Lag Bolts

Lag Screws: Heavy-Duty Fasteners for Structural Wood Connections

Lag screws—also commonly called lag bolts—are large-diameter wood fasteners designed for heavy-duty structural connections where exceptional holding power is required. Despite the interchangeable terminology, "lag screw" is the technically correct designation since these fasteners create their own mating threads in wood rather than passing through pre-threaded holes like true bolts. Manufactured to ASME B18.2.1 specifications, lag screws deliver the reliable performance that demanding applications require.

Lag Screw vs Lag Bolt: Understanding the Terminology

The terms "lag screw" and "lag bolt" are used interchangeably throughout the construction industry, though fastener engineers prefer "lag screw" as the accurate designation. The confusion arises because lag screws feature a bolt-style hex head but function as screws—they tap their own threads into wood rather than engaging pre-formed threads like conventional bolts. The hex head enables high-torque installation with wrenches or socket drivers, providing the mechanical advantage needed to drive these substantial fasteners into dense lumber.

This hybrid nature—combining the drive capability of a bolt head with the thread-forming function of a screw—makes lag screws uniquely suited for structural wood connections where through-bolting is impractical or unnecessary. They excel in applications requiring strong single-sided fastening into solid wood or engineered lumber products.

Hex Head vs Square Head Lag Screws

Lag screws are available with two distinct head styles: hex head and square head. Hex head lag screws dominate modern construction due to compatibility with standard socket sets and wrenches. The six-sided design provides multiple engagement points, reducing the risk of wrench slippage during high-torque installation.

Square head lag screws represent the traditional design, still specified for restoration projects, timber frame construction, and applications where period-correct hardware is required. The four-sided head offers excellent grip with adjustable wrenches and provides a distinctive aesthetic favored in exposed fastener applications. Both head styles meet identical strength requirements under ASME B18.2.1.

Gimlet Point Design for Efficient Installation

Quality lag screws feature a gimlet point—a sharp, self-centering tip that initiates penetration into wood fibers without the wandering common to blunt-point fasteners. This conical point design guides the screw into pilot holes accurately, reduces installation effort, and minimizes wood splitting by progressively displacing fibers rather than wedging them apart abruptly.

The gimlet point works in conjunction with the screw's coarse thread pitch to pull the fastener into the wood efficiently. This self-feeding action reduces the driving torque required and helps ensure consistent seating depth across multiple fasteners in the same connection.

Pilot Hole Requirements: Critical for Performance

Proper pilot holes are essential for achieving rated load capacity with lag screws. Unlike self-drilling screws designed for thin materials, lag screws require pre-drilled holes sized to match the fastener diameter and wood species. The pilot hole should equal approximately 60-75% of the shank diameter for softwoods and 75-85% for hardwoods.

For the threaded portion, pilot hole diameter should match the root diameter of the threads. A properly sized pilot hole allows threads to cut cleanly into surrounding wood, maximizing the shear area that resists withdrawal forces. Undersized holes increase driving torque and risk splitting; oversized holes compromise thread engagement and reduce holding power significantly.

In critical structural connections, many building codes and engineering specifications require two-stage pilot holes: a smaller diameter through the threaded zone and a larger clearance hole through the unthreaded shank portion. This ensures the fastener pulls mating members tightly together while maximizing thread engagement in the base material.

Wood Connection Capacity and Design Values

Lag screw connection capacity depends on multiple factors: fastener diameter, length, thread engagement, wood species, grain orientation, and loading direction. Design values published in the National Design Specification (NDS) for Wood Construction provide allowable loads for common configurations, with adjustments for specific gravity, moisture content, and duration of load.

Withdrawal resistance—the force required to pull the screw from wood along its axis—increases with thread engagement depth and wood density. Lateral load capacity—resistance to forces perpendicular to the fastener axis—depends on fastener diameter and the bearing capacity of connected members. Connections subject to combined loading require analysis considering both failure modes.

For maximum connection strength, lag screws should penetrate the receiving member by a minimum of 4 times the shank diameter, with thread engagement of at least 7 thread pitches beyond the point. Greater penetration increases capacity proportionally up to practical limits.

Hot-Dip Galvanized Lag Screws for Pressure-Treated Lumber

Hot-dip galvanized (HDG) lag screws are mandatory for connections involving pressure-treated lumber, which contains corrosive preservative compounds that rapidly degrade unprotected steel fasteners. The hot-dip galvanizing process creates a thick, metallurgically-bonded zinc coating that provides both barrier protection and galvanic (sacrificial) corrosion resistance.

Current pressure treatment formulas—including ACQ (Alkaline Copper Quaternary), CA (Copper Azole), and MCQ (Micronized Copper Quaternary)—are significantly more corrosive to steel than the older CCA treatments they replaced. Building codes explicitly require HDG or stainless steel fasteners for these applications. Using plain steel or electroplated fasteners in treated lumber voids warranties, violates code requirements, and creates serious structural safety risks as fasteners corrode and lose strength.

For marine environments, coastal exposure, or high-moisture applications, stainless steel lag screws (Type 304 or Type 316) provide superior corrosion resistance beyond what galvanizing offers.

Ledger Board Attachment for Deck Construction

Lag screws are the standard fastener for attaching deck ledger boards to house rim joists—one of the most critical connections in residential deck construction. This connection transfers the full dead and live load of the deck's house-side edge into the building structure. Connection failure at the ledger is the leading cause of deck collapses, making proper fastener selection and installation essential.

The International Residential Code (IRC) specifies minimum lag screw requirements for ledger connections based on deck dimensions and joist spans. Typical specifications call for 1/2" diameter hot-dip galvanized lag screws at 16" on-center spacing, staggered between top and bottom rows. Fasteners must penetrate the band joist by at least 1-1/2" excluding the point.

Best practices include: using washers to distribute bearing loads, applying flashing tape or metal flashing behind ledgers to prevent moisture intrusion, ensuring pilot holes are properly sized, and verifying the rim joist provides adequate material for fastener engagement. Hollow or deteriorated rim joists require alternative connection methods.

ASME B18.2.1 Standards Compliance

All quality lag screws conform to ASME B18.2.1: Square, Hex, Heavy Hex, and Askew Head Bolts and Hex, Heavy Hex, and Askew Head Screws (Inch Series). This standard establishes dimensional tolerances, head geometry, thread specifications, and marking requirements that ensure interchangeability and predictable performance across manufacturers.

Specifying ASME-compliant lag screws guarantees the fasteners meet established engineering assumptions for load calculations and will interface correctly with standard tools and accessories. When structural capacity matters, always verify fasteners meet applicable ASTM material standards (A307 for carbon steel) in addition to ASME dimensional specifications.

Selecting the Right Lag Screw

Proper lag screw selection requires matching fastener properties to application demands: diameter and length for required load capacity, material and finish for environmental exposure, and head style for installation method and aesthetics. Our inventory includes the full range of sizes, materials, and finishes needed for residential, commercial, and industrial wood construction projects.