Although screws are simple devices, subtle variations in their design affect how they drive, how they hold, what they hold and where they work best. Each type is engineered for specific applications — even the "all-purpose" ones — and should be chosen for more than just its size. Among wood screws alone, there are hundreds of options, each with features that determine their strengths in fastening.
Who hasn't cursed at a wood screw because it stripped out, cammed out, busted, corroded, failed to hold or just wouldn't drive? Often these failures are due to the wrong choice of thread sizeand configuration or the head type, the screw's metal type or coating or its tip shape. Problems can also be due to misapplication (the wrong size pilot hole or driver bit, etc.). To be screw savvy, consider these fastener features.
Anatomy — Despite the plethora of differences in their features, all wood screws have these parts.
Profiles — Flathead screws are best for counterbore and countersink applications; washer-head, panhead, oval-head and hex-head screws typically remain above the wood surface.
Size — Wood screws (for woodworking and cabinet and furniture construction) can be as short as 1/4 in. or as long as 3-1/2 in. Their gauge is identified by the numbers zero through 12 (skipping 11), but 6, 8 and 10 are most common. Both of these screws are considered 1-1/2 in. long, but panhead screws are measured from the underside because they are intended to sit above the wood surface.
Driver type — The first wood screws had slotted heads to work with flat-tip drivers. To prevent a screwdriver from slipping out of the slot, the Robertson (square-drive) head was invented in 1908. Square-drive heads make it easier to seat the driver, although they do not allow you to fudge on the bit size; Robertson screws specify (and often supply) a certain size bit. Phillips head screws are somewhat forgiving on the driver size, but they turn much better when the bit fits. Star-drive screws most easily catch and hold a bit, saving time on production work.
Custom curves — To again improve screw technology, designers have tweaked the underside of the screwhead. Some, like the decking screw above, are bugle-shape (rather than a straight bevel), which helps prevent blowout or bubbling of the wood. Special nibs (or saw teeth) in the underside of a screw (center) help the head cut into the wood to automatically create a clean countersink. A flat underside (right) helps to distribute the screw's grip over a larger area.
Width — Comparable to a finishing nail, a trim-head screw (right) is an option when you want to create an invisible connection without having to add plugs to hide the fastener. It leaves an easy-to-fill hole in the wood, but it does not offer the holding power of a larger head size (left). You would choose the latter when hanging cabinets, for example.
Threads — Wood screws vary greatly in thread design, especially in depth and spacing. The differences are significant to each application: Coarse (larger) threads are made for use in softwood; fine (smaller and more closely spaced) threads work best in hardwood. Screws with extra-coarse threads are designed for use with particleboard. Two other variations are high-low threads (found on general-purpose screws for most types of wood) and serrated threads, such as those on Spax and Quick-Screws. These screws' teeth help to cut into the wood, so they are easier to drive.
Shank — The root of the shank (and the wood type) determine the size of the drill bit you use for boring a pilot hole. If you're working with softwood, choose a bit slightly smaller than the root; for hardwood, match the pilot bit to the size of the root. On some wood screws, the thread starts at the neck, but most often, the top one-third of the shank is smooth. This design prevents the screw from biting into the first layer of wood as it pulls two pieces together (cross threading).
Tips — The shape of the pointed end is also engineered for each fastener type. Some are rounded; others have a longer, narrower shape. Auger points (called self-tapping) save you from having to drill a pilot hole, a convenience for DIYers and a major boon to production crews. These drilling tips actually clean out the hole as they drive in, rather than wedging their way into the material.
Metals — When shopping for screws, keep in mind that the metal they're made of affects application and performance. For example, if you want the look of a brass screw but need a strong bond, choose steel with a brass coating. (And when real brass is used for a decorative connector, remember that it is soft and requires a pilot hole.) TIP: When using brass screws, first "prime" the hole with a steel screw.
For exterior projects, stainless steel is recommended because it stays strong and doesn't stain. Zincaluminum can also be used outdoors, but zinc screws are only for interior applications. For the hardest metal, look for heat-treated screws.
Use coated screws (such as powdercoated) if you're making something with treated wood, which is corrosive to steel. Besides providing color, coatings on screws can prevent corrosion or (in the case of lubricated finishes) make installation easier. Lacquer is used on brass to prevent tarnishing.
Boring, boring, boring The terms pilot hole, clearance hole, countersink and counterbore are often used interchangeably, even though they refer to four boring steps, each with a very different purpose (see illustration above).
- Pilot: A hole (no larger than the root diameter) made to guide the screw
- Clearance: A wider hole bored only in the top piece of wood to prevent the threads from biting into the first layer as it's being drawn tight to the adjoining wood
- Countersink: A beveled hole large enough to receive the head of a flathead screw so it is flush with the wood or slightly below the surface
- Counterbore: A straight-side hole deep enough to allow for insertion of a wood plug to cover the head of the screw
Manufacturers offer new designs and specialty screws to improve performance and application. Pocket-hole screws were created with self-tapping points, unthreaded necks and wide, flat heads for greater holding power in pocket joinery. QuickScrews' Spiral Point Funnel Head Screw — a recent innovation — can be driven into medium-density fiberboard or melamine without requiring a pilot or clearance hole.