Estimate the shear and withdrawal capacity of lag screws in wood based on bolt diameter, embedment, and wood species.
Lag screws (lag bolts) are large-diameter screws used for heavy-duty wood-to-wood and wood-to-steel connections. They provide both shear resistance (perpendicular to the shank) and withdrawal resistance (along the shank axis). Common applications include deck ledgers, beam-to-post connections, and structural hardware mounting.
This calculator estimates the reference design capacities for lag screws in wood based on the NDS (National Design Specification) methodology. The capacity depends on the lag screw diameter, the depth of thread embedment in the main member, the wood species (specific gravity), and the direction of loading relative to grain.
Lag screw design values have been significantly updated in recent NDS editions. The yield-limit equations now govern, and design values may differ from older tables. Always verify critical connections with current NDS values or an engineer.
Accurate calculation of this value helps construction professionals plan projects more effectively, reduce material waste, and ensure compliance with building codes and industry standards.
Knowing lag bolt capacity is essential for designing connections that meet code. This calculator provides quick reference values to help you select the right diameter and quantity for your connection force. This quantitative approach replaces rule-of-thumb estimates with precise calculations, minimizing material waste and reducing the likelihood of costly change orders during construction.
Withdrawal: W = W_ref × Embedment × Diameter (per NDS Table 12.2A) Shear (single): Z depends on bolt diameter, wood SG, side member, and bearing direction Adjusted: Z' = Z × CD × CM × Ct × CΔ × Ceg × Cdi
Result: Z = 340 lbs shear, W = 330 lbs/in withdrawal per bolt
A 3/8″ lag screw with 3″ thread embedment in Douglas Fir-Larch (SG = 0.50): reference shear Z ≈ 340 lbs (wood-to-wood, side member ≥1.5″). Withdrawal W ≈ 330 lbs per inch of thread embedment × diameter.
Lag screw shear (Z) values are determined by the yield-limit equations in NDS Chapter 12. These equations consider four possible failure modes: bearing in the side member, bearing in the main member, fastener bending at the shear plane, and a combination. The minimum value from all modes governs.
Several factors reduce lag screw capacity: end-grain factor (Ceg = 0.75 for withdrawal), group action factor (Cg) for rows of 4+ fasteners, geometry factor (CΔ) for edge/end distance, and wet-service factor (CM). All apply to the reference design value.
Modern structural screws (Simpson SDS, GRK RSS) are replacing traditional lag screws in many applications. They install faster (no pre-drilling in most softwoods), have published design values, and are recognized by the IRC for specific connections like deck ledgers.
IRC Table R507.9.2 specifies 1/2″ diameter lag screws with a minimum 2-3/4″ embedment into the band joist, or 3/8″ through-bolts with nuts and washers. The number and spacing depend on the joist span.
Yes. Washers are required to prevent the lag screw head from pulling into the wood. Use a flat washer sized for the bolt diameter (e.g., 3/4″ washer for a 3/8″ lag).
Yes, but use hot-dipped galvanized or stainless steel lag screws. The ACQ and CA-B preservatives in modern pressure-treated wood corrode plain steel. Check the preservative label for fastener requirements.
Lag screws thread into wood—no nut is needed on the back side. Through-bolts pass completely through both members with a nut and washer on the back. Through-bolts generally have higher shear capacity but require access to both sides.
Minimum thread penetration is 4D (4 times diameter) for shear and withdrawal. For a 1/2″ lag: 4 × 0.5 = 2″ minimum. For full design value, 7–10D penetration is ideal.
Impact drivers work but make it harder to control torque. Over-driving strips the hole and reduces capacity. Use a socket on a slow-speed impact or a ratchet wrench for best results.