connection.

Head-to-Head Comparison

Feature	Lag Bolt	Carriage Bolt
How it works	Wood screw threads grip fibers	Bolt passes through, nut on back
Access needed	One side only	Both sides required
Head type	Hex head — driven with wrench	Round head — tighten nut only
Hole requirement	Pilot hole slightly smaller than shank	Clearance hole same diameter as bolt
Shear strength	Good	Higher — full bolt diameter in shear
Tension strength	Good — threads resist pullout	Excellent — nut and washer bear on full face
Aesthetics	Hex head visible	Smooth round head — cleaner look
Best for	One-sided access, ledger boards	Two-member connections, railings

When to Use Lag Bolts

Ledger boards to rim joists: Attaching a deck ledger to the house rim joist — one of the most common lag bolt applications. IRC R507.9 prescribes 1/2" lag bolt diameter and spacing for this connection.
One-sided access: Any connection where the back side is inaccessible — attaching to an existing wall, connecting to a beam that can't be reached from behind.
Structural screws alternative: Modern structural screws (GRK RSS, Simpson SDS) are often a better choice than lag bolts — they drive faster, require no pre-drilling, and have ICC-ES ESR load ratings. See our guide to structural screws vs lag bolts.
Hardware attachments: Attaching post bases, joist hangers, and other metal connectors to wood where a through-bolt is not practical.

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When to Use Carriage Bolts

Railing posts to deck framing: The round head sits flush against the post face; tightening the nut from inside the framing pulls the connection tight. Standard for deck railing post attachment.
Beam-to-post connections: Connecting a beam to the side of a post — pass the bolt through both members and tighten a nut and washer on the far side.
Gate construction: Fence gate hinges and hardware typically use carriage bolts through the gate frame for a clean look and high shear resistance.
Playground and outdoor structures: The smooth round head and anti-spin square neck make carriage bolts ideal where people contact the fastener — no sharp hex head to catch on.

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Pilot Hole Requirements

Fastener	Size	Pilot Hole (Softwood)	Pilot Hole (Hardwood)
Lag Bolt	1/4"	3/16"	7/32"
Lag Bolt	3/8"	5/16"	11/32"
Lag Bolt	1/2"	7/16"	15/32"
Carriage Bolt	1/4"	1/4" clearance	1/4" clearance
Carriage Bolt	3/8"	3/8" clearance	3/8" clearance
Carriage Bolt	1/2"	1/2" clearance	1/2" clearance

For lag bolts, the pilot hole is drilled smaller than the bolt diameter so the threads can bite. For carriage bolts, the hole is the same diameter as the bolt for a snug fit.

Corrosion and Coating Requirements

For outdoor use and pressure-treated lumber, the coating requirement is critical:

Pressure-treated lumber (ACQ, CA): Use hot-dipped galvanized (HDG) or stainless steel. ACQ treatment is highly corrosive to standard zinc-plated fasteners — they will fail within 1-3 years outdoors.
Coastal and marine environments: Use Type 316 stainless steel. Hot-dipped galvanized is insufficient within 1 mile of saltwater.
Interior dry applications: Standard zinc-plated lag bolts and carriage bolts are adequate.

Related Guides

Structural Screws vs Lag Bolts — When modern structural screws are better
Joist Hangers vs Toenailing — Load ratings and code requirements
Deck Ledger Attachment — Lag bolt spacing and code requirements
Screw Size Selector — Find the right fastener for any application

Frequently Asked Questions

Can I use carriage bolts instead of lag bolts for a deck ledger?

Only if you have access to both sides of the rim joist — which is rarely possible on an existing house. IRC R507.9 permits both lag screws and bolts for ledger connections. Lag bolts are standard because they only require access from the deck side. If the rim joist is accessible from inside a crawl space or basement, carriage bolts or through bolts are a structurally superior option.

Do lag bolts need washers?

Yes — always use a washer under the lag bolt head. Without a washer, the hex head can pull through the wood under load or when wood shrinks and the connection loosens over time. Use a hardened washer sized to match the bolt diameter.

How long should lag bolts be for deck ledger attachment?

IRC R507.9 requires lag bolt penetration into the band joist or rim joist to be at least 1.5 inches for 1/2" diameter lags. A typical installation through a 2x ledger (1.5" actual) into a 2x rim joist (1.5" actual) requires a minimum 3" lag bolt — most installers use 3.5" to 4" for additional margin.

What is the difference between a lag bolt and a lag screw?

There is no difference — lag bolt and lag screw refer to the same fastener. The term lag bolt is more commonly used in the US; lag screw is technically more accurate since it has wood screw threads rather than machine bolt threads. Both terms refer to the hex-head large wood fastener described in this guide.