When should I use carriage bolts vs lag bolts?

Use carriage bolts when you can access both sides for through-bolting. Use lag bolts when you can only access one side. Carriage bolts are generally stronger.

Drill the hole to match the bolt diameter exactly.

Carriage Bolts

Hardware & Fasteners

Bolts

Budget: $0.20-$0.75 each (zinc-plated small) | Mid-range: $0.75-$2 each (galvanized medium) | Premium: $2-$6 each (stainless or large)

Carriage Bolts: Smooth Dome-Head Bolts for Through-Fastening (2026)

Prepared by Lumberama's Building Materials Team

|Last updated May 25, 2026|Sources & methodology

Carriage bolts feature a smooth, rounded dome head with a square shoulder that locks into wood to prevent spinning during tightening. They pass through pre-drilled holes and secure with a nut and washer on the back side. Ideal for applications where a smooth, tamper-resistant head is desired.

Key Features

Smooth dome head for safety and aesthetics; Square shoulder prevents spinning in wood; Through-bolt for maximum strength; Zinc
galvanized
and stainless options; Sizes from 1/4" to 3/4" diameter

Specifications

Diameters: 1/4"

5/16"

3/8"

1/2"

5/8"

3/4" | Lengths: 1" to 12" | Material: Grade 2 steel

hot-dipped galvanized

18-8 stainless | Head: dome with square neck | Thread: UNC coarse

Applications

Deck railing posts; Fence gate hardware; Playground equipment; Dock and pier construction; Wood-to-wood through-bolting

Installation Overview

Drill through-hole matching bolt diameter. Insert carriage bolt from finished side. Place washer and nut on opposite side. Tighten nut with wrench while square shoulder grips wood to prevent spinning.

Pros & Cons

Pros

✓ Smooth head is safe and attractive; Square shoulder prevents spinning; Through-bolt provides maximum strength; Tamper-resistant from finished side

Cons

✗ Require through-hole (no blind fastening); Nut must be accessible from back; Square shoulder can split thin wood; Slower installation than screws

Top Brands

Hillman

Everbilt

Portland Bolt

Code Requirements

Must meet ASTM A307 Grade A for structural applications. Galvanized required for treated lumber. Washer required under nut. Bolt length must provide minimum thread engagement in nut.

Buying Tips

Buy carriage bolts with matching nuts and washers; Galvanized is essential for any outdoor or treated lumber application; Slightly countersink the dome head for a flush finish on railings