Screw Fundamentals

While there are many different types of screws, they all have one thing in common: a spiral thread that wraps around the length of the shank, plus a head that contains a drive­—the recess used with a driver to install the screw, below. Screws may seem pretty basic, but understanding the purpose of each feature proves key to selecting the right screw.

Wood screws come in several head styles, with flathead being the most common. Screws with this head style are driven flush with the surface of the wood or countersunk and covered with a plug or plastic cap.

Other common wood-screw head styles include round, oval, panhead, washer-head, and trimhead. Each head style serves a unique purpose.

The dashed line, below, indicates how to determine screw length: Measure from the top face of flathead screws, and from the underside of the head on screws with domed tops.

Flathead: Flat on top with a conical underside, a flathead screw distributes the head's clamping force outward in all directions. Most projects in WOOD® magazine call for this type.

Roundhead & Ovalhead: These screws feature a decorative head that sits above the face of the wood and a flat underside that creates clamping force straight down around the screw.

Panhead & Washer-head: Panhead and washer-head screws use a wide head, flat on the underside, to spread clamping force over a larger area to increase holding power.

Trimhead: Trimhead screws' small diameter makes them less visible and easier to hide with putty or wood filler. These screws are commonly used for joining cabinets and installing decking.

In addition to the head style, screws come in multiple drive styles. Slotted drives have been around the longest. While not as common for project building today, these screws come in handy for period pieces or when you wish to match existing screws in a project.

The Phillips drive improves on slotted screws with a cross-shaped opening that provides more surface area for the driver tip to grip. Phillips drivers are available in different sizes, and you should pair them with the appropriate drive size.

Robertson, or square-drive, screws use a square recess in the head that matches a square-tipped driver to prevent the drive tip from camming out as easily. Like Phillips screws, square drivers come in a few different sizes.

As drill/drivers became more powerful and impact drivers more common, even square drives have grown more susceptible to camming out. Star drives (often referred to as Torx, the name of one brand) combat that with a six-lobed, star-shaped opening. The matching driver, also made in several sizes, engages in these lobes over a much larger surface area to grip into the screw's drive and not cam out.

You may also find a combination of these drives, such as slotted and Phillips combo, or square and Phillips combo.

Slotted: Slotted drives, the oldest style, take a straight screwdriver. Drive these screws by hand becausethe slip-prone straight blade is easily misaligned.

Phillips: Phillips drives use a pointed, crossed driver that aligns and stays engaged better than a slot. Under heavy load, the driver tends to rise and cam out.

Square: A square drive grips the driver tightly and is less likely to cam out than a Phillips drive. Square drives gained favor as power drill/drivers became common.

Star: Star drives provide multiple contact points for the driver to reduce camming out. This drive is more common in construction screws than in wood screws.

Wood screws begin their lives as round wire, and the threads are either cut or rolled. Screws with cut threads start out with wire of a larger diameter, and the threads are cut into the metal. Most wood screws are only partially threaded; the threads don't run the entire length of the shank. This allows the screw to "let go" of the entry piece so it doesn't fight against itself as it draws the workpieces together tightly, the threads biting into only the second piece as the head clamps down on the first. 

This creates the traditional wood screw look. A cut-thread design also explains why two holes are specified for traditional wood screws: a pilot hole that the threaded portion bites into, and a larger shank hole that gives clearance around the unthreaded upper shank.

Screws with rolled threads use a smaller wire diameter. Squeezing the wire between rollers forms the threads. As a result, the shank of the screw is smaller in diameter than the threaded portion. This method costs less to produce because smaller-diameter wire is used to make the screws.

Construction screws and some other types also incorporate special features at the tip, such as cutting flutes that make them self-tapping. 

Cut: Screws with cut threads taper from the underside of the head to the tip, with threads that are almost the same diameter as the shank.

Rolled: Rolled-thread screws have a narrower shank with threads that protrude and don't require a shank hole, just a pilot hole.

Wood screws are sized using a gauge numbering system that ranges from 0000 to 32. The higher the number, the larger the screw—both in shank diameter and in head size. Despite this wide range, sizes 4 through 14 are the ones most commonly available. And for furniture construction, sizes 6, 8, and 10 cover most needs. You can learn more about screw sizing in the chart, below.

Each size of screw is also available in a number of different lengths to accommodate different wood thicknesses. When joining two boards face-to-face, choose a length that will penetrate at least halfway into the thickness of the second piece. When driving a screw into end grain, choose a screw that will penetrate the end grain at least 1-1/2" to 2" to prevent the screw from pulling out.   

Wood screws are typically made of mild steel, but you'll also find them in brass, stainless steel, and silicon bronze. Screws for indoor use may be bare metal or have a coating, such as zinc. Use brass screws—which may be solid brass or brass plated—for decorative applications such as hardware.

Use stainless steel and silicon bronze screws for outdoor applications because they don't rust. (Boatbuilders prefer silicon bronze.) Many outdoor screws are made from mild steel, and then given an anti-corrosion coating that helps them last in damp conditions while keeping the cost lower than silicon bronze or stainless.