If you’re comparing lag screw vs lag bolt, the most important fact is that these terms usually refer to the same general type of heavy-duty wood fastener. In modern technical and engineering references, lag screw is the more precise term. The phrase lag bolt is still extremely common in hardware stores, construction discussions, product listings, and everyday speech.
A lag screw has a large head, a relatively thick body, and coarse threads designed to engage wood. Unlike an ordinary bolt, it does not normally require a nut on the other side. A traditional lag screw is installed by turning it into a properly prepared hole, while a conventional through-bolt passes through the connected materials and is secured with a nut.
The terminology becomes confusing because manufacturers and suppliers may use “lag bolt” as another name for a hex-head lag screw. Even the American Wood Council has used the terms together in technical material, including the phrase “lag screws, also known as lag bolts.”
So, lag screw vs lag bolt is usually not a comparison between two completely different fasteners. The more useful comparison is between a traditional lag screw and a conventional bolt, while understanding why the name “lag bolt” exists.
This guide explains the terminology, construction, strength, installation, applications, washers, pilot holes, common mistakes, and how to choose the right fastener for wood projects.
Quick Answer
Lag screw and lag bolt are commonly used as interchangeable names for a heavy-duty, hex-head wood fastener. Technically, lag screw is the preferred term because the fastener is driven into a prepared hole and does not normally use a nut.
A traditional lag screw can connect wood members without requiring access to the opposite side. A conventional through-bolt, by contrast, passes completely through the materials and uses a nut and washer.
The key distinction is therefore:
- Lag screw: screws into wood and normally does not require a nut.
- Lag bolt: commonly another name for a lag screw.
- Through-bolt: passes through the assembly and is tightened with a nut.
- Structural wood screw: a modern alternative that may offer easier installation depending on the application and product specification.
For structural work, don’t choose a fastener based only on whether someone calls it a “lag screw” or “lag bolt.” Diameter, length, material, thread design, wood species, installation method, load, spacing, and the applicable engineering requirements all matter.
What Is a Lag Screw?
A lag screw is a heavy-duty threaded fastener commonly used to connect wood components. Traditional lag screws typically have a hexagonal head that can be tightened with a wrench or socket.
They are substantially heavier than ordinary wood screws and are intended for applications where a stronger mechanical connection is needed.
A traditional lag screw generally has:
- A hex head
- A relatively large diameter
- Coarse threads
- A pointed end
- A threaded section
- An unthreaded shank on many designs
- A washer under the head in many applications
The threads engage the wood and provide resistance to withdrawal. The body of the fastener also contributes to resistance against lateral forces when the connection is properly designed.
Traditional lag screws are commonly used to attach substantial pieces of wood to one another or to connect wood to other materials when the connection has been designed for that purpose.
How a Lag Screw Works
A lag screw works by creating a mechanical connection between the fastener and the surrounding material.
The threaded section engages the wood fibers. The head presses against the outer member, while the fastener transfers forces through the connected materials.
For many traditional lag-screw installations, a pilot hole is required. The pilot-hole arrangement is important because the shank and threaded portions of the fastener interact differently with the wood.
A properly prepared hole can:
- Reduce splitting
- Make installation easier
- Reduce unnecessary installation torque
- Help the threads engage correctly
- Reduce the possibility of damaging the wood
- Improve consistency between installations
The correct hole size is not simply “one size fits all.” It depends on the fastener dimensions, wood species, connection design, and applicable requirements.
What Is a Lag Bolt?
Lag bolt is a widely used common name for what is technically called a lag screw.
This is one of the biggest sources of confusion when researching fasteners.
When someone asks for a lag bolt at a hardware store, they may be handed a hex-head lag screw. Product catalogs may also label the same general product as a lag bolt.
The word “bolt” can make people expect a nut, but a traditional lag fastener does not work like an ordinary through-bolt.
A conventional bolt typically works by passing through a hole and being secured with a nut. A lag screw, on the other hand, develops its connection by engaging threads in the wood or another suitable material.
Why Is It Called a Lag Bolt?
The term developed through common construction and hardware usage. “Lag bolt” has become a familiar name for the large, coarse-threaded fasteners traditionally used in heavy wood construction.
However, terminology isn’t always consistent between manufacturers, retailers, tradespeople, and technical documents.
That’s why you may see:
- Lag screw
- Lag bolt
- Hex lag screw
- Lag fastener
- Wood lag screw
used to describe closely related products.
For technical writing and engineering documentation, lag screw is generally the clearer term.
Lag Screw vs Lag Bolt: Are They Actually Different?
In most everyday hardware contexts, no.
A typical product sold as a “lag bolt” may simply be a traditional hex-head lag screw.
| Feature | Lag Screw | Lag Bolt |
| Common meaning | Heavy-duty threaded wood fastener | Common name for a lag screw |
| Usually uses a nut | No | No, when referring to a lag screw |
| Typical head | Hex head | Hex head |
| Main use | Wood connections | Same general applications |
| Threads engage | Wood or suitable material | Wood or suitable material |
| Technical terminology | More precise | Common but less precise |
| Pilot hole | Often required | Often required when it refers to a traditional lag screw |
| Washer | Commonly used | Commonly used |
| Can mean a through-bolt? | No | Not normally |
The important point is that “lag bolt” does not automatically mean a conventional bolt.
If a project specification calls for a bolt, don’t substitute a lag screw simply because someone informally calls it a lag bolt. The connection may require a specific fastener type.
Lag Screw vs Regular Bolt
This is the comparison that often matters more in real construction.
A conventional bolt and a lag screw have different installation methods and connection behavior.
Lag Screw
A traditional lag screw:
- Threads into the material
- Usually does not require a nut
- Can often be installed from one accessible side
- Is commonly used in wood connections
- Uses a washer under the head in many applications
- Often requires a properly sized pilot hole
Through-Bolt
A conventional through-bolt:
- Passes through the connected materials
- Requires a nut
- Normally uses washers
- Requires access to both sides during installation
- Can provide a strong clamped connection
- Is frequently used where through-bolting is appropriate for the design
This difference is especially important when you’re working on an existing structure.
If you can only reach one side of a timber assembly, installing a traditional through-bolt may be impractical. A properly specified lag screw may be easier because it can be installed from one side.
If both sides are accessible and the design calls for a through-bolted connection, using a lag screw simply because it is easier may not be appropriate.
Key Difference: Screw vs Bolt
The basic naming distinction comes from how the fastener creates its connection.
A screw is generally designed to engage its mating material through threads.
A traditional bolt is generally used with a nut or another internally threaded component and is primarily used to clamp materials together.
However, real-world fastener terminology is not perfectly consistent.
That’s why a hex-head lag fastener can be called a “lag bolt” even though its technical behavior is screw-like.
The terminology becomes even more confusing because modern fastener designs don’t always fit simple textbook definitions.
For that reason, when buying or specifying a fastener, look at the actual product specifications rather than relying solely on the word “bolt.”
How Lag Screws Differ From Ordinary Wood Screws
A lag screw and an ordinary wood screw are both threaded fasteners, but they are not simply interchangeable versions of the same product.
Traditional lag screws are generally:
- Larger
- Heavier
- Designed for substantial wood connections
- Frequently installed with hex heads
- Used with washers in many applications
- Installed with specific pilot-hole requirements
Ordinary wood screws are available in many sizes and head styles and are commonly used for lighter-duty fastening.
There are also modern structural wood screws designed specifically for structural applications. These products can provide an alternative to traditional lag screws in some projects, but they should be selected according to their individual specifications and approvals rather than assuming every structural screw is equivalent to every lag screw.
Why This Matters
A fastener’s apparent size doesn’t tell you its actual structural capacity.
Two screws with the same diameter can have different:
- Steel strength
- Thread geometry
- Head strength
- Thread length
- Coating
- Corrosion resistance
- Installation requirements
- Approved applications
So the question should not simply be, “Which screw looks stronger?”
The correct question is, “Which fastener is specified and rated for this connection?”
Where Are Lag Screws Commonly Used?
Traditional lag screws have many applications in wood construction and woodworking.
Common examples include:
Deck Construction
Lag screws have historically been used to connect deck ledgers and other substantial wood components.
Deck connections are particularly important because they can transfer significant loads between the deck and the building.
The American Wood Council’s deck guidance includes specific provisions for lag screws and bolts, including requirements concerning diameter, placement, washers, pilot holes, spacing, and penetration.
Do not assume that any lag screw is acceptable for a deck ledger simply because it is large enough. Structural deck connections must follow the applicable design and code requirements.
Timber Connections
Lag screws can be used for connecting substantial timber members when the connection has been designed appropriately.
Wood-to-Wood Connections
They are commonly used where one wood member needs to be securely attached to another.
Examples can include:
- Framing connections
- Heavy-duty woodworking
- Timber assemblies
- Structural reinforcement applications
- Mounting substantial wooden components
Wood-to-Metal Connections
Some lag screws can be used to attach wood components to metal components or hardware, but the suitability depends on the connection design and the fastener specifications.
A metal bracket with predrilled holes may be designed specifically for a particular screw diameter and length.
Why Are Washers Used With Lag Screws?
A washer is commonly placed under the head of a traditional lag screw.
The washer provides a larger bearing surface between the fastener head and the wood.
This can help distribute the clamping force over a larger area and reduce the tendency of the head to sink excessively into the wood.
A washer can also help protect the surface around the fastener head.
However, the correct washer is part of the connection design. Simply adding a random washer does not automatically make a connection stronger.
For structural connections, follow the washer type and dimensions specified by the applicable design, fastener manufacturer, connector manufacturer, or code requirement.
Do Lag Screws Need Pilot Holes?
Traditional lag screws commonly require properly sized pilot holes.
This is one of the most important installation differences between a traditional lag screw and some modern self-drilling structural screws.
A pilot hole helps the fastener enter the wood without unnecessarily damaging the surrounding material.
For traditional lag screws, the hole arrangement can involve different diameters for the unthreaded shank and threaded portion.
The correct dimensions depend on factors such as:
- Lag screw diameter
- Thread design
- Wood species
- Wood density
- Connection geometry
- Fastener length
- Structural requirements
A single generic pilot-hole size should therefore not be applied to every lag screw.
Why an Oversized Hole Can Be a Problem
If the hole is too large for the threaded section, the threads may not engage the wood effectively.
That can reduce the connection’s ability to resist withdrawal.
Why an Undersized Hole Can Be a Problem
If the hole is too small, installation can require excessive force.
That can:
- Damage the wood
- Increase splitting risk
- Increase installation torque
- Damage the fastener
- Make proper installation difficult
The goal is not simply to make installation easier. The hole needs to be appropriate for the specific fastener and connection.
Can You Drive a Traditional Lag Screw With an Impact Driver?
It depends on the fastener and installation requirements.
Modern structural screws may be specifically designed for installation with power tools, including impact drivers, according to manufacturer instructions.
Traditional lag screws are different.
For a structural connection, you should follow the manufacturer’s installation instructions and applicable engineering requirements rather than assuming that an impact driver is acceptable.
Overdriving a fastener can damage the wood or connection. Under-tightening can also prevent the connection from functioning as intended.
For traditional lag screws, installation is often performed by turning the fastener with a wrench or socket after the appropriate holes have been prepared.
Lag Screw Strength: What Actually Determines It?
People often ask whether lag screws are “strong.”
The more useful answer is that lag screw strength depends on the complete connection, not just the fastener itself.
Important factors include:
Diameter
A larger diameter generally provides a larger cross-sectional area, but diameter alone does not determine connection capacity.
Length
The length must provide adequate penetration into the supporting material while accounting for the thickness of the attached member and any washers or hardware.
A longer fastener isn’t automatically better.
Threaded Length
The amount of threaded engagement matters because the threads interact with the wood.
Wood Species
Different woods have different mechanical properties.
A connection in dense hardwood may behave differently from a connection in a lower-density softwood.
Wood Condition
Moisture, deterioration, splitting, decay, and damage can affect connection performance.
Edge and End Distance
Fastener placement matters.
A fastener installed too close to an edge or end can increase the risk of splitting or reduce connection capacity.
Spacing
Multiple fasteners cannot always be placed arbitrarily close together.
The wood between fasteners needs to be capable of transferring the forces without causing premature failure.
Steel Strength
The fastener itself must have adequate mechanical properties for the application.
Installation Quality
Even a correctly specified fastener can perform poorly if it is installed incorrectly.
Lag Screw Pull-Out Strength vs Shear Strength
Two important concepts in fastener connections are withdrawal and lateral resistance.
Withdrawal
Withdrawal refers to a force trying to pull the lag screw out of the wood along the direction of the fastener.
For example, imagine a connection where a force is trying to pull a mounted component directly away from a wood member.
Thread engagement and wood properties are particularly important in withdrawal behavior.
Lateral Loading
Lateral loading acts across the fastener rather than directly along its length.
For example, if two wood members are connected and one tries to slide sideways relative to the other, the fastener experiences lateral forces.
Lateral connection behavior depends on factors such as:
- Fastener diameter
- Fastener bending strength
- Wood bearing strength
- Member thickness
- Connection geometry
- Number of fasteners
- Spacing and edge distance
This is why saying “a 1/2-inch lag screw can hold X pounds” without specifying the connection is misleading.
There is no single universal load capacity for every installation.
Full-Diameter vs Reduced-Diameter Lag Screws
Traditional lag screws can also differ in their body design.
A full-diameter body lag screw has an unthreaded shank that is larger than the root diameter of the threaded section.
A reduced-diameter body lag screw has a shank closer to the root diameter of the threaded section.
This distinction matters in structural design because fastener geometry affects how the connection behaves.
The American Wood Council’s technical material distinguishes between full-diameter and reduced-diameter lag screws and explains that design values can differ depending on the fastener configuration.
Therefore, two lag screws that have the same nominal diameter and length may not necessarily have identical structural characteristics.
Lag Screw Materials and Finishes
Lag screws are available in different materials and finishes.
Common options include:
- Carbon steel
- Zinc-plated steel
- Hot-dip galvanized steel
- Stainless steel
- Other coated or treated fasteners
The appropriate choice depends on the environment.
Indoor Use
A standard carbon-steel fastener may be suitable for many dry indoor applications.
Outdoor Use
Outdoor applications require attention to corrosion resistance.
Rain, humidity, condensation, treated lumber chemicals, and coastal environments can all affect fastener corrosion.
Pressure-Treated Wood
Fasteners used with pressure-treated wood may require corrosion-resistant materials or coatings compatible with the treatment chemicals.
Don’t assume that a fastener is appropriate simply because it is labeled “outdoor.”
The fastener manufacturer and project requirements should determine the appropriate corrosion resistance.
Lag Screws and Deck Construction
Deck construction is one area where terminology can cause serious confusion.
People may casually talk about using “lag bolts” to attach a deck ledger, but a deck ledger connection is a structural connection and should not be treated like an ordinary DIY fastening task.
Applicable deck guidance may specify:
- Fastener diameter
- Fastener length
- Fastener placement
- Vertical spacing
- Horizontal spacing
- Edge distance
- End distance
- Washer requirements
- Pilot-hole dimensions
- Minimum penetration
- Wood species or material requirements
The American Wood Council’s prescriptive deck guidance specifically discusses lag screws and bolts as different fastening options within ledger connections.
This illustrates an important point: although “lag bolt” may be used informally as a synonym for “lag screw,” technical documents can distinguish lag screws from actual bolts.
Always follow the specific requirements for the connection rather than relying on casual terminology.
When Should You Use a Lag Screw?
A traditional lag screw may be appropriate when:
- You’re making a substantial wood connection.
- The connection is designed for a lag screw.
- Access to the opposite side is limited.
- A threaded connection into wood is appropriate.
- The required diameter and length are available.
- The installation can follow the required pilot-hole procedure.
- The fastener has appropriate corrosion resistance.
- The connection meets the applicable structural requirements.
For nonstructural woodworking, a traditional lag screw may also be selected when its large size and hex head suit the project.
When Should You Use a Through-Bolt Instead?
A through-bolt may be appropriate when:
- Both sides of the connection are accessible.
- The design specifically calls for bolts.
- A nut-and-washer connection is desired.
- The materials can be drilled completely through.
- The connection requires the characteristics of a bolted assembly.
Through-bolts can be particularly useful in substantial structural connections where the connection has been designed around the bolt, nut, washers, and connected materials.
Do not substitute a lag screw for a specified through-bolt without confirming that the alternative is acceptable.
When Is a Structural Wood Screw Better?
Modern structural wood screws have become common alternatives to traditional lag screws in many applications.
Depending on the product, structural screws may offer features such as:
- Easier installation
- Torx or other drive systems
- Self-drilling or self-tapping points
- Reduced need for traditional pilot-hole preparation
- Manufacturer-tested structural capacities
- Specialized coatings
- Various head designs
- Purpose-specific applications
However, “structural screw” is a product category rather than a guarantee that every structural screw can replace every lag screw.
The product must be selected according to its published specifications and approved applications.
A structural screw designed for one connection should not automatically be substituted into another connection.
Common Lag Screw Installation Mistakes
1. Skipping the Pilot Hole
Traditional lag screws often require pilot holes.
Driving a large lag screw directly into wood without appropriate preparation can increase the risk of splitting and installation damage.
2. Using the Wrong Pilot-Hole Size
Too small can cause excessive resistance and damage.
Too large can reduce thread engagement.
3. Choosing the Wrong Length
The screw needs adequate penetration into the supporting material.
But excessive length can create other problems, including unwanted penetration through the member or interference with other components.
4. Ignoring Wood Species
Pilot-hole requirements and connection performance can vary with wood density.
5. Over-Tightening
More torque does not automatically mean a stronger connection.
Over-tightening can crush the wood beneath the washer or damage the connection.
6. Under-Tightening
A loose connection can prevent the fastener and connected members from behaving as intended.
7. Omitting Required Washers
If the connection calls for a washer, don’t omit it.
The washer can be an important part of the load-transfer arrangement.
8. Using a General-Purpose Fastener for Structural Work
A large screw from a hardware bin isn’t automatically a structural fastener.
Structural applications require appropriate fastener specifications and connection design.
9. Ignoring Corrosion
A fastener that works indoors may not be appropriate outdoors or with certain treated lumber.
10. Placing Fasteners Too Close Together
Fastener spacing affects how forces are transferred through the wood.
11. Installing Too Close to an Edge
Insufficient edge distance can increase the risk of splitting and affect connection performance.
12. Treating “Lag Bolt” as a Conventional Bolt
This is one of the most common terminology mistakes.
If a specification calls for a bolt and nut, don’t assume “lag bolt” is an equivalent product.
How to Choose the Right Lag Fastener
When selecting a lag screw, consider the complete connection.
Step 1: Identify the Materials
Are you connecting:
- Wood to wood?
- Wood to metal?
- Wood to another structural material?
The answer affects fastener selection.
Step 2: Determine the Load
Ask whether the connection experiences:
- Withdrawal
- Shear
- Lateral movement
- Tension
- Combined loading
A structural engineer or applicable design standard may be necessary for significant loads.
Step 3: Select the Required Diameter
Diameter should be based on the connection requirements rather than appearance.
Step 4: Select the Length
Make sure the fastener provides the required penetration without causing unwanted interference.
Step 5: Check the Fastener Material
Choose a material or coating suitable for the environment.
Step 6: Check the Installation Requirements
Confirm:
- Pilot-hole requirements
- Washer requirements
- Installation tool
- Torque or tightening requirements
- Minimum embedment
- Spacing
- Edge distance
Step 7: Follow the Connection Design
For structural applications, the connection design should take precedence over general rules of thumb.
Lag Screw vs Lag Bolt: Simple Decision Guide
If you’re standing in a hardware store and wondering what to buy, use this basic approach:
If you need a heavy-duty threaded fastener that screws into wood:
Look for a lag screw or a product commonly labeled a lag bolt, then verify the specifications.
If your plans call for a bolt passing completely through the wood with a nut:
You need the specified through-bolt assembly, not simply a lag screw.
If your plans specify a structural wood screw:
Use the specified structural screw or an approved equivalent.
If you’re working on a structural deck, beam, ledger, or load-bearing connection:
Don’t select the fastener based only on the name. Follow the applicable plans, code provisions, engineering requirements, and manufacturer specifications.
Are Lag Bolts Stronger Than Lag Screws?
No meaningful strength comparison can be made simply from the names because lag bolt and lag screw commonly describe the same type of fastener.
If someone says that a lag bolt is stronger than a lag screw, first determine what products they are actually comparing.
Strength depends on:
- Fastener diameter
- Steel grade
- Fastener geometry
- Thread design
- Embedment
- Wood species
- Wood member dimensions
- Load direction
- Spacing
- Edge distance
- Installation quality
- Connection configuration
The name alone does not establish capacity.
Are Lag Screws Better Than Regular Bolts?
Not universally.
A lag screw can be more convenient when you need to install a fastener from one accessible side and the connection is designed for a lag screw.
A through-bolt can be appropriate when the connection is designed around a bolt, nut, and washers and both sides are accessible.
Neither should be called universally “better.”
The correct fastener is the one that matches the connection requirements.
Can You Replace a Lag Screw With a Bolt?
Not automatically.
A lag screw and a conventional bolt create connections differently.
Replacing one with the other can change:
- Load capacity
- Installation requirements
- Washer requirements
- Connection geometry
- Required access
- Spacing requirements
- Wood stresses
- Overall structural behavior
For a nonstructural project, substitution may sometimes be practical, but for a structural connection, follow the project design and applicable requirements.
Can You Replace a Lag Bolt With a Lag Screw?
If “lag bolt” is being used to mean a traditional lag screw, then the two names may refer to the same product.
But if someone uses “lag bolt” to mean a different bolted connection, you need to identify the actual fastener before making a substitution.
This is why looking at the product dimensions and specifications is more reliable than looking at the product name alone.
What Size Lag Screw Should You Use?
There is no single correct lag screw size for every project.
The required size depends on the connection.
Important considerations include:
- Thickness of the attached member
- Thickness of the supporting member
- Load
- Fastener spacing
- Wood species
- Required embedment
- Edge distance
- Connection type
- Applicable design requirements
For simple nonstructural projects, the manufacturer’s recommendations can guide selection.
For structural applications, use the applicable design requirements rather than relying on a generic “use a larger screw” rule.
Do Lag Screws Need Nuts?
Traditional lag screws do not normally use nuts.
They are designed to engage the material through their threads.
A conventional bolt generally uses a nut when making a through-bolted connection.
This is one of the easiest ways to distinguish a traditional lag screw from a conventional through-bolt.
A washer may be used under a lag screw head, but a washer is not the same thing as a nut.
Do Lag Screws Need Washers?
Many traditional lag-screw applications use washers, particularly where the washer is specified to distribute force under the hex head.
However, the exact washer requirement depends on the application.
For structural connections, use the washer type and dimensions specified by the design or manufacturer.
What Is the Difference Between a Lag Screw and a Structural Screw?
A traditional lag screw is a conventional heavy-duty wood fastener with a long history in wood construction.
A modern structural wood screw is engineered for particular structural applications and may offer different installation characteristics.
Structural screws may have:
- Specialized threads
- Specialized heads
- Proprietary coatings
- Self-drilling points
- Published load capacities
- Specific installation instructions
They can be useful alternatives to traditional lag screws, but one should not assume that any structural screw is interchangeable with any lag screw.
FAQs
Is a lag bolt the same as a lag screw?
In common hardware terminology, yes. “Lag bolt” is frequently used as another name for a lag screw, particularly for large hex-head wood fasteners. In technical writing, lag screw is generally the clearer term. Always check the actual product specifications when a particular fastener is required.
Do lag screws require nuts?
No. Traditional lag screws are designed to engage threads in the wood or another suitable material and normally do not require a nut. A conventional through-bolt usually passes through the connected materials and is secured with a nut and washers.
Are lag screws stronger than wood screws?
Traditional lag screws are generally larger and designed for substantial connections, but strength cannot be determined from the name alone. Modern structural wood screws can also be engineered for significant loads. Fastener capacity depends on the product and the complete connection.
Should you predrill for a lag screw?
Traditional lag screws generally require appropriately sized pilot holes. The correct hole dimensions depend on the fastener and material. Always follow the applicable installation instructions rather than using one generic pilot-hole size for every project.
Can you use lag screws outdoors?
Some lag screws are suitable for outdoor applications, but the fastener must have appropriate corrosion resistance for the environment. Outdoor exposure, moisture, treated lumber, and coastal conditions can all affect fastener selection.
Are lag screws good for decks?
Lag screws can be used in certain deck connections when the connection is designed for them. Deck ledger connections are structural and have specific requirements for fastener size, spacing, penetration, washers, and installation. A generic lag screw should not be assumed to satisfy those requirements.
What is the difference between a lag screw and a bolt?
A traditional lag screw threads into the material and normally does not require a nut. A conventional bolt generally passes through the materials and is secured with a nut. The term “lag bolt,” however, is commonly used as another name for a lag screw.
Can you use a lag screw instead of a structural screw?
Not automatically. A structural screw and a traditional lag screw can have different specifications and approved uses. If a project specifies a structural screw, use the specified product or an approved equivalent rather than assuming a lag screw is interchangeable.
Conclusion
The lag screw vs lag bolt question is mostly a terminology issue. In everyday hardware language, lag bolt commonly means lag screw, and the two names may describe the same hex-head, coarse-threaded fastener designed for substantial wood connections.
The more important distinction is between a lag screw and a conventional through-bolt. A lag screw engages the material with its threads and normally does not require a nut. A through-bolt passes through the assembly and is tightened with a nut and washers.
For basic woodworking, the terminology may not cause much trouble. For structural applications such as decks, ledgers, timber connections, and load-bearing assemblies, however, the exact fastener specification matters. Diameter, length, thread configuration, material, corrosion resistance, pilot-hole requirements, spacing, embedment, and the properties of the connected wood all affect the connection.
So, when choosing between a “lag screw” and a “lag bolt,” don’t focus only on the name. Identify the actual fastener, understand the connection it needs to make, and follow the specifications for that application.
