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Wood Screws
Wood screws are used to fasten wood to wood or attach objects to wood and offer a pleasing, finished appearance. They can be removed more easily than nails so entire assemblies can be dismantled or fixtures dismounted with relative speed. Although lateral resistance (shear) may only be slightly better than similarly sized driven nails, wood screws offer greater withdrawal resistance. For example, a 20d (20 penny) common nail (0.192" diameter) has a withdrawal load design value of 42 lbs./in. of penetration in the side grain of wood having a specific gravity of 0.48 (such as Douglas Fir, South). In comparison, a #10 wood screw (0.190" diameter) has a withdrawal design value of 124 lbs./in. of thread penetration under the same conditions.
THREADS and POINT
Wood screws and have sharp crested, coarse, widely spaced threads (see Table 1 for the number of threads per inch for each screw size). Being a thread-forming-type screw, they create their own mating threads during installation—no nut is needed. A gimlet point allows wood screws to start threading into wood easily: "A gimlet point is a threaded cone point usually having a point angle of 45 to 50 degrees." Threading is standard right-hand, so turn the screw clockwise to install.
Screws with cut threads are tapered and about two-thirds of their length is threaded—the remaining one-third is an unthreaded section of the shank adjacent to the head, called the body. Wood screws with rolled threads are not tapered and "at least four times the basic screw diameter or two-thirds of the nominal screw length, whichever is greater" is threaded. For screws with cut threads, the thread crest diameter is no larger than the body diameter, and the body diameter is larger than same-sized screws with rolled threads. For screws with rolled threads, the thread crest diameter is larger than the body diameter, and the body diameter is smaller than same-sized screws with cut threads. The unthreaded body is said to help prevent two wood pieces from separating as they are screwed together.
SIZES
Wood screws range in diameter from #0 to #24; the larger the number, the larger the size and, unlike wire gauge, #0 is the smallest screw size. Sizes from #2 to #14 are commonly available. The number size does not relate to any measurement—use Table 1 to determine basic diameter in inches.
LENGTHS
Lengths range from 1/4" to 6"; 5" or so is the longest typically found and larger screw sizes are offered in the longer lengths. Sizes less than 1" are in 1/8" increments; lengths from 1" to 3" are in 1/4" steps; and screws longer than 3" increase in length by 1/2". Wood screws are measured like other screws: "The length of a headed fastener is the distance from the intersection of the largest diameter of the head with the bearing surface to the extreme point..." (the bearing surface is the part of the head that contacts the mating surface). For flat heads, length is measured from the top of the head to the tip of the thread; measure oval heads from the edge where the dome top meets the sloping bearing surface to the threaded tip; and round heads are measured from under the head to the screw's tip.
DRIVE STYLES
Drive styles include slotted, cross recessed (Phillips) and square recessed. Due to the tendency of cam out (the driver bit slips or twists out of the drive recess), slotted screws are often not used with power drivers (the chance of damaging the screw head, driver bit or marring the wood surface is too great). Consequently, use Phillips and square drive wood screws with power drivers but hand drive slotted screws (square drive is more resistant to cam out than Phillips). Driver bit sizes are listed in Table 2.
HEAD TYPES
Three head types are common: flat (countersunk), oval (countersunk) and round. The head angle (conical bearing surface) of flat and oval heads is 82°. Although ANSI Standard B18.6.1-1981 (R2003) provides specifications for pan heads, pan head wood screws are largely non-existent. Flat heads are probably the most popular because their head, when countersunk, is flush with the mating surface. In addition, flat head screws can be easily concealed using a color-coordinated-plastic screw cover (see below); filler or a wood plug can be used if the head is countersunk below the surface. Oval and round heads are considered decorative and often left exposed. Refer to Table 1 for a summary of average head diameters.
Wood Screw Dimensions, Sizes, Head Diameters & Threads per Inch
Table 1. Wood screw dimensions, nominal sizes and average head diameters, in decimal and fractional inches, and threads per inch. (Note: Fractional size is decimal size rounded to nearest 64th.) Click on link above to see table.
MATERIALS and FINISHES
Common materials are unhardened steel (usually zinc plated), stainless steel, brass and silicon bronze. Zinc plating offers moderate corrosion resistance for steel. Brass provides better protection, while silicon bronze and stainless steel are known for their superior protection against corrosion. Stainless steel and silicon bronze are usually recommended if the screws will be used with pressure preservative treated wood such as "ACQ" (Alkaline Copper Quaternary)—check local building codes and contact your lumber supplier for recommendations. (Hot dip galvanized screws may also be used with treated wood but galvanizing is not a common finish for wood screws.) In marine environments, "stainless steels are subject to potentially severe pitting-corrosion attack when immersed in salt water, without free-oxygen," which makes silicon bronze a preferred material because stainless steel needs oxygen to create its self-healing, corrosion-resisting chromium oxide film. In terms of physical strength, steel, stainless steel and silicon bronze are all stronger than brass (brass is characterized as a soft material). Steel is magnetic, stainless steel may be slightly magnetic, and brass and silicon bronze, being nonferrous (not made of iron), are nonmagnetic. Stainless steel, brass and silicon bronze wood screws are more expensive than steel.
DESIGNATION
Wood screws are typically described as follows:
- Nominal Size (a number from #0 to #24; #0 is smallest)
- Screw Length (in inches; see above for instructions on how to measure)
- Drive Style (slotted, Phillips or square)
- Head Type (flat or oval countersunk, or round)
- Fastener Name (Wood Screw)
- Material (steel, stainless steel, brass and silicon bronze)
- Protective Finish (zinc plated if steel)
Examples:
- #10 x 1 1/2" Slotted Round Head Wood Screw, Brass
- #4 x 7/8" Phillips Flat Head Wood Screw, Zinc Plated (because zinc plating usually means that the fastener is made of steel, the word "steel" is often omitted in the description)
Wood Screw Pilot Holes & Countersink Sizes & Driver Bit Sizes
Table 2. Wood screw pilot holes and countersink sizes, in inches, and driver bit sizes. Use #3 Phillips driver bit for #10 flat and oval heads and #2 bit for #10 round heads. (NOTE: Body pilot hole size is average body diameter of screw, based on Cut or Rolled threads screw type, rounded to nearest 64th; and countersink size is maximum head diameter rounded up to nearest 8th [#0 and #24 rounded up to nearest 4th].) Click on link above to see table.
PILOT HOLES and COUNTERSINKING
To prevent the wood from splitting, an appropriately sized pilot hole is suggested (see Table 2 for untapered pilot hole sizes in soft and hard woods). There are differing opinions as to the depth of pilot holes. One source suggests you drill the pilot hole half to two-thirds of the screw embedment depth in soft wood. In hard wood, you most likely will need to drill the hole as deep as the threads extend. Another source recommends the depth to be the body length plus three-quarters of the thread length in soft and medium-hard woods, and the body plus the full thread length in extremely hard wood. It has also been suggested that soap or wax be used to lubricate wood screws when installing in hard wood (be sure to remove any exposed residue before applying a wood finish).
If you are using tapered drill bits, follow the manufacturers recommendation regarding pilot hole size and depth. Drilling a tapered hole too deep may compromise the screw's holding strength because fewer threads may be fully engaged in the wood.
Use an 82° countersink to create a countersunk recess (a beveled opening) for flat and oval heads. The body diameter of the countersink should be equal to or greater than the head diameter to ensure that a uniform bearing surface is created (see Table 2 for suggested countersink sizes). Special bits are available that will drill the pilot hole and countersunk recess in a single operation. A different bit is needed for each screw size.
SUMMARY
Wood screws are summarized in Table 3.
Wood Screw Typical Characteristics
Table 3. Summary of typical wood screw characteristics including sizes, head types, drive styles, materials and finishes. Click on link above to see table.
MORE INFORMATION
Dome-shaped plastic screw covers, available in different colors, hide the countersunk heads of Phillips and square drive flat heads. Countersunk finishing washers can be used with flat and oval heads and flat washers with round heads. Lock washers are not used with wood screws.
Sometimes used in place of wood screws, sheet metal screws (also known as tapping screws) are fully threaded and not tapered. Flat head sheet metal screws, especially Phillips drive, are commonly supplied with packaged barrel bolts, corner braces, hasps, hinges, mending plates, utility pulls and other hardware items. Withdrawal resistance of sheet metal screws is said to be about 10% better than wood screws of the same diameter and threaded length. In denser woods, such as oak, it increases to about 16%, and decreases to about 5% better in lighter woods, like redwood.
Other fasteners that use wood screw threads:
- Lag screws are heavy-duty wood screws and are available in larger sizes, both in terms of screw diameter and length, and usually have a hex head to facilitate higher installation torque.
- Acoustical lag screws have wood screw threads opposite a flat end with a hole. Used in tie-wire applications, like suspended ceilings, they are also popular for wire guiding.
- Dowel screws are wood screw threaded studs used to attach wood pieces and joints end-to-end.
- Hanger screws have wood screw threads on one end and machine screw threads on the other to accept a hex nut or coupling nut.
Printable wood screw reference charts and information:
FF:WDSCRWS v2.1
