Load Distribution in Wood Connections: How Lag Screws and Through Bolts Actually Transfer Weight

A homeowner building a deck ledger board or a pergola beam faces a structural fork in the road that dictates how the entire assembly handles weight. Driving a lag screw into a post transfers the load through the friction of steel threads gripping wood fibers, while drilling a hole and passing a through bolt transfers that same load by compressing the wood directly against the smooth steel shank.[6]

That mechanical difference determines whether a backyard structure survives a heavy winter snow load or slowly pulls away from its supports. The American Wood Council’s National Design Specification (NDS) governs these connections, and the math separates fasteners into two distinct categories: those that rely on bearing, and those that rely on withdrawal resistance.[3][6]

Through bolts represent the bearing category. When a 1/2-inch galvanized steel bolt passes entirely through two pieces of lumber and is secured with a nut and washer, the wood itself acts as a crushing boundary. The downward load pushes the steel shank against the wood fibers inside the hole, distributing the weight evenly across the entire depth of the timber.[1]

The USDA Forest Service details this mechanism in Chapter 8 of its Wood Handbook. "Bolted connections are typically designed based on the bearing strength of the wood under the bolt shank," the agency notes, explaining that the diameter of the bolt and the density of the wood species dictate the joint's absolute failure point.[2]

To achieve that bearing strength, the installation must be precise. A through bolt requires a clearance hole drilled exactly 1/16-inch to 1/32-inch larger than the bolt diameter. If the hole is too tight, the wood splits during seasonal expansion; if it is too loose, the joint slips before the bolt engages the wood fibers, transferring a sudden shock load to the assembly.[1][2]

Lag screws operate on a completely different physical principle. Instead of passing through the entire assembly, a lag screw bites into the main member. The unthreaded shank near the head provides some bearing resistance, but the primary holding power comes from the friction of the threads cutting into the wood grain.[4][5]

Component Solutions Group, an industrial fastener supplier, highlighted this distinction in a December 2025 technical bulletin. They noted that lag screws are engineered specifically for "blind" connections—situations where a builder cannot access the back side of the joint to attach a nut and washer.[4]

The engineering math behind lag screws is notoriously complex. In a July 2025 design guide, the engineering platform CalcBook explained the underlying mechanics. "The lateral load capacity of a lag screw is determined by the yield limit equations, which account for the bending yield strength of the fastener and the dowel bearing strength of the wood members," the authors wrote.[5]

Those yield limit equations, formalized in the 2015 NDS update published by the American Society of Civil Engineers (ASCE), reveal a significant performance gap. When attaching a standard 1.5-inch thick side member to a 3.5-inch thick main member, a 1/2-inch through bolt provides approximately 40 percent more lateral shear resistance than a 1/2-inch lag screw of equivalent length.[3][6]

That 40 percent deficit exists because the lag screw does not engage the full depth of the main member. To prevent the screw from snapping during installation, builders must drill two distinct pilot holes: a clearance hole for the unthreaded shank, and a smaller pilot hole for the threaded portion.[5][6]

If the threaded pilot hole is too large, the friction is lost, and the screw pulls out under tension. If it is too small, the torque required to drive the screw exceeds the torsional strength of the steel, shearing the head off before the joint is tight. The NDS mandates that the threaded portion must penetrate the main member by at least six to eight times the diameter of the screw to achieve its rated capacity.[3][5]

ConnForge Knowledge, a structural engineering resource, emphasized in a May 2026 brief that the choice between the two fasteners often comes down to the direction of the load. Through bolts excel at resisting shear forces—weight pushing straight down, like gravity acting on a floor joist. Lag screws are highly susceptible to withdrawal forces—weight pulling the fastener straight out of the wood, like wind lifting a roof truss.[1]

For a homeowner replacing a deck ledger board, this physics lesson has immediate financial consequences. Building codes increasingly restrict the use of lag screws in ledger attachments due to historical failures where water intrusion rotted the thread friction points. Through bolts, backed by heavy steel washers, maintain their bearing strength even if the surface wood begins to soften.[2][6]

The transition from lag screws to through bolts in critical residential connections reflects a broader shift in structural engineering. As the 2015 NDS incorporated new standards for cross-laminated timber and engineered wood products, the reliance on thread friction gave way to the predictable, quantifiable bearing strength of smooth steel shanks.[3]