Everything you need to know about screws

There are many screws in the world. The concept was invented by Archimedes in the 3rd century B.C. when he invented the screw drive inside a pipe used for raising water. The name, 'screw', has been given to any cylinder that has spiral, helical threads running up the shaft. Because screws were invented so long ago, they have had many years to mutate. For centuries, screws were all hand made and extremely expensive. It wasn't until the industrial revolution that machinists were able to devise tools precise and strong enough to cut accurate, measurable threads consistently, allowing for mass production. Screws then became cheaper and the competition for the best screw began. The hardware isle is huge and often the most confusing. Screws, in combination with driver drills, are now rivaling nails in popularity. Screws provide a much tighter and far stronger fastener than that of nails and with the new ease of installation, screws have become the fastener of choice.

The dry wall industry was one of the first to start using screws in large quantities. Drywall is one of the best fire retardants in any structure. It is sometime doubled up where dwellings share a common wall to help prevent the spread of fire and screws are used to attach it because they are far better drywall fasteners in a fire than are nails. When a building heats up and things start to swell, nails will back right out and the drywall falls off the walls but not screws. Screws hold the drywall firmly to the studs in a fire. It is only in science fiction that screws will back out of their little nest without a screwdriver.

What do you get and why? Screws can be divided into types and then descriptive features. Because the term, 'screw', is so general it can be very confusing when looking for what you need for any given project. Various types of screws have been designed for fastening different types of materials together. Some common names are wood screws, sheet metal screws, drywall screws, deck screws and masonry screws. Often, it is a specialized thread that makes the difference or it could be the colour. There are confusing names like 'machine screw' which is really a bolt but, since it has screw threads on it, it can be called a screw. Then there are Lag bolts which are really very large wood screws but have a hex head for driving the screw in and so people have begun calling them bolts. Often, local names differ but if you ask for a fastener by name of material first, it'll at least get you started. Screws generally refer to threaded cylinders with a point. Bolts are threaded cylinders with a blunt end and take a nut.

Once you've determined that what you want is a wood screw, then there are a bunch of choices. It's sort of like calling the telephone company and getting their multiple choice directory that goes on and on. The first thing you should ask yourself is, "Will this screw be exposed to the elements - especially water, sunlight and air?" If it will, then your screw should be treated for such conditions. There are several ways to treat iron screws to help them resist corrosion. The most common and cost effective one is to coat it with zinc either by electroplating or by dipping it in molten zinc creating a zinc/iron alloy on the surface. Some screws have a coloured ceramic coating (sometimes seen in deck screws). Hot dipped zinc-coated screws are usually considered the best for preventing rust.

If what you want is a yellow coloured wood screw, then what you need is a brass screw which eliminates the corrosion issue altogether. There are acidic conditions that aren't suitable for brass, but under typical conditions that would cause iron to corrode throughout, the same conditions will only discolour brass. The trouble with brass is that it is soft and difficult to drive into wood. So, along comes iron with a brass plate on the surface. This is good as long as the surface of the brass is never penetrated allowing the iron to corrode to pieces. Traditional woodworkers often prefer solid brass because it is traditional.

Stainless steel is the best to use in conditions where the screw will be exposed to severe conditions. The only trouble is it costs about the same as a handmade screw did during the middle ages.

Now, you know that you need a corrosion resistant wood screw. The next multiple choice in our directory is what type of head you want. There are five basic types of heads. Flat head screws are used for countersinking the screw head flat to the surface or sometimes below the surface and covered with wooden plugs or wood filler. Oval headed screws are designed to sit on the surface of the wood and are most often used with a trim washer that holds it up off the surface of the wood. The third kind is a round-headed screw which will sit on the surface and is often used with a flat washer. A trim headed screw is designed to replace a finish nail and will leave a larger hole but has far more strength than a nail. Bugle headed nails are similar to the flat heads but are designed to sit just on or slightly below the surface of softwood or drywall pulling it down slightly.

The next question to answer is what type of driver you prefer to use. The one used for centuries was the slot head. It is old and venerable and the least effective. The X-shaped slot or Phillips slot is another type and is far more efficient than the straight slot. The square socket or Robertson headed screw is the easiest one to drive in. All three can be found in household items in need of repair, but it is your choice on new projects and replacement screws. Occasionally hex headed screws are used for specialized applications.

So, you've determined the type of screw by the material, wood. It is to be used outdoors so it is treated for that. The easiest type of head to use is the square head so, we'll go with that. Finally, how long should it be. Some people say that the length should be at least three times the thickness of the material penetrated. So, for attaching a piece of half inch plywood to a 2x4 use at least a one and a half inch screw.

The truth is that a screw is a fastener and even if it is designed to fasten wood to wood, it could also fasten sheet metal to wood. Screws have become very specialized but that isn't to say that they have to be used exclusively for the purpose they were designed. Usually pan headed screws are designed for metal, but in certain cases they can look good on wood too. Don't be limited by the name. In most cases, however, the screw that has the material's name in it's title is usually the best one for the job. So, happy screwing.