What size pilot hole for lag bolts?

Generally 60-75% of the bolt shank diameter. For a 3/8" lag bolt, drill a 1/4" pilot hole. Check manufacturer tables for exact sizing.

Yes, always use a flat washer under the lag bolt head to distribute load and prevent wood crushing.

Lag BoltsGenerating image…

Hardware & Fasteners

Bolts

Budget: $0.30-$1 each (zinc-plated 1/4"-5/16") | Mid-range: $1-$3 each (galvanized 3/8"-1/2") | Premium: $3-$8 each (stainless or large diameter)

Lag Bolts: Heavy-Duty Lag Screws for Structural Wood Connections (2026)

Prepared by Lumberama's Building Materials Team

|Last updated May 25, 2026|Sources & methodology

Lag bolts (lag screws) are heavy-duty fasteners with a hex head and coarse wood-thread shank, used for high-strength wood-to-wood and wood-to-metal connections. They require a pre-drilled pilot hole and are driven with a socket wrench. Common in deck ledger boards, structural framing, and heavy equipment mounting.

Key Features

Hex head for wrench driving; Heavy coarse threads for wood; Sizes from 1/4" to 3/4" diameter; Galvanized and stainless options; High shear and withdrawal strength

Specifications

Diameters: 1/4"

5/16"

3/8"

1/2"

5/8"

3/4" | Lengths: 1" to 12" | Material: zinc-plated steel

hot-dipped galvanized

stainless | Head: hex | Thread: gimlet point

coarse

Applications

Deck ledger board attachment; Structural beam connections; Heavy equipment mounting to wood; Playground equipment assembly; Post-to-beam connections

Installation Overview

Drill pilot hole sized per bolt diameter (typically 60-75% of shank diameter). Add washer under head. Drive lag bolt with socket wrench or impact driver with socket adapter. Do not over-tighten to avoid crushing wood fibers.

Pros & Cons

Pros

✓ Very high strength connections; Hex head allows maximum torque; Available in large sizes; Proven structural fastener

Cons

✗ Require pre-drilled pilot holes; Can split wood if too large; Slower to install than screws; Heavy and expensive in bulk

Top Brands

Hillman

Grip-Rite

Simpson Strong-Tie

Code Requirements

IRC Table R507.9.1.3 specifies lag screw requirements for deck ledger connections. Pilot holes required per NDS. Hot-dipped galvanized required for treated lumber. Washer required under head.

Buying Tips

Always pre-drill pilot holes to prevent splitting and ensure full thread engagement; Use flat washers under every lag bolt head; Hot-dipped galvanized is required for any treated lumber or exterior application