structural lag screws
Heat-treated steel strength
No pre-drilling. No wasted time.
Full diameter shank.
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Sinsun Structural Lag Screws: Engineered for Heavy-Duty Wood Connections
Sinsun structural lag screws deliver 1,200+ lbf ultimate load capacity. Heat-treated steel, IBC/IRC compliant, and engineered for framing, ledger boards, and heavy timber connections. No pre-drilling required on most models.
Get a Quick QouteSinsun Structural Lag Screws: The Evolution of
Heavy-Duty Fastening
Heavy-Duty Fastening
For decades, using traditional lag bolts meant making too big pilot holes, using more than one tool, and spending hours of extra time. Sinsun has made a difference. Our structural lag screws are designed for professionals who need strength and speed. They have the holding power of 1/2" lag bolts and the speed of modern construction screws.
Construction screws and heavy-duty wood screws are made of hardened, heat-treated steel to ensure that they are as strong and durable as possible. Standard lag bolts need two separate drilling operations: one for the shank and one for the threads. Sinsun structural lag screws, on the other hand, have advanced thread geometry that lets them drive directly into wood with little or no pre-drilling.
Sinsun doesn't just make fasteners. We design connections based on metallurgical science, third-party testing, and international building codes. Our ISO-certified facilities ensure that every structural lag screw meets or exceeds ASTM standards for shear strength, withdrawal resistance, and durability.
| Parameter | Sinsun Structural Lag Screws | Traditional Lag Bolts |
| Installation Speed | 2x faster | Slow—requires multiple drilling steps |
| Pre-Drilling Required | No (most applications) | Yes—pilot hole + clearance hole |
| Material Grade | Heat-treated high-carbon steel | Varies—often generic steel |
| Drive Type | Torx/Hex (no cam-out) | Hex head only |
| Code Compliance | IBC/IRC compliant | Varies by manufacturer |
| Cost | Higher upfront, lower installed cost | Lower material cost, higher labor cost |
| Withdrawal Resistance | 1,200+ lbf (5/16" diameter) | Comparable but requires precise installation |
Real-world example: In ASTM D1761 testing, Sinsun 5/16" x 6" structural lag screws demonstrated 1,326 lbf average ultimate load in wood-to-wood connections—exceeding the performance of many 1/2" diameter lag bolts .
Applications
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Attachment for the deck ledger board
The most important part of any deck's structure is the ledger connections. Sinsun structural lag screws are made to meet or exceed IRC standards for attaching ledgers. Each application has its own engineered fastener patterns and load tables.
Why Sinsun: The full 1/2" diameter bearing surface spreads loads without breaking wood fibers.
Installation: Most pressure-treated wood doesn't need to be pre-drilled.
Note: Always use the manufacturer's engineered patterns, not the IRC lag bolt spacing.
Get A Quick QouteMulti-Ply Beam Assembly
Building up beams from multiple layers of dimensional lumber requires fasteners with exceptional shear strength. Sinsun structural lag screws provide the holding power to keep beam plies acting as a single structural unit.
Recommended size: 3/8" or 1/2" diameter with full thread engagement.
Spacing: Per engineered design or NDS guidelines.
Advantage: Faster than through-bolting, stronger than nails
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Applications
Applications
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Connections for Deck Posts
Wind and use put constant sideways pressure on railings and deck posts. Not only are loose railings annoying, they can also be dangerous. Sinsun structural lag screws make strong connections that don't wobble or pull out.
Recommended: 5/16" x 4" to 6" for connecting posts to beams.
Ultimate load tested: 1,196 lbf (5/16" x 4").
Tip for installation: Use washers to keep heads from sinking into softwoods
Get A Quick QouteAttach the stair stringer
Stair stringers have to stay stiff even when they are carrying heavy loads. Sinsun structural lag screws give stairs the shear strength they need to be safe.
Minimum size: 3/8" wide for stairs in homes.
Placement: 2" from the edges to keep it from splitting.
Load capacity: More than what a typical home needs for live load.
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Applications
Applications
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Heavy construction and timber framing
Sinsun structural lag screws are stronger than regular lag bolts and are easier to install and look better when used on exposed timber frames, barns, and post-frame construction.
Available sizes: up to 8 inches long and 5/8 inches wide.
You can choose between hot-dip galvanized or stainless steel.
Design values: Based on how NDS 2024 does its math.
Get A Quick Qoute| Diameter | Length | Thread Length | Drive Type | Head Style | Ultimate Load (lbf) | Material |
| 5/16" | 2-1/2" | Full thread | T-30 Torx | Flat/Hex | 903 | Heat-treated steel |
| 5/16" | 3-1/8" | Full thread | T-30 Torx | Flat/Hex | 840 | Heat-treated steel |
| 5/16" | 4" | Full thread | T-30 Torx | Flat/Hex | 1,196 | Heat-treated steel |
| 5/16" | 5-1/8" | Full thread | T-30 Torx | Flat/Hex | 1,314 | Heat-treated steel |
| 5/16" | 6" | Full thread | T-30 Torx | Flat/Hex | 1,326 | Heat-treated steel |
| 3/8" | 3" to 8" | Partial thread | T-40 Torx | Hex washer | Available upon request | Heat-treated steel |
| 1/2" | 4" to 10" | Partial thread | 3/4" hex | Hex washer | Available upon request | Heat-treated steel |
| Screw Diameter | Softwood Pilot Hole | Hardwood Pilot Hole | Drill Bit Size |
| 5/16" | Optional | 1/4" (6.4mm) | #7 or 1/4" |
| 3/8" | 9/32" (7.1mm) | 5/16" (8.0mm) | 9/32" or 5/16" |
| 1/2" | 5/16" (8.0mm) | 3/8" (9.5mm) | 5/16" or 3/8" |
Rule of thumb: Pilot hole diameter should be approximately 70-80% of the thread's outer diameter . Depth should be slightly shorter than screw length.
Installation Guide
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Tools Required
Impact driver or electric wrench for production work.Torx bits (T-30, T-40) or socket wrench for hex heads.ANSI-compliant safety glasses.Optional: Drill with pilot bit for hardwood applications. -
Detailed instructions for installation
The wood should be aligned correctly and clamped if necessary to set up the connection.To drive the screw, use a moderate speed T-30 or T-40 bit.Keep the angle at 90° to the wood surface for the strongest hold.Put the seat in the right place. Stop when your head is level with the wood.Check to make sure everything works and there is no visible damage. -
Things to Avoid When Installing
Not pre-drilling in hardwoods can cause splitting and a 40% loss of strength.Wrong bit size: strips drive recess and breaks screw.Too much tightening strips wood threads and makes it less strong.Installation at an angle: up to 30% less resistance to withdrawal.Edge placement too close: wood splits and the connection fails.
Material & Coating Options
Hot-Dip Galvanized (HDG)
Best for: Heavy timber, industrial applications, freshwater exposure
Specification: ASTM A153 Class C
Thickness: 2.0-3.5 mils minimum
Lifespan: 30+ years in typical outdoor applications
304 Stainless Steel
Best for: Coastal applications (up to 1 mile from saltwater)
Corrosion resistance: 1,500+ hours salt spray
Standards: ASTM F1575, ISO 3506
Appearance: Bright, mill finish
316 Stainless Steel (Marine Grade)
Best for: Direct saltwater exposure, docks, marinas
Corrosion resistance: 5,000+ hours salt spray
Molybdenum content: 2-3% for chloride resistance
Applications: Within sight of breaking waves
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Material traceability: There are mill certificates for every batch.
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100% dimensional check—Thread gauges checked to ASME B18.2.1
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Verification of heat treatment—testing the hardness of every production run
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Testing in batches—testing random samples according to ASTM standards
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ISO 9001 facility—certified for quality management
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Made in ISO-certified factories where quality is checked at every step of the process.
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FAQ: Structural Lag Screws
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Q: Do structural screws meet the rules for building?
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Q: Can I use structural screws without drilling first?
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Q: What is the difference between lag screws with full and reduced diameters?
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Q: Are structural screws stronger than lag bolts?
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Q: Are structural screws okay to use to attach ledger boards?
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Q: How much torque do I need to use to install?
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Q: Can I use Sinsun structural screws with pressure-treated wood?
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Q: What kind of drive bit do I need?
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