Thru Bolt: Complete Guide to Types, Sizes, and Installation (2026)

A thru bolt is a fastener that passes completely through the materials being joined and is secured with a nut on the opposite end — providing a clamping force that distributes load across both faces of the connection.

If you’ve spent any time in structural construction, bike maintenance, or door hardware installation, you’ve encountered a thru bolt — even if you didn’t know it by name. The term covers a surprisingly wide range of fasteners unified by one mechanical principle: the bolt goes all the way through, and a nut does the work on the back side.

That sounds simple. The complications start when you need to pick the right diameter, grade, length, and coating for a specific load — and when you need to know whether a thru bolt is actually the right fastener for your application in the first place. This guide walks through all of it, starting from the basics and ending with a decision framework you can use on a real job.

What Is a Thru Bolt?

A thru bolt passes completely through all materials in a connection — wood, steel, concrete formwork, or any combination — and is tightened from the exposed end using a nut and typically one or more washers. This is the defining difference from a lag bolt or screw anchor, where the fastener threads directly into the base material rather than passing through it.

The mechanical advantage is load distribution. When you tighten a thru bolt, the clamping force is spread over the washer bearing area on both the bolt head side and the nut side. A lag bolt, by contrast, concentrates withdrawal resistance entirely on the threads embedded in wood or masonry, which is why thru bolts are required by code in many structural wood connections.

Thru Bolt vs. Through Bolt — Same Thing?

Yes, completely. “Thru bolt” is the shorthand spelling used in hardware catalogs, building product specs (notably FastenMaster’s ThruLOK line), and most U.S. trade literature. “Through bolt” is the more formal engineering term you’ll find in ASTM bolt standards and structural engineering specifications. They refer to the identical fastener concept. Throughout this guide we use both interchangeably — just as the industry does.

How a Thru Bolt Works (Load Path Explained)

When a thru bolt is properly installed and torqued:

1. Clamping force is generated by pre-tension in the bolt shank — the same principle as any bolted joint.
2. Shear loads (forces perpendicular to the bolt axis) are transferred by bearing between the bolt shank and the connected materials.
3. Tension loads (forces pulling the joint apart) are resisted by the nut and washer assembly on the through side.

The washer is not optional. Without it, the nut or bolt head can pull into soft materials (especially Douglas fir or SPF lumber) under load, reducing the effective bearing area and weakening the connection. According to the American Wood Council’s National Design Specification (NDS), washers are mandatory on both sides of a bolt connection in wood framing unless the connected member is metal.

Table 1 — Thru Bolt vs. Similar Fasteners

Types of Thru Bolts

Not all thru bolts are the same fastener dressed differently — the head geometry, thread engagement, material spec, and use case vary meaningfully by application.

Structural Thru Bolts for Wood Construction

The most common thru bolt in residential and light commercial construction is a hex head machine bolt passing through lumber members with a standard hex nut and 3-inch round washer (or 3×3 square plate washer). The National Design Specification governs the engineering of these connections: bolt diameter, species-adjusted bearing values, end and edge distances, and minimum embedment when a lag bolt substitutes.

Common diameters in wood framing: ½ inch (13 mm), ⅝ inch (16 mm), and ¾ inch (19 mm). The ½-inch thru bolt is the workhorse of deck ledger and beam connections; ⅝-inch and larger appear in girder trusses, post bases with tension loads, and multi-ply LVL or PSL beams.

FastenMaster’s ThruLOK is a branded structural thru bolt system worth mentioning: it uses a patented asymmetric thread pattern that engages wood fibers similarly to a lag bolt, but with a full hex head and no need for a nut — the threaded end grips the far-side wood member. It’s technically a hybrid, but it’s marketed and specified as a thru bolt replacement in many deck and timber frame codes.

Concrete Anchor Thru Bolts

In concrete and masonry applications, “thru bolt” often describes a sleeve anchor or through-bolt wedge anchor that expands inside a pre-drilled hole. The bolt passes through the fixture into the concrete, and the wedge or sleeve mechanism locks it in place as the nut is tightened. These are fundamentally different from wood-framing thru bolts — they use expansion rather than clamping — but the name sticks.

We’ve seen a lot of confusion on jobsites between wedge anchors and through-bolts in wood. They look similar from the exposed-hex-head end. The tell: a concrete anchor typically has a partial thread and visible expansion sleeve near the tip; a wood thru bolt is fully threaded or has a smooth shank with threads only at the nut end.

Bicycle Thru Axle Bolts

A bicycle thru axle (also called a thru bolt axle) is a hollow bolt that passes through the fork dropout or rear dropout and through the wheel hub, replacing the traditional quick-release skewer. The diameter is standardized by the fork manufacturer — common sizes are 12×100mm (front), 12×142mm (rear), 15×100mm (front), and 20×110mm (front on downhill/enduro).

The advantage over a quick release: the thru axle eliminates lateral flex in the dropout, which improves braking consistency and eliminates wheel-walk under hard braking. Shimano’s technical documentation on thru axle compatibility is authoritative on spec interoperability — you generally cannot swap a 12mm thru axle fork for a 15mm without replacing the hub or using an adapter.

Door Hardware Thru Bolts

Door thru bolts are barrel bolts or vertical-throw locks mounted by passing a long machine screw through the door stile and engaging a receiver mounted flush on the face. The most familiar form is the surface-mounted patio door bolt — a two-piece set with a bolt body through the door thickness and a keep on the frame.

Security deadbolt thru bolts are a different category: they use a long machine screw (typically M6×80–150mm) that passes through the lock body and tightens against a reinforcing plate on the far face of the door stile, preventing the lock from being pried out under forced entry.

Table 2 — Thru Bolt Types Comparison

Thru Bolt Size Chart and Selection Guide

Getting the size right is where most purchasing errors happen. Oversizing wastes money and may create bearing stress concentrations in soft lumber. Undersizing is a code violation in structural applications and a failure waiting to happen in concrete.

Diameter and Length Dimensions

For wood construction thru bolts, the governing dimensions are:

• Diameter: typically ½″, ⅝″, or ¾″ for framing. The NDS sets minimum bolt diameter as a function of member thickness — generally ½″ is minimum for two-member connections, ⅝″ for three-member connections under significant shear.
• Length: bolt length = sum of all member thicknesses + washer stack (typically ¼″–⅜″ per side) + nut engagement (at least 1× diameter). For a 3-ply 2×10 beam with washers and nut, a ½″×9″ bolt is the standard order.
• Thread length: most structural machine bolts are partially threaded — smooth shank spans the main members, threads only at the nut end. This is important: the smooth shank transfers shear by bearing, not by thread engagement.

For concrete anchor thru bolts, length selection follows the manufacturer’s embedment depth chart. Minimum embedment for a standard ½″ wedge anchor in normal-weight concrete (3,000 psi) is typically 3¼″ — the bolt must be long enough for the fixture thickness plus that embedment plus nut and washer.

Grade and Material Standards

For most residential deck and framing work, SAE Grade 5 or ASTM A307 is specified by code. Grade 8 is overkill for wood — the wood fails before the bolt does in shear. Where you do need Grade 8 or A325 is in steel-to-steel connections and steel moment frames. Per OSHA 29 CFR 1926 Subpart R, structural steel erection requires high-strength fasteners (A325 or A490) in slip-critical connections.

How to Choose the Right Thru Bolt for Your Application

Work through these four questions in order:

1. What materials are being connected? Wood-to-wood → standard hex bolt, partial thread, steel washer each side. Wood-to-concrete → anchor bolt or wedge anchor. Steel-to-steel → high-strength bolt per AISC spec.
2. What load type dominates? Shear (perpendicular to bolt) → prioritize diameter. Tension (along bolt axis) → prioritize grade and nut engagement. Combined → engineer specifies.
3. What’s the exposure? Interior dry → zinc plated or hot-dip galvanized. Exterior above ground → hot-dip galvanized (ASTM A153 Class C, minimum). In contact with pressure-treated lumber → hot-dip galvanized or stainless (ACQ-treated wood is corrosive to standard zinc plate).
4. What does the local code or engineer’s spec say? Always the final word. IRC Section R507 governs deck connections; IBC Chapter 23 governs engineered wood. If there’s a structural drawing, it specifies diameter, grade, and washer size explicitly — don’t substitute.

Thru Bolt Applications in Construction and Industry

Wood Beam and Girder Truss Connections

Multi-ply LVL beams, girder trusses, and built-up headers are among the highest-traffic applications for structural thru bolts. When you nail-laminate multiple 2× members into a beam, nails alone transfer only shear — they can’t resist the tendency of individual plies to roll or separate under load. Thru bolts clamp the assembly, and under the NDS bolt design values, a single ½″ × 8″ thru bolt through a 3-ply 2×10 SPF header can carry approximately 1,800 lbf of shear parallel to grain (more across grain).

In practice, most LVL manufacturers void their warranties if thru bolts are not installed at the pattern and spacing shown in their span tables — typically 24″ o.c. for beams up to 12 feet, 16″ o.c. for longer spans or higher loads.

Deck Post and Ledger Board Fastening

The IRC’s 2024 update tightened requirements for deck ledger connections, one of the most common structural failure points in residential decks. A ½″ thru bolt through a 2×ledger and band joist — as opposed to a lag bolt — is now preferred (and in some jurisdictions required) for ledger connections to engineered lumber rim boards, because lag bolt withdrawal values drop sharply in LVL compared to dimensional lumber.

Deck post base connections with uplift resistance — resisting wind or seismic loads trying to pull the post off the beam — are another thru bolt application. A post cap with ½″ thru bolts through the beam and post provides both positive bearing and positive tension connection. Relying on toenails alone for this connection is a code violation in most current editions of the IRC.

Door Lock and Hardware Installation

Security thru bolts for doors serve a different structural purpose: they prevent a lock body from being pried or kicked out of the door stile under forced-entry attack. A standard cylindrical lockset is held by two #10 wood screws through the face of the strike plate — a pry bar removes this in seconds. A thru bolt kit replaces those screws with M6 or M8 machine bolts that pass completely through the door and tighten against a large-bearing backplate on the interior face. The bolt head is typically concealed or tamper-resistant on the exterior face.

Most residential security upgrades combine door thru bolts with a door frame reinforcement kit — the bolt does no good if the frame itself splinters. According to a summary of door security testing by the Department of Homeland Security, reinforced door hardware including thru bolts can increase forced-entry resistance from 1–2 seconds with standard hardware to 3–5 minutes against a motivated attacker — enough to trigger alarm response in most systems.

How to Install a Thru Bolt Correctly

Tools and Materials Required

• Drill / hammer drill: standard drill for wood; rotary hammer for concrete
• Drill bit: for wood, twist bit at bolt nominal diameter (½″ bit for ½″ bolt); for concrete, SDS carbide bit at anchor diameter per manufacturer
• Socket wrench or impact driver with appropriate socket
• Torque wrench: required if the connection is engineered (spec will call out foot-pounds)
• Washers: one per side, minimum. 3×3 square plate washers for wood; round hardened washers for steel
• Nuts: standard hex for most applications; nylon-insert lock nuts where vibration or withdrawal is a concern

Step-by-Step Installation in Wood

1. Mark and drill the hole. Drill perpendicular to the member face. Drift (hole misalignment) greater than 1/16″ creates a bending moment on the shank — avoid it. In multi-ply beams, drill through all plies in a single pass with a long bit rather than drilling each ply separately (staging errors are common when drilling each member independently).
2. Insert the bolt from the head side. The smooth shank should fill the hole snugly — a sloppy fit means you used too large a drill bit and the bearing area is reduced. ½″ bolt → ½″ hole.
3. Install a washer on the nut side, then the nut. Hand-tighten first.
4. Torque to specification. For general framing without a specific torque spec: snug-tight is defined as the maximum torque achievable with a standard wrench by a single person without a breaker bar — typically 25–40 ft-lbs for a ½″ bolt in wood. Do not overtighten into wood. Crushing the wood fibers under the washer reduces bearing area and can split the member.
5. Verify bolt does not protrude excessively. Bolt end should extend at least half a thread beyond the nut. Bolt overhang > 3× diameter suggests the bolt is too long and the smooth shank length is wrong for the assembly thickness.

Common Installation Mistakes

• Skipping washers on one or both sides. The most common error. The washer dramatically increases the bearing area — the difference between a ½″ washer (0.2 in²) and a 3×3 plate washer (9 in²) is the difference between a point load and a distributed load. Always use washers on both sides in wood.
• Using a fully-threaded bolt where a partially-threaded one is specified. Full-thread bolts have reduced shear area at the threaded section — the minor diameter (root of threads) is smaller than the nominal. In shear-critical connections, this matters.
• Cross-threading the nut on first engagement. This is especially easy with long bolts. Thread the nut on by hand for at least 3–4 turns before applying torque.
• Overtorquing into ACQ-treated lumber. Pressure-treated wood is softer when wet (freshly treated) and compresses easily. If you’re installing thru bolts in green-treated wood, expect to retighten after the lumber dries (typically 60–90 days).

Thru Bolt vs. Competing Fastener Types

Thru Bolt vs. Carriage Bolt

Both are through-fasteners requiring a nut — the mechanical principle is identical. The difference is the head geometry:

In practice: if you can reach both sides with tools, a thru bolt (hex head) is stronger in shear due to head bearing area. If one side is inaccessible (inside a hollow post, behind a ledger), a carriage bolt’s self-locking square shoulder is the right call.

Thru Bolt vs. Anchor Bolt

Anchor bolts are embedded in concrete — cast-in-place (J-bolt) or post-installed (epoxy or mechanical anchor). They are not technically “through” fasteners because the bottom end is not accessible; they pass through a fixture plate and are secured by a nut on top. Despite the name confusion, anchor bolt design is covered by ACI 318 Appendix D and involves concrete cone breakout, pullout, and pryout failure modes that don’t apply to through-bolts in wood.

Are Through Bolts Better Than Anchors?

It depends entirely on the base material and load type. For connecting structural elements to concrete, a properly installed mechanical wedge anchor or epoxy anchor typically provides higher tension and shear values than a cast-in-place J-bolt of the same diameter, because the post-installed anchor can be placed precisely and inspected after installation. For wood-to-wood connections, through-bolts (thru bolts) are in a different category — there’s no concrete anchor equivalent for connecting two timber members.

The direct comparison that matters: in wood-to-concrete sill plate connections (sill plate bolted to foundation), a cast-in J-bolt and a post-installed anchor thru-bolt are both legitimate — code allows both. The post-installed anchor wins when the foundation was poured without bolt placement (retrofit construction) or when the bolt spacing must be adjusted from the original layout.

Future Trends in Structural Fastening (2026+)

High-Strength Thru Bolts for Seismic and Wind Zones

Post-2024 IBC updates have pushed higher load requirements for structural connections in seismic design categories D and E — particularly for multi-story wood-frame buildings. This is accelerating adoption of ⅝″ and ¾″ thru bolt systems in wood framing applications that traditionally used ½″. Simpson Strong-Tie’s HD (holdown) and HHUS (hanger) products increasingly specify ⅝″ thru bolts at post base and top connections specifically because the ½″ bolt value is insufficient for the increased seismic design forces.

According to FEMA P-807 guidance on soft-story wood-frame buildings, the majority of unretrofitted multi-unit wood-frame buildings in high-seismic zones have inadequate connection hardware — thru bolt upgrades are a primary retrofit tool. This creates sustained B2B demand for ⅝″ hot-dip galvanized thru bolts in the retrofit market.

Corrosion-Resistant Coatings and Material Upgrades

ACQ and CA pressure-treated lumber — the current standard for exterior decking — is more corrosive to zinc-plated fasteners than the now-discontinued CCA treatment. Hot-dip galvanized (HDG) is the minimum acceptable coating for ACQ-contact applications; in coastal environments (within 1 mile of saltwater), stainless steel 316 is the only reliable long-term choice.

The trend we’re tracking: manufacturers are moving toward mechanically galvanized thru bolts (cold bonded zinc, ASTM B695 Class 55) as an alternative to HDG for applications where the thick HDG coating creates fit issues in tight tolerances. Mechanically galvanized bolts run 20–30% thinner coating than HDG but are more uniform — useful in connections where the hole must be drilled close to bolt OD. Per ASTM B695 specification data, Class 55 mechanical galvanizing provides 55 g/m² zinc deposit, roughly equivalent to HDG Class C (1.85 oz/ft²) in salt-spray performance.

Frequently Asked Questions About Thru Bolts

What is a thru bolt used for?
A thru bolt connects structural members by passing completely through them and using a nut and washer for clamping. Common applications include wood beam connections, deck ledger fastening, concrete fixture attachment, bicycle wheel retention (thru axle), and door security hardware.

Are through bolts better than anchors?
For wood-to-wood structural connections, there is no equivalent anchor — thru bolts (or lag bolts) are the only practical option. For wood-to-concrete connections, a properly specified post-installed anchor often outperforms a cast-in J-bolt in both tension and shear, because placement is controllable and the expanded mechanism provides a mechanical lock. Neither is categorically “better” — the right choice depends on base material, load direction, and whether the installation is original construction or retrofit.

Can through bolts be removed?
Yes. Thru bolts are one of the few fully reversible structural fasteners. Back off the nut, remove the washer, and tap the bolt back through with a drift punch if it has seized. Seized bolts in exterior wood connections (after years of water infiltration and wood compression) may require penetrating oil (allow 30 minutes) and a bolt extractor if the head rounds. Concrete wedge anchors are more difficult — once expanded, the wedge mechanism is permanent, and removal requires cutting the bolt flush and leaving the sleeve.

What is the difference between a thru bolt and a lag bolt?
A lag bolt (lag screw) threads directly into the base material — the tip is a wood-cutting thread, and there is no through-hole or nut. A thru bolt passes completely through and requires access to both sides for installation. Thru bolts generally provide higher shear and tension values because load is transferred through the full shank cross-section and distributed over washers on both sides. Lag bolts are used when only one side is accessible.

What size thru bolt do I need for deck framing?
For most residential deck ledger connections to dimensional lumber rim joists, ½″ thru bolts on the pattern specified by IRC Table R507.9.1.3(1) is standard — typically alternating high/low at 16″ or 18″ o.c., depending on ledger depth and joist span. For LVL or engineered rim boards, ½″ thru bolts are often mandatory over lag bolts due to lower lag withdrawal values in LVL. Always confirm with the local building department — some jurisdictions adopt code amendments that change fastener requirements.

What grade thru bolt should I use?
For wood structural framing: ASTM A307 or SAE Grade 5, hot-dip galvanized for exterior. For steel-to-steel: ASTM A325 or A490 per AISC. For concrete anchors: check the manufacturer’s spec — most use ASTM F1554 Grade 36 for cast-in, ASTM F593 stainless for coastal.

How tight should a thru bolt be?
In engineered connections, follow the torque spec on the structural drawing — typically 25–40 ft-lbs for ½″ in wood, 60–80 ft-lbs for ⅝″. In non-engineered wood connections (decks, fences), snug-tight: the washer should be firmly seated and not spin, the wood surface should not be visibly crushing under the washer. Overtightening splits lumber and reduces the bearing area you’re trying to maximize. In steel connections per AISC, torque-controlled tightening follows the specification’s pretension table.

Conclusion

The thru bolt is one of the oldest and most reliable fastener principles in construction — a bolt, a hole, and a nut on the other side. The complexity isn’t in the concept; it’s in selecting the right combination of diameter, length, grade, coating, washer spec, and torque for a specific load condition and environment.

For structural wood connections, start with the NDS or your local code table, confirm the grade and coating requirements for your lumber type and exposure, and install with proper washers on both sides — that covers 90% of applications correctly. For concrete, defer to the anchor manufacturer’s load tables for your concrete strength and fixture thickness. For specialty applications (bicycle thru axles, door security hardware), follow the manufacturer’s spec, which will be tighter and more specific than any general guide can be.

If you’re sourcing thru bolts in bulk for a project or distribution, contact our team at productionscrews.com — we stock structural hex bolts, carriage bolts, wedge anchors, and specialty through-bolt assemblies across the full range of grades, coatings, and sizes, with next-day availability on common specs.