Simpson Lag Bolts vs Lag Screws: Understanding the Differences

When you're standing in the fastener aisle or placing an order for structural connections, the terms "lag bolt" and "lag screw" get thrown around like they're completely different products. Add Simpson Strong-Tie into the mix, and you might think there's some special category of Simpson lag bolts that differs from their lag screws. In actuality, these are all the same. The proper technical term is "lag screw," regardless of the manufacturer, but Simpson Strong-Tie doesn't manufacture traditional lag screws.

Instead, they've developed structural screws that often replace lag screws in many applications. This distinction matters when it comes to performance, installation efficiency, and code compliance.

Watch this short video below, made by Simpson Strong-Tie, to better understand what the lag screw alternative is.

Let's clear this up once and for all. The two terms "lag bolts" and "lag screws" are used interchangeably to refer to the same type of fastener. The confusion arises because these fasteners exhibit characteristics of both bolts and screws; however, technically speaking, they should be referred to as lag screws.

A bolt is a fastener with a machine thread that can accept a nut. A bolt is assembled correctly and tightened by rotating the nut. A screw, on the other hand, is a fastener that is properly installed by rotating the head of the fastener. Since lags are not used with a nut and installed by turning the head of the fastener, the proper technical term would be "lag screw".

Even ASME B18.2.1, which sets the standards for construction fasteners, refers to these fasteners as screws rather than bolts. So whether you call them lag bolts or lag screws, you're talking about the same product – but "lag screw" is technically correct.

The real issue for construction professionals isn't what to call them, but understanding what Simpson Strong-Tie actually offers and how their products compare to traditional lag screws from other manufacturers.

Here's where things get interesting:

Simpson Strong-Tie doesn't manufacture traditional lag screws. Instead, they focus on engineered structural screws that often outperform conventional lag screws in key applications. Their flagship product line includes the Strong-Drive SDWS series, which represents an evolution beyond basic lag screw technology.

Simpson Strong-Tie doesn’t manufacture traditional lag screws. Instead, they focus on engineered structural screws that often outperform conventional lag screws in key applications.

The Strong-Drive SDWS (Structural Wood Screw) line includes several variants designed for different applications:

SDWS TIMBER Screws (Exterior Grade): These are the heavy-duty structural fasteners most commonly used to replace traditional lag screws. The Simpson Strong-Tie Strong-Drive SDWS Timber screw is designed to provide an easy-to-install, high-strength alternative to through-bolting and traditional lag screws.

Key Features of SDWS TIMBER Screws:

• Bold thread design provides superior holding power
• Patented 4CUT™ tip ensures fast starts, reduces installation torque, and eliminates the need for pre-drilling in most applications
• Under-head nibs offer outstanding control when seating the large washer head
• Available in 0.220" diameter with various lengths
• Double barrier coating equivalent to hot dip galvanization
• provides the strength of a 1/2" lag bolt

Allowable shear load values for SDWS screws vary depending on the type of wood being fastened. Because wood species differ in density and strength, Simpson Strong-Tie provides separate charts for Douglas Fir-Larch and Southern Pine (DFL/SP) and for Spruce-Pine-Fir and Hem-Fir (SPF/HF) . These charts help builders, engineers, and inspectors choose the right fastener length and thread engagement based on wood member thickness, ensuring safe, reliable, and code-compliant connections in structural applications.

This chart shows the allowable shear loads for SDWS screws installed in Douglas Fir-Larch (DFL) and Southern Pine (SP) lumber. It lists the tested shear capacities based on screw length, thread length, and the thickness of the wood side member. Use this table to determine the proper screw size and ensure strong, code-compliant connections in heavy-duty wood-to-wood applications.

This chart provides allowable shear load values for SDWS screws used in Spruce-Pine-Fir (SPF) and Hem-Fir (HF) lumber. Because these species are generally lower in density compared to DFL/SP, the allowable loads are slightly reduced. Use this table to help select the right screw length and thread engagement to maintain structural integrity when fastening in softer wood materials.

SDWS FRAMING Screws: Explicitly designed for framing applications where rapid installation and consistent performance are priorities.

SDWS Stainless Steel Variants: Featuring a heavy-duty 0.276-inch-diameter shank, SawTooth point, and flat washer head, the SDWS Stainless Steel Timber Screw is the ideal choice for building in corrosive environments where Type 316 stainless steel construction is required. 

To understand why Simpson's approach matters, you need to know what traditional lag screws bring to the table. These fasteners have been the go-to choice for heavy-duty wood connections for decades, and they remain a staple in construction.

Standard Lag Screw Characteristics:

• Hex heads requiring socket or wrench installation
• are at least one inch long and 1/4 inch thick
• Coarse thread design for strong wood engagement
• Available in diameters from 1/4" to 1" or larger
• Require pre-drilled pilot holes for proper installation

Installation Requirements:

Traditional lag screws demand respect during installation. Pre-drill a hole slightly smaller than the diameter of the screw's threads to facilitate easier insertion into the material. This helps prevent splitting and ensures a secure fit. The pilot hole should typically be 60-70% of the screw's root diameter, and the depth should be roughly half the threaded engagement length.

Material and Coating Options:

1. Plain steel for interior applications
2. Galvanized coating for moderate exterior exposure
3. Hot-dip galvanized for structural exterior applications
4. Stainless steel for corrosive environments
5. Various specialty coatings for specific applications

Load Characteristics: Traditional lag screws rely on their coarse threads and substantial diameter to develop holding power. Their load capacity depends on factors like wood species, moisture content, thread engagement length, and installation quality. Unlike engineered fasteners, you typically need to calculate allowable loads using generic tables and safety factors to ensure the structural integrity.

The real-world performance gap between traditional lag screws and Simpson's structural screws becomes apparent in several key areas that directly impact job site efficiency and long-term reliability.

Installation Efficiency: This is where Simpson's engineered approach shines. Eliminates the need for pre-drilling in most applications, allowing you to drive SDWS screws directly into most wood materials without the time-consuming step of creating a pilot hole. For a crew installing multiple connections, this translates to significant time savings.

Traditional lag screws require careful pilot hole drilling, which means:

• Additional time for each connection
• Extra tools and drill bits
• Potential for misaligned holes
• Risk of splitting if pilot holes are incorrectly sized

Load Performance and Engineering: Simpson's structural screws are accompanied by ICC-ES evaluation reports that provide specific engineered load values. This eliminates guesswork and provides the documentation needed for code compliance and inspection approval. Traditional lag screws typically require referencing generic load tables and applying safety factors, which can result in over-engineering or potential under-capacity situations.

Consistency and Quality Control: The manufacturing standards for Simpson's structural screws include consistent metallurgy, precise thread geometry, and quality control testing. Traditional lag screws from various manufacturers can exhibit significant variations in materials and dimensions, which affect performance predictability.

Cost Analysis: While Simpson structural screws cost more per piece than commodity lag screws, the total installed cost often favors the engineered fasteners when you factor in:

1. Reduced installation time

2. Elimination of pilot hole drilling

3. Lower risk of installation errors

4. Fewer fasteners are needed due to higher load ratings

5. Reduced callbacks and warranty issues

Framing and Structural Work: For structural framing where building code compliance is critical, Simpson's SDWS screws offer clear advantages. The ICC-ES evaluation reports provide the engineered values inspectors want to see, and the consistent installation eliminates variables that can cause problems during inspection.

Traditional lag screws remain suitable for structural applications, but you must consult code tables for allowable loads and ensure that proper installation procedures are followed. This approach works well for straightforward connections where the engineering is well-established.

HVAC Equipment Installation: HVAC professionals frequently encounter situations involving equipment mounting where vibration, thermal cycling, and consistent clamping force are critical considerations. Simpson's structural screws provide more predictable performance in these applications, especially when mounting heavy equipment or ductwork supports.

The self-drilling capability is particularly valuable when working in tight spaces or when access for pilot hole drilling is limited. Traditional lag screws remain viable for lighter-duty HVAC applications where cost is the primary concern.

Electrical Panel and Equipment Mounting: Electrical work often requires structural-grade fasteners for mounting panels and supporting equipment. The documentation provided with Simpson's structural screws can simplify inspection approval, especially in commercial applications where engineered fastener values are specified.

For residential electrical work where generic lag screws meet code requirements, traditional options provide adequate performance at a lower cost.

Retrofit and Repair Applications: Retrofit work presents unique challenges where existing conditions may not align with standard installation procedures. Simpson's self-drilling structural screws can adapt better to situations where existing holes don't match new fastener requirements or where access is limited.

Traditional Lag Screw Installation: Begin with accurate pilot hole sizing – this is crucial for both optimal performance and preventing wood splitting. The pilot hole diameter should be 60-70% of the screw's root diameter, and the depth should be approximately half the threaded engagement length. Use sharp drill bits to create clean holes that allow proper thread engagement without overstressing the wood fibers.

Drive the lag screw with steady, controlled force. Over-tightening can strip threads or damage the wood, while under-tightening compromises the connection strength. Use washers under hex heads when specified to distribute loads and prevent embedment into the wood surface.

Simpson Structural Screw Installation: Simpson's self-drilling design simplifies installation but still requires attention to proper technique. Verify that your material combination is compatible with the self-drilling capability – some dense materials or specific applications may still benefit from pilot holes.

Use the recommended drive bit type and ensure your drill has adequate torque capacity. The patented 4CUT™ tip ensures fast starts, reduces installation torque, and eliminates the need for pre-drilling in most applications. However, proper bit engagement remains critical for avoiding cam-out or head damage.

Follow the manufacturer's torque specifications as outlined in the evaluation reports. Over-driving can compromise the connection, while under-driving may not achieve full load capacity.

The choice between traditional lag screws and Simpson structural screws isn't just about the fastener itself – it's about understanding your project requirements, crew capabilities, and long-term performance expectations.

For most construction professionals, Simpson's Strong-Drive SDWS series offers compelling advantages that justify the higher material cost through improved installation efficiency and engineered performance documentation. The self-drilling capability alone can save significant time on projects with multiple connections, and the ICC-ES evaluation reports simplify code compliance and inspection approval.

Traditional lag screws retain their value in applications where cost is the primary concern and installation conditions permit proper pilot hole preparation. They remain a proven solution for many construction applications and continue to meet code requirements when properly installed and maintained.

The key is matching the fastener choice to your specific situation: project requirements, crew experience, time constraints, and performance expectations. Both options have earned their place in construction, but understanding their fundamental differences helps ensure you're using the right fastener for each application.

Whether you call them lag bolts or lag screws, the goal remains the same: creating reliable connections that meet code requirements and perform consistently over time. Simpson Strong-Tie's engineered approach represents an evolution in fastener technology that addresses many traditional limitations, but the fundamental principles of proper fastener selection and installation remain unchanged.

Consider your project needs, evaluate the total installed cost, and choose the fastener that best balances performance, efficiency, and budget requirements. Your connections are only as good as the fasteners you choose and how well you install them.