What Screws Should I Use?

July 19, 2013 at 8:32 pm
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Whilst the basic principle varies vary little from screw to screw, their suitability and applications can vary significantly, so picking the right screw for the job is often critical to its success. Screws can vary in the style and number of threads that they have (depending on the object intended to be screwed into), the style of the head, both in terms of the driver that will be used and the shape of the head itself and the material that the screw is made from.

All of these differences make the screws suitable for their intended purpose. For example, the sharp point, cut tip and self-countersinking ridges on some wood screws allow these screws to be driven into soft woods very quickly without a pilot hole, whilst minimising  the damage done by the head as it makes contact with the wood, preventing splitting. However, these screws would give a very poor finish when hanging plasterboard as the heads would cut too far into the board, and the shiny finish would not be ideal for plaster to adhere to.

The guide below will help you determine which screw (or combination of screws) you should be using for your project.

Self Tapping Woodscrews - strictly speaking, this should really be known as self drilling as a pilot hole is not required for these screws as they'll 'tap' themselves. That's not to say that a pilot hole isn't advisable to avoid splits in the wood though. These screws can have several features, such as very sharp, slashed tips and serrated edges to the thread, which act to drill into the wood as the screw itself turns.

If you're using this type of screw in an outdoor application, it's important to check their coating for exterior suitability as directed by the manufacturer. For example, the silver screws (unless they're stainless steel) are generally unsuitable for use outdoors. Yellow tropicalized screws are suited to exterior use. Be aware that damage to the head recess will give the screw am opportunity to rust, so take extra care when driving in.

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Self Tapping Screws - used in metal, the screw will cut its own hole in some very soft or thin metals, or the intended hole is not required to be 'tapped' first. The screw will cut its own thread as it is turned into the hole.

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Drywall / Plasterboard Screws - commonly black or silver in colour, the black phosphorous surface is both rust resistant and intended to help the plaster or 'mud' stick to the screw head. The screw is tapered to help it self tap into the plaster board and the head of the screw is curved on the underside to prevent it punching too far into the plaster board.

These screws are very brittle, so must not be used for load bearing applications as they can (and will) snap off.

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Decking Screws - these screws are designed with load bearing strength and rust resistance (for exterior applications) in mind. You will also find variations in the screw head's recess, as some decking screws are square drive to prevent the screwdriver slipping out, or the screw head rounding out.

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Brass Countersunk Screws - a traditional woodscrew, these screws are typically single threaded and will self tap into softer woods. Pilot holes are necessary in harder woods, as these screws are liable to heating up and the heads shearing off. The slotted head recess is also not as strong as the Phillips, Prodrive or Pozidriv equivalents, so will fail under excessive torque.

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Timberlok Screws - these are very heavy duty screws, often very long and with a self-countersinking hex head, designed for fastening very large pieces of wood such as railway sleepers. It is sometimes not necessary to pre-drill holes to accommodate these screws, but for use with hardwoods such as railway sleepers, it is unavoidable. When working with sleepers, it may even be necessary to lubricate the screw itself with grease or oil for it to be driven easily into the wood.

The screws are coated to prevent rusting.

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Machine Screws - this type of screw has a tight, machine thread which is designed for use with a nut, or tapped hole, not for driving into wood or similar. They are available with a variety of head shapes and recesses, from flat to pan shapes, with slotted, Phillips, Torx and Allen recesses all common.

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Security Screws - the important part of this type of screw is the head recess, which can come in a variety of configurations, such as Torx with a central protrusion so that it cannot be undone without a special type of screwdriver. Obviously not an entirely foolproof solution, but it does make the screws harder to undo than standard slot or Phillips head recesses.

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Masonry Screws - used with rawl plugs which are forced against the edges of the drilled hole to provide the necessary anchorage, this type of screw is designed to fix wood, metal or other components to stone, bricks or concrete..

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Flooring Screws - intended to be driven in at a 45 degree angle without pre-drilling, these screws have a number of features such as a countersunk heads and a serrated thread which ensure quick and accurate penetration.

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Roofing Screws - this type of screw is designed to fasten roofing sheets (in a variety of materials) to timber or steel purlins. Typically the head will be wide, with a captive washer in order to distribute the clamping pressure evenly where the screw touches the sheet. This prevents damage to the roofing sheet and pooling round the screw head whilst holding the sheet securely. Some roofing screws will have a rubber edge to the washer, or a rubber/plastic cap to prevent water ingress around the screw.

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