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Calculate lag screw load capacity, pilot hole size, and installation requirements. Get withdrawal capacity, lateral capacity, minimum penetration depth, and spacing requirements based on NDS standards.
Minimum required: 5.00 in
Per screw: 1594 lbs
Withdrawal (axial):
p = 16,600 × SG^1.84 × d^0.75
Lateral (shear):
p = 6,600 × SG^1.2 × d
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Lag screws (also called lag bolts) are large fasteners designed for heavy-duty wood connections. They have a hex head and wood screw threads, providing excellent holding power in wood.
Lag screws are commonly used in:
Load Capacity
High withdrawal and lateral load capacity
Material
Usually galvanized or stainless steel
Head Type
Hex head for wrench installation
Diameter Range
1/4" to 1" or larger
| Nominal Diameter | Decimal Diameter | Pilot Hole Diameter | Pilot Hole (mm) |
|---|---|---|---|
| 1/4" | 0.2500 in | 0.1875 in | 4.8 mm |
| 5/16" | 0.3125 in | 0.2500 in | 6.3 mm |
| 3/8" | 0.3750 in | 0.3125 in | 7.9 mm |
| 7/16" | 0.4375 in | 0.3594 in | 9.1 mm |
| 1/2" | 0.5000 in | 0.4375 in | 11.1 mm |
| 5/8" | 0.6250 in | 0.5625 in | 14.3 mm |
| 3/4" | 0.7500 in | 0.6875 in | 17.5 mm |
| 7/8" | 0.8750 in | 0.8125 in | 20.6 mm |
| 1" | 1.0000 in | 0.9375 in | 23.8 mm |
Pilot hole diameters are recommended for hardwood. For softwood, use 50-75% of the lag screw diameter.
Inspect wood for knots, checks, or defects. Avoid placing fasteners directly over or adjacent to large knots.
Mark fastener locations, maintaining minimum spacing of 4d (where d = screw diameter) and 1.5d from edges.
Drill pilot hole to required depth (minimum 10d). Use proper pilot hole diameter for your wood species and screw size.
Place washer on screw before installation. Use washer to distribute load and prevent head sinking into wood.
Use wrench or socket on hex head. Turn slowly to avoid stripping and to ensure proper seating. Do not overtighten.
Ensure full penetration depth is achieved for design capacity. Minimum penetration = 10 × diameter.
Lag screws have wood screw threads and don't require nuts, while bolts need washers and nuts on both sides. Lag screws are simpler to install but have less load capacity than properly installed bolts.
For hardwood, use 75% of the screw diameter. For softwood, use 50% of the screw diameter. This prevents splitting and ensures proper threading into the wood.
Use galvanized steel, stainless steel, or hot-dip galvanized for outdoor/wet environments. For dry indoor use, standard carbon steel is acceptable. Always match fastener material to expected conditions.
Yes, but capacity is lower due to reduced specific gravity. Always consult NDS standards and reduce allowable loads according to wood species. Penetration depth is especially critical in softwood.
Minimum penetration is 10 times the screw diameter. For 1/2" screw, that's 5 inches into the wood member (not including pilot hole in outer member). Full design capacity requires this minimum.
Total capacity = per-screw capacity × number of screws. However, NDS standards require minimum spacing and edge distances be maintained. Consult structural engineer for layout.
Yes, always use a washer. The washer distributes the load over a larger area, prevents the hex head from sinking into the wood, and significantly improves holding power.
Withdrawal capacity is when the screw is pulled straight out. Lateral capacity is when the screw resists perpendicular forces (shear). Lateral capacity is typically much higher than withdrawal capacity.
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