Fastener Guide: Screws, Nails, and Bolts - When to Use Each
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Fasteners hold things together mechanically with no glue, no welding, and no waiting for cure times. 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. This guide explains when to use each one and how to pick the right size, material, and coating for every project.
When to Use Nails
Nails excel at resisting shear forces, which are 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 because hardened steel is brittle under lateral stress.
Common nails have a thick shank and a wide, flat head for maximum holding power in framing. The thick shank resists bending and the wide head distributes load across the wood surface. These are the nails you see in wall framing, roof sheathing, and structural connections. They are not pretty, but they are strong.
Finish nails have a small, barrel-shaped head that can be set below the wood surface with a nail set and then filled with wood putty. Use them for interior trim, baseboard, window casing, and any visible woodwork where nail heads should disappear. They are available in 15-gauge (thicker, stronger) and 16-gauge (thinner, less splitting) for pneumatic nailers.
Brad nails are thin 18-gauge wire nails for lightweight trim, small moldings, and assemblies where splitting is a concern. They leave a tiny hole that barely needs filling. Brad nailers are the standard tool for attaching thin stock, shoe molding, and decorative trim.
Nail sizing uses the penny (d) system: 8d nails are 2.5 inches long, 10d are 3 inches, and 16d are 3.5 inches. For framing connections, 16d common nails are the standard specified by building codes. For subflooring, 8d ring-shank nails prevent squeaks because the rings mechanically grip the wood fibers and resist being pulled back out.
Ring-shank and spiral-shank nails have dramatically higher withdrawal resistance than smooth-shank nails. The textured shank grips wood fibers and resists being pulled straight out. Use ring-shank nails anywhere withdrawal is a concern: subfloors, siding, roofing, and decking. A ring-shank nail can have three to four times the withdrawal resistance of a smooth nail of the same size.
When to Use Screws
Screws excel at resisting withdrawal forces, which are loads that try to pull the fastener straight out of the material. This makes them the right choice for cabinets, shelving, deck boards, hinges, hardware mounting, 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 for access panels, removable trim, adjustable components, or future repairs, screws are the only practical choice among common fasteners. Nails damage wood on removal. Bolts require back-side access. Screws drive in and back out cleanly.
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 tasks: mounting hardware, assembling jigs, attaching brackets. A #10 x 3 inch screw handles deck boards, heavy shelving, and more substantial structural connections.
Drive types matter for practical use. Phillips (#2 is the most common) is universal but cams out under high torque, stripping the head. Square drive (Robertson) resists cam-out and holds the screw on the bit, making one-handed driving easy. Star drive (Torx) handles the highest torque without stripping. For deck screws and structural screws, star drive or square drive is far less frustrating than Phillips.
Self-drilling screws (deck screws, drywall screws, general-purpose construction screws) have a sharp point that cuts its own pilot hole in softwood. For hardwood (oak, maple, walnut), always pre-drill pilot holes to prevent splitting. The pilot hole should match the screw's root diameter (the solid shaft between the threads), and the clearance hole in the outer piece should allow the threads to pull the joint tight without binding.
Drywall screws have a bugle-shaped head that seats flush with the drywall paper face without tearing it. They are designed specifically for attaching drywall to studs and are not appropriate for structural wood-to-wood connections. The hardened steel is brittle and snaps under shear loads. Never substitute drywall screws for wood screws or structural screws.
When to Use Bolts
Bolts with nuts clamp two pieces together with controllable, measurable force. Unlike screws that thread into wood fibers, 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 evenly by washers on both faces. Bolts are the standard for heavy structural connections, equipment mounting, and joints that must withstand high loads from multiple directions.
Carriage bolts have a smooth dome head and a square shoulder that locks into a round hole in wood. Once inserted, they cannot be turned from the front side. You tighten from the nut side only. This makes them standard for deck railings, playground equipment, fence rails, and any connection where a protruding hex head is undesirable or where tampering should be difficult. The smooth head also eliminates snagging hazards.
Lag bolts (lag screws) are heavy-duty screws with a hex head driven by a wrench or socket. They substitute for through-bolts when you cannot access the back side of the connection. Ledger boards attached to house framing, beam-to-post connections, and heavy shelving brackets commonly use lag bolts. Always pre-drill the pilot hole. Driving a lag bolt without a pilot hole splits the wood and reduces holding strength.
Machine screws and bolts use standard thread sizes (1/4-20, 3/8-16, 1/2-13, and so on) and require either tapped holes or nuts. They are the standard for metal-to-metal connections, equipment mounting, electrical panel covers, and any application requiring precise, repeatable clamping force. The first number is the diameter, the second is threads per inch. Coarse thread (fewer threads per inch) is stronger; fine thread (more threads per inch) allows more precise adjustment.
Material and Coating Selection
The environment where the fastener lives determines what material it needs to be made from. Choosing wrong here leads to corrosion failure, staining, and structural weakness.
Bare steel is the cheapest and strongest option for interior use where moisture is not a factor. It rusts immediately in outdoor or damp conditions. Never use bare steel fasteners in exterior, marine, or wet applications.
Galvanized (zinc-coated) fasteners resist corrosion for outdoor use. Hot-dipped galvanized has a thicker, rougher zinc coating than electroplated (also called bright zinc or electro-galvanized) and lasts significantly longer. Hot-dipped galvanized fasteners are required by code for deck framing, exterior structural connections, and outdoor trim. Check local building codes, because some pressure-treated lumber (ACQ treatment) requires specific galvanization levels or stainless steel due to the corrosive chemicals in the treatment.
Stainless steel resists corrosion without any coating. It is more expensive than galvanized but lasts far longer in salt air, constant moisture, and direct contact with ACQ-treated lumber, which corrodes zinc coatings. Stainless steel is required for coastal construction, marine applications, and any environment where long-term corrosion resistance is critical. Type 304 stainless is adequate for most residential use. Type 316 stainless is the marine grade for saltwater exposure.
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 over time. The less noble metal dissolves preferentially, creating holes and weakened joints. Use matching fastener and material metals, or isolate dissimilar metals with plastic washers, nylon sleeves, or sealant.
Fastener Sizing Rules
Getting the length right is straightforward. The fastener should penetrate the receiving piece (the one you are fastening into) by at least 1 to 1.5 times the thickness of the piece being attached. If you are screwing a 3/4-inch board to a framing member, the screw needs to pass through the 3/4-inch board and then penetrate at least 1 inch into the framing. That means a 2-inch screw at minimum.
For nails in framing, the penetration requirement is typically at least 1.5 inches into the receiving member. A 16d (3.5-inch) common nail through a double top plate penetrates 2 inches into the stud below, well above the minimum.
For bolts, the nut should engage the full thread length. A bolt that barely catches the nut has almost no clamping strength. Use washers on both sides to distribute force and prevent the bolt head and nut from pulling into the wood.
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 through ductile bending. A screw, being hardened and brittle, shears off suddenly with no warning. Building codes specify nails for connections where ductile failure (gradual bending) is safer than brittle failure (sudden 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 gauges. In hardwood (oak, maple, walnut), always pre-drill. The dense grain splits without a pilot hole, and the screw may snap from the resistance. 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, always check code tables for specific requirements that may exceed this general rule.