Screws provide more strength and holding power than nails. Additionally, if something needs to be disassembled, screws can easily be removed.
Like nails, screws are available with different coatings to deter rust. They are manufactured with four basic heads and different kinds of slots. Flathead screws are almost always countersunk into the material being fastened so the head of the screw is flush with (or lower than) the surface. Oval-head screws are partially countersunk, with about half the screw head above the surface. Roundhead screws are not countersunk; the entire screw head lies above the surface. Fillister-head screws are raised above the surface on a flat base to keep the screwdriver from damaging the surface as the screw is tightened.
Most screws have slot heads and are driven with slotted, or standard, screwdrivers. Phillips-head screws have crossed slots and are driven with Phillips screwdrivers. Screws are measured in both length and diameter at the shank, which is designated by gauge number from 0 to 24. Length is measured in inches. The length of a screw is important because at least half the length of the screw should extend into the base material.
Here's a tip: To prevent screws from splitting the material, pilot holes must be made with a drill before the screws are driven.
For most home repair purposes, wood screws will suffice. Sheet metal screws, machine screws, and lag screws also come in various types. If you're trying to replace one of these screws, take an old screw with you to the hardware store. Following are some of the most common screw types:
- Wood Screws: Wood screws are usually made of steel, although brass, nickel, bronze, and copper screws should be used if there is potential for corrosion. For optimum wood screw performance, you need to drill first.
- Sheet Metal Screws: Use this type of screw to fasten pieces of metal together. Sheet metal screws form threads in the metal as they are installed. There are several different types of sheet metal screws. Pointed panhead screws are coarse-threaded; they are available in gauges from 4 to 14 and lengths from 1/4 inch to 2 inches. Pointed panheads are used in light sheet metal. Blunt panhead screws are used for heavier sheet metal; they are available in gauges from 4 to 14 and lengths from 1/4 inch to 2 inches. Both types of panhead screws are available with either plain or Phillips-head slots.
- Roundhead Screws: Partial-tapping roundhead screws have finer threads; they can be used in soft or hard metals. They are available in diameters from 3/16 inch to 11/4 inches. Self-tapping roundhead screws are used for heavy-duty work with thick sheet metal and are available in diameters from 1/4 inch to 2 inches and in lengths from 1/8 to 3/4 inch. Both types of roundhead screws are available with either plain or Phillips-head slots.
- Machine Screws: Machine screws are blunt-ended screws used to fasten metal parts together. They are commonly made of steel or brass. Like other fasteners, they are also made with coatings -- brass, copper, nickel, zinc, cadmium, and galvanized -- that help deter rust. Machine screws are manufactured with each of the four basic types of heads -- flathead, ovalhead, roundhead, and fillister-head -- and with both plain and Phillips-head slots. They are typically available in gauges 2 to 12 and diameters from 1/4 inch to 1/2 inch and in lengths from 1/4 inch to 3 inches.
- Lag Screws: For light work, lead, plastic, or fiber plugs (called anchors) can be used to hold screws. But for larger jobs and more holding power, lead expansion anchors and lag screws are used. Lag screws are heavy-duty fasteners. They are driven with a wrench and are used primarily for fastening to masonry or wood framing. The anchors are inserted into holes drilled in the masonry, and the lag screws are driven firmly into the anchors.
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