Standard deck screws are generally not structural and should not be used for connections that bear weight or resist significant forces. Using the wrong fastener for structural components is a common dilemma that often leads to unsafe structures. While screws are excellent for securing decking boards to joists, relying on them for the main framing elements violates standard building practices. This compromises the safety and longevity of the entire structure.
Understanding Structural Deck Connections
Structural connections are the joints responsible for transferring the deck’s weight down to the ground and resisting lateral forces like wind and seismic activity. These connections include the ledger board attachment to the house, the beam-to-post connection, and the joist-to-beam connection. Fasteners in these joints must resist two primary types of mechanical stress: shear and withdrawal.
Shear force is a side-to-side stress that attempts to slice the fastener in two. This force is applied when the vertical load tries to pull the connected members past each other. Withdrawal, or tension force, is a straight pulling force that tries to yank the fastener out of the wood, such as when wind tries to lift the deck. Structural failure occurs when fasteners lack the necessary strength to resist these forces simultaneously.
The Mechanical Limitations of Deck Screws
Standard deck screws, including those coated for use with treated lumber, are designed to resist withdrawal force, but they have low resistance to shear force. Their small diameter shank and brittle composition make them prone to snapping under side-to-side stress. This brittleness often results from the hardening process used to increase corrosion resistance. When subjected to a lateral load, the screw’s thin body can fracture abruptly, leading to sudden connection failure.
The primary function of a standard deck screw is to hold the decking boards tightly to the joists, resisting the upward pull of warped lumber. These screws excel at this task due to their threads, which grip the wood fibers tightly along their entire length. This design feature does not translate into high shear strength, which requires a thicker, more ductile metal shank that can bend significantly before breaking.
General-purpose deck screws are not engineered or tested to establish specific shear strength ratings, meaning they lack the performance data required by building codes for use in load-bearing applications. Using them in structural connections like beam-to-post joints is a code violation and a significant safety hazard. Standard decking screws remain unsuitable for the demands of a deck’s framing.
Proper Fasteners for Load-Bearing Applications
For connections that bear the deck’s weight, builders must use fasteners specifically engineered for high shear and withdrawal resistance. The traditional structural fasteners are through-bolts, which pass entirely through the wood members and are secured with a washer and nut. Through-bolts offer superior strength for ledger boards, especially in resisting tension and shear forces.
Lag screws, which are heavy-duty screws with a thick shank and hex head, are also approved for structural use when installed with a specific pattern and diameter, usually 1/2-inch. Newer alternatives include specialized structural wood screws that are significantly thicker and stronger than standard deck screws, often featuring a wider washer head and an ICC-ES (International Code Council Evaluation Service) report that specifies their approved load ratings and installation requirements. These high-performance structural screws are designed to replace traditional lag screws. Regardless of the fastener chosen, it must be used exactly according to the manufacturer’s specifications and local building code to ensure the deck’s stability and safety.