Fastener Guide: Screws, Nails, and Bolts — When to Use Each
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Fasteners hold things together mechanically — no glue, no welding, no waiting for cure. Each type has strengths that make it the right choice for specific situations and weaknesses that make it wrong for others. Choosing the correct fastener is not about preference; it is about physics. Nails resist shear. Screws resist withdrawal. Bolts handle both under high loads.
When to Use Nails
Nails excel at resisting shear forces — loads that try to slide the joined pieces past each other. This is why building codes specify nails for structural framing: wall sheathing, joist connections, rafter ties, and subfloor attachment. A nail bends under shear load but does not snap. A screw in the same situation can shear clean through.
Common nails have a thick shank and a wide head for maximum holding power in framing. Finish nails have a small head that can be set below the surface and filled. Brad nails are thin wire nails for trim and molding where splitting is a concern.
Nail sizing uses the penny (d) system: 8d (2.5 inches), 10d (3 inches), 16d (3.5 inches). For framing connections, 16d common nails are the standard. For subflooring, 8d ring-shank nails prevent squeaks because the rings resist withdrawal better than smooth shanks.
Ring-shank and spiral-shank nails have dramatically higher withdrawal resistance than smooth nails. The textured shank grips wood fibers and resists being pulled straight out. Use these anywhere withdrawal is a concern — subfloors, siding, and decking.
When to Use Screws
Screws excel at resisting withdrawal forces — loads that try to pull the fastener straight out. This makes them the right choice for cabinets, shelving, deck boards, hinges, and any connection that could be pulled apart rather than slid apart.
Screws also allow disassembly. If you might need to take the joint apart later (access panels, removable trim, adjustable components), screws are the only practical choice among common fasteners.
Wood screws come in numbered gauges (#6 through #14 most commonly) that indicate shank thickness, and lengths from 1/2 inch to 6 inches. A #8 x 1-1/4 inch screw handles most general woodworking and interior carpentry. A #10 x 3 inch screw handles deck boards and heavier structural connections.
Self-drilling screws (deck screws, drywall screws) have a sharp point that cuts its own pilot hole in softwood. For hardwood, pre-drill pilot holes to prevent splitting. The pilot hole should match the shank diameter (the unthreaded portion of the screw), and the clearance hole in the outer piece should allow the threads to pull the joint tight.
When to Use Bolts
Bolts with nuts clamp two pieces together with controllable force. Unlike screws that thread into wood, bolts pass through both pieces and are secured by a nut on the back side. This creates the strongest mechanical joint because the clamping force is distributed by washers on both faces.
Carriage bolts have a smooth dome head and a square shoulder that locks into a round hole. Once inserted, they cannot be turned from the front — you tighten from the nut side only. Standard for deck railings, playground equipment, and any connection where a protruding bolt head is undesirable or where tampering should be difficult.
Lag bolts (lag screws) are heavy-duty screws with a hex head driven by a wrench. They substitute for through-bolts when you cannot access the back side. Ledger boards, beam connections, and heavy shelving brackets commonly use lag bolts. Always pre-drill the pilot hole.
Machine screws and bolts use standard thread sizes (1/4-20, 3/8-16, etc.) and require tapped holes or nuts. They are the standard for metal-to-metal connections, equipment mounting, and any application requiring precise, repeatable clamping force.
Material and Coating Selection
Bare steel is the cheapest and strongest option for interior use. It rusts immediately outdoors. Never use bare steel fasteners in exterior or wet applications.
Galvanized (zinc-coated) fasteners resist corrosion for outdoor use. Hot-dipped galvanized has a thicker coating than electroplated and lasts longer. Required for deck framing, exterior trim, and outdoor structural connections. Check code requirements — some pressure-treated lumber requires specific galvanization levels or stainless steel.
Stainless steel resists corrosion without a coating. More expensive than galvanized but lasts longer in salt air, constant moisture, and contact with ACQ-treated lumber (which corrodes zinc). Required for coastal and marine applications.
Never mix dissimilar metals. A stainless screw in aluminum, or a galvanized nail near copper flashing, creates galvanic corrosion that eats one of the metals. Use matching materials or isolate with plastic washers and sealant.
Frequently Asked Questions
Why do building codes require nails instead of screws for framing?
Nails bend under shear loads; screws snap. In a structural connection like a joist hanger or shear wall, the forces try to slide the connected members past each other. A nail deforms and absorbs this energy. A screw, being hardened and brittle, shears off suddenly with no warning. Building codes specify nails for connections where ductile failure (bending) is safer than brittle failure (snapping).
Do I need to pre-drill for screws?
In softwood (pine, cedar, fir), self-drilling screws typically do not require pre-drilling for #8 and smaller. In hardwood (oak, maple, walnut), always pre-drill — the dense grain splits without a pilot hole. Near the end of any board, pre-drill regardless of wood species to prevent splitting. The pilot hole should be slightly smaller than the screw's root diameter.
What length fastener do I need?
The fastener should penetrate the receiving piece by at least 1 to 1.5 times the thickness of the piece being attached. Screwing a 3/4-inch board to a framing member needs a screw that passes through the 3/4-inch board plus penetrates at least 1 inch into the framing — so a 2-inch screw minimum. For structural connections, check code tables for specific requirements.