Wood screws are the usual way of making a secure fixing in carpentry. But they come in a very wide variety of shapes, sizes and finishes. Here’s how to choose the right screw for the job.
To be able to pick exactly the screw you need for a particular purpose, it helps to know what each part of the screw does.
The thread is the spiral which actually pulls the screw into the wood and holds it there. Most have the same profile, but chipboard screws combat the material’s crumbly quality with their shallower spiral; and some have a double thread, which means the screw won’t wander off-centre, and can be driven into the wood twice as fast. Most wood screws have about two-fifths of the length unthreaded, forming a shank, but chipboard screws are threaded all the way up to the head for better grip.
Screw heads come in at least nine shapes, but wood screws use only three. Countersunk is the commonest. This name describes how the screw head fits into the surface of what you’re fixing: that is. In a hole with sloping sides. Such ‘countersunk holes’ are common in hinges and other fittings. In timber they have to be made with a special drill bit or hand tool – except where the screw cuts its own way in. as may happen in softwood.
You often need an ordinary countersunk head, because it lies flush with the surface. But the raised countersunk head looks more handsome, and is frequently used with metal fittings. The round head is useful when fixing a metal fitting without countersunk screw holes to wood, perhaps with a washer underneath it to spread the pressure.
On all screws either a slot or a recess is cut into the head so that the screwdriver tip can engage in it. Recessed heads need special screwdrivers, but they offer a more positive grip for the tip, so that it won’t slip out in use and perhaps scar the work. On the other hand, they make it harder to get screws out if they’re damaged or clogged with paint.
On wood screws the Pozidriv recess has now given way to the similar-looking Supadriv type. Each has its own screwdriver tip shape, but you can get away with using a Pozidriv screwdriver for both, or even a Phillips screwdriver . Recessed heads need fewer sizes of screwdriver to fit the range of screw gauges.
Accessories and unusual screws
Screws for special purposes include the clutch-head screw, which can’t be undone by a thief, or by you . The coach screw, for heavy work, has a square head and is tightened with a spanner. The mirror screw is inserted in the usual way: then a shiny chrome-plated dome is screwed into the head in turn, covering it and making a decorative feature. You can also get several types of snap-on cover for similar effects.
A screw cup or collar fits under a countersunk or raised head. Enhancing the appearance and spreading the load: it also enables screws to be easily withdrawn, and to hold thin materials such as hardboard and plastic laminate.
A screw socket is similar but flush with the surface.
Sizes and materials
How big is a screw? Its length -the amount hidden beneath the timber surface – ranges from 5mm to 150mm . The gauge – the diameter of the shank – has a number from 0 to 32; 4,6,8,10 and 12 are the commonest. A No 14 screw, for example, is 6.3mm in diameter. With practice you’ll soon be able to tell the gauge of a screw without having to measure it against others . Remember that you can have the same length in different gauges and the same gauge in different lengths.
And what are screws made of? Steel is the commonest and cheapest metal, but it has disadvantages — it’s not pretty, it rusts at the slightest sign of water , and it stains certain hardwoods such as oak. Sycamore and afrormosia.
Luckily there are quite a few alternatives. Steel itself comes with several different coatings, from nickel plate to black japanning . However, most are purely decorative: the exception is bright zinc plate, which is corrosion-resistant as well as attractive. When painted, it survives well outdoors.
Of other metals, brass is fairly corrosion-proof, but weak. Aluminium , stainless steel and silicon bronze are virtually corrosion-free. Stainless steel is the strongest of these, but is expensive.
So even the smallest screw has a long title, which you have to spell out fully when buying. Remember to give all the following details, preferably in this order: length. Gauge number, head type, material. Recess type , and finish .