Screws are the most common mechanical fasteners in construction. Unlike nails, which hold by friction, screws carry ridges called threads that bite into the hole and pull the fastener deeper as it turns. That grip makes a joint stronger under tension and easier to take apart later. Before you buy, review the basics of structural screws, code compliance, and fastener selection, because the gap between a general-purpose screw and a rated structural fastener is often the line between a connection that holds and one that fails.
Every screw is built from five parts: the drive, where the driver bit sits; the head, which bears on the surface; the shank between head and threads; the threads themselves; and the tip, which starts the hole. Screws work in wood, drywall, metal, concrete, brick, and machinery, and each material favors a different thread pitch, head shape, and coating.
How a Screw Is Built: Anatomy and Performance
The five parts work together, and small changes to any one change how the fastener behaves. The head controls seating and bearing surface, the shank length sets thread engagement in the second board, and the tip decides whether the screw starts itself or needs a pilot hole. When loads get heavy, a direct comparison of structural screws vs. lag bolts for heavy-duty construction connections explains when each fastener earns its place.
The Drive Dictates the Tool
The drive is the shape cut into the head and determines which bit you need. Phillips drives dominate because they are inexpensive and accept a bit from any angle, but they cam out under load. Square (Robertson) drives grip the bit so well that one-handed driving works. Torx drives transfer the most torque with almost no cam-out, so structural and deck screws use them. Hex drives take a socket or nut driver and appear on lag bolts and masonry screws.
Why Cam-Out Matters
Cam-out is the bit slipping out of the drive under load, rounding the recess and stripping the head. Phillips drives cam out before the bit breaks; Torx and square drives do not.
- Phillips: standard household screws, prone to cam-out under load
- Square (Robertson): strong bit grip, used in framing
- Torx: highest torque transfer, common on structural screws
- Hex: driven with a socket, standard on lag bolts
- Slotted: old flathead style, easily damaged
Threads: Coarse or Fine
Thread pitch is the spacing between ridges and sets how the screw grabs. Coarse threads remove more material per turn and hold well in softwood and drywall. Fine threads have a tighter pitch that grips denser materials like hardwood and metal. A drywall screw for wood studs has coarse threads; for metal studs it uses fine threads that tap into thin steel.
| Part | What It Does | Why It Matters |
|---|---|---|
| Drive | Recess for the driver bit | Sets cam-out and tool needed |
| Head | Top that bears on the surface | Stops pull-through, sets depth |
| Shank | Smooth section between head and threads | Lets the screw pull boards together |
| Threads | Ridged spiral around the shaft | Grip the hole, convert rotation to pull |
| Tip | Leading end of the screw | Self-starting or needs a pilot hole |
Wood Screws and Drywall Screws for Framing and Finish Work
The two families homeowners handle most are wood screws and drywall screws. Both are engineered for wood-frame construction, but differently. A wood screw has a smooth shank and threads only on the lower half, so the threaded end drags the bottom board up and clamps the joint. Drywall screws are hardened, brittle, and threaded nearly full length, so they sink a panel without cracking it but are a poor choice for load-bearing connections. For structural work like a deck ledger, the field tests behind lag screws vs. structural screws for ledgers have changed how framers spec the connection.
Drywall Screws: Coarse Thread or Fine Thread
Drywall screws come in two thread pitches. Coarse-thread screws are meant for wood studs, the aggressive thread biting into framing lumber and setting quickly. Fine-thread screws are for metal studs, the tighter pitch tapping into thin steel without splitting the flange. Both have a bugle head that recesses flush with the panel, and both are heat-treated, so they are hard but brittle and snap under shear loads.
Wood Screws: From Trim to Heavy Framing
Traditional wood screws have a tapered body, smooth shank, and sharp point, in slotted, Phillips, square, or Torx drives. They are sold by gauge and length, from #4 for small hardware up to #14 for heavy wood connections. Countersinking the head flush is standard for trim and furniture work, and a pilot hole prevents splitting in hardwoods and near board ends.
- Drill a pilot hole about 75 percent of the screw diameter in hard wood
- Countersink the mouth of the hole so the head seats flush
- Drive at moderate speed and stop when the head is flush
- Seal exterior heads after driving
Sheet Metal Screws, Self-Tapping Screws, and Machine Screws
Metal fastening is different. Sheet metal screws are fully threaded with sharp, widely spaced threads that cut into thin steel and ductwork. Self-tapping screws form their own threads in a pre-drilled hole; self-drilling screws carry a drill-point tip that makes the hole and taps the threads in one pass. Vibration is the enemy of threaded fasteners in machinery, so check the guidance on whether to glue screws or use thread locking compounds before relying on a plain screw in a high-shake assembly.
Self-Tapping vs. Self-Drilling
The two terms are often mixed up. A self-tapping screw has a sharp thread that cuts mating threads as it enters an existing pilot hole. A self-drilling screw adds a flute-tipped point that drills its own hole in thin metal. For steel up to about 1/8 inch thick, self-drilling screws save a step.
Machine Screws and Bolts
Machine screws have straight-sided threads with a constant diameter and no taper, so they cannot cut their own threads. They are driven into a tapped hole or paired with a nut and measured by diameter and threads per inch. They show up in hinges, brackets, and equipment mounts, where a nut on the far side carries the load.
Masonry Screws, Lag Bolts, and Structural Screws for Heavy Loads
When the base material is concrete, brick, or block, ordinary wood screws will not bite. Masonry screws are hardened and feature a sharp thread that cuts into a drilled hole, plus a coating that resists concrete’s alkaline chemistry. For wood-to-wood connections carrying real loads, lag bolts have been standard for a century, but structural screws now compete head to head. The engineering case, including choosing the right fastener for heavy-duty connections, rests on published load tables rather than habit.
Concrete and Masonry Screws
Masonry screws tap their own threads into a pilot hole drilled with a carbide bit. The hole size must match the screw, and the hole should run about 1/4 inch deeper than the embedment depth so dust settles out of the bottom. Minimum embedment in concrete is usually 1 inch, and never drive the screw into an oversized hole, because the threads need solid material to grip.
Lag Bolts: The Traditional Heavy-Duty Wood Fastener
A lag bolt is a large screw with a hex head and coarse threads, driven into a pilot hole with a wrench. It is sized by diameter and length, from 1/4 inch to 3/4 inch or larger. Because the shank is smooth, a lag bolt pulls members together the way a wood screw does, and it needs a properly sized pilot hole to avoid splitting.
Structural Screws: The Modern Alternative
Structural screws are heat-treated, code-rated fasteners with Torx or hex heads, designed to replace lag bolts in many connections without pre-drilling. They are tested to published load values, marked with approvals, and accepted by code for ledgers, posts, and railing attachments.
When Structural Screws Beat Lag Bolts
Structural screws win on installation speed, because most skip the pilot hole and wrench, and on the withdrawal and shear values in engineered tables. Lag bolts still hold an edge in very large diameters and where a through-bolt with a nut is impractical.
- Drill with a carbide-tipped bit sized to the screw
- Blow or brush the dust out of the hole
- Drive the screw until the head seats, without over-torquing
- Do not reuse the hole; move over at least 1 inch
Deck Screws, Exterior Coatings, and Corrosion Resistance
Outside, corrosion decides how long a fastener lasts. Deck screws are wood screws redesigned for outdoor service, with a self-tapping tip, coarse threads, and a corrosion-resistant finish. Plain zinc is fine indoors, galvanized or ceramic-coated screws resist moisture longer, and stainless steel is the standard near salt water. Deck screws carry a nib under the head that countersinks itself without splitting the board, so they suit decks and fences. For a wider reference list, a roundup of the types of screws used around the house covers everything from finishing nails to lag bolts.
Coating Grades and Where Each One Fits
Coating choice follows exposure. Standard zinc is a thin sacrificial layer for dry indoor work. Hot-dip galvanizing adds a thick coating for pressure-treated lumber, though zinc can react with the copper in it. Ceramic and polymer coatings slide through the wood and add a moisture barrier. Stainless steel, in 304 or 316 grades, shrugs off coastal salt air for decades.
Deck Screw Design Details
Deck screws are engineered around one job: holding boards down without splitting. The sharp point starts in the board face, the self-countersinking nib reams a seat for the head, and the coarse threads pull the board tight to the joist. Most are #8 or #10 gauge, and the length should sink at least 1 inch into the joist.
Fastener Selection in Practice: From Thread Lube to Structural Connections
With the families mapped, selection comes down to four questions: the material, the load, the exposure, and the driving tool. Installation details matter, and one of the most overlooked is lubrication: greasing screws with the right lubricants for easier and stronger fastening cuts driving torque and reduces stripped heads.
Specialty Screws That Skip the Screwdriver
A few fastener families never see a driver bit. Hammer-drive screws are hardened pins driven with a hammer into masonry or steel. Double-ended screws have threads at both ends and join two parts, as in furniture connectors. Hanger bolts are half wood screw and half machine thread, mounting legs and hardware where a nut engages on the far side.
Putting Fastener Choice to Work
The proof of a good fastener choice is a joint that survives weather and time. A freestanding pergola is a good example: the frame resists wind uplift through structural screws and metal connectors, and the build shows how fastener selection follows the load path from post base to beam top.