Carriage Bolts vs Lag Bolts: Sizes, Pilot Holes & Uses

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.

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.

The Square Neck: Why a Carriage Bolt Does Not Spin

Everything that makes a carriage bolt different from an ordinary bolt comes down to the short square section directly under the domed head. When the nut is tightened from the far side, that square is pulled down into the wood and cuts its own seat. Once seated, the bolt cannot rotate — which is the whole point, because the round head gives a wrench nothing to grip.

This is why a carriage bolt is a genuinely one-person fastener. You are not holding the head with one wrench and turning the nut with another; you tighten the nut and the wood itself holds the bolt still.

Three consequences follow from that, and all three catch people out:

• The hole must be a snug fit. Drill the through hole at the bolt diameter, not oversized. If the hole is loose, the square neck has nothing solid to bite and the bolt spins the moment you put a wrench on the nut.
• No washer under the head. A flat washer under a carriage bolt head holds the square neck off the wood and stops it seating — the exact failure it is designed to prevent. The washer belongs on the nut end only.
• It needs wood, or a matching square hole. Through steel plate or thin metal hardware, there is nothing for the square to embed into. In that situation a hex bolt is the correct fastener, not a carriage bolt.

Seating the square is a deliberate step, not something that happens on its own: after pushing the bolt through, tap the head with a hammer until the shoulder pulls flush with the surface. In dense or dry hardwood it may need several firm strikes.

Head-to-Head Comparison

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.

Hot-dip galvanized is the requirement outdoors and in pressure-treated lumber, and washers matter more than people expect — the head will crush into softwood without one. This kit ships both, and the listing covers 3/8" and 1/2" in several lengths.

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.

Both kits include the nuts and washers, which matters because a carriage bolt is useless without a nut and the washer belongs on that end. Pick the diameter by load: 1/2" for beam-to-post and railing posts, 3/8" for gates and lighter framing.

Pilot Hole Requirements

A lag bolt needs two different holes, not one. Skipping the second is the most common lag bolt mistake.

The rule behind the numbers: the clearance hole matches the shank diameter exactly, so the smooth upper section passes through the top board without binding — that is what pulls the two members tight. The lead hole goes into the receiving member only, and must be smaller than the thread root diameter so the threads have solid fibre to cut into. Per NDS, the lead hole runs 40–70% of shank diameter in softwood (specific gravity 0.5 or less) and 65–85% in dense hardwood (specific gravity above 0.6). Denser wood needs the larger hole, or the lag snaps before it seats.

Carriage bolts are simpler: one hole, straight through both members, matching the bolt diameter. The square neck under the head is what stops the bolt spinning — it needs the hole to be a snug fit, not oversized, so the square can bite into the wood as the nut is tightened.

How to Install a Lag Bolt

1. Clamp or support the members in final position. A lag bolt pulls two pieces together but does not align them — anything out of position when you start stays out of position.
2. Drill the clearance hole through the top member only, at the full shank diameter, so the unthreaded shank passes freely.
3. Drill the lead hole into the receiving member, using the size from the table above, to at least the depth the threads will reach. Clear the chips by pulling the bit out once or twice.
4. Fit a washer under the head. The head will crush into softwood without one, and the lost clamping force never comes back.
5. Turn it in with a wrench or socket — never drive it with a hammer. You should feel steady, significant resistance the whole way.
6. Stop when the washer is snug against the wood. Continuing past that point strips the threads out of the wood, and a stripped lag hole cannot be repaired in place.

Bar soap or wax on the threads makes driving much easier in dense material and does not reduce the connection’s design value.

How to Install a Carriage Bolt

1. Drill one hole straight through both members at the bolt diameter. Keep it snug and keep it square to the face.
2. Push the bolt through from the finished side — the side the round head should show on.
3. Tap the head with a hammer until the square neck pulls fully into the wood and the head sits flush. This is the step people skip.
4. Add a washer, then the nut, on the back side. The washer spreads the load and stops the nut sinking into the wood.
5. Tighten the nut only. If the bolt starts turning with the nut, the square neck is not seated — back off, tap the head again, and continue.

How to Remove a Carriage Bolt

Removal is where the design works against you: there is no head to grip. Work through these in order.

1. Try the nut alone first. If the square neck is still seated, the nut backs off normally and the bolt taps out from behind.
2. If the bolt spins with the nut, the square neck has stripped its seat — common in weathered or rotted wood. Pry the head outward with a flat bar while turning the nut; pulling the square back into fresh wood is often enough to hold it.
3. Grip the head with locking pliers. Clamped hard onto the dome, they will usually hold long enough to break the nut free. This marks the head permanently.
4. Cut the nut off with a grinder or reciprocating saw with a metal blade if it is seized with rust, then drive the bolt out from the nut side.
5. Cut the bolt itself between the members as a last resort, using a metal-cutting reciprocating blade in the joint line.

Carriage Bolt vs Hex Bolt vs Through Bolt

These three overlap enough to cause real confusion, and “through bolt” is not a separate product at all — it describes any bolt passing fully through both members with a nut on the far side. Both carriage and hex bolts can be through bolts.

The practical split: use a carriage bolt when the head faces somewhere people or clothing pass, and when you want a one-wrench job. Use a hex bolt when either member is steel, when you need a strength grade above A307, or when the head will be inaccessible once assembled.

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.

Grades and Strength: What You Can Actually Buy

One fact narrows most of this decision: carriage bolts are effectively only made in ASTM A307 Grade A, a low-carbon steel with a 60,000 psi minimum tensile strength. Grade 5 and Grade 8 carriage bolts are not a normal stocked item. If a connection needs a higher-strength fastener, the answer is a hex bolt or a structural screw — not a stronger carriage bolt, because there generally is not one.

Lag screws sit in a similar place: commodity lag screws are low-carbon steel to A307 or SAE J429 Grade 1. They are not high-strength fasteners, and their real limit is usually the wood rather than the steel — withdrawal capacity depends on thread engagement and wood density, so the same lag holds substantially less in SPF framing than in denser Southern Yellow Pine.

For connections where strength is the governing factor rather than convenience, modern structural screws carry published ICC-ES evaluation report values, which neither commodity lag screws nor carriage bolts do. See structural screws vs lag bolts for that comparison, and bolt grades explained for what the grade markings mean.

Common Mistakes

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
• Screw Size Chart — Complete sizing for every screw type, including lag screws
• Bolt and Wrench Size Chart — Wrench size for every bolt, SAE and metric

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. For the full explanation of why the names differ and what the standards specify, see lag bolt vs lag screw.

Do carriage bolts need washers?

Yes — on the nut end. A washer there spreads the clamping load and stops the nut pulling into the wood. Do not put one under the head: it prevents the square neck from seating, which is what stops the bolt spinning.

How do you keep a carriage bolt from spinning?

Seat the square neck properly. Drill the hole at bolt diameter rather than oversized, then tap the head with a hammer until it sits flush before touching the nut. If it still spins, the seat has stripped — pry the head outward with a flat bar while turning the nut.

What size hole do you drill for a carriage bolt?

The same diameter as the bolt, straight through both members. A 1/2" carriage bolt takes a 1/2" hole. Unlike a lag bolt there is no separate pilot — and unlike a lag bolt, oversizing it stops the fastener working.

Are carriage bolts stronger than lag bolts?

In shear, generally yes: the full bolt diameter crosses the joint line and the nut clamps both members. A lag bolt relies on thread grip in the receiving member, so its capacity depends on wood density and thread engagement. The trade-off is access — a carriage bolt needs both sides.

What grade are carriage bolts?

Almost always ASTM A307 Grade A, a low-carbon steel rated to 60,000 psi minimum tensile. Grade 5 and Grade 8 carriage bolts are not commonly stocked, so if you need a higher grade the answer is a hex bolt rather than a stronger carriage bolt.