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How to choose the right screw for the job

Adam_W
Valued Contributor

HowToChooseScrew.jpg

 

Screws are essential fasteners for many home improvement projects, and are quick and easy to use thanks to the power and convenience of modern drill drivers.

 

The tough part can be choosing what screw to use for the job. Whether you’re a complete beginner or seasoned professional, when you go to select a screw you’re faced with a huge array of choices.

 

Here’s our guide to selecting the right screw for your project so you end up with a great result every time.

 

Durability and appearance

Screws come in many different kinds of metals and finishes. In most cases the finish is not merely cosmetic: it is determined by the kind of metal the screw is made from and the type of surface treatment it has received. This determines what the screw can be used for, and how durable and resistant to corrosion it will be.


It is important to be aware of material compatibility issues, too. Some galvanised screws will be rapidly corroded by certain types of treated pine, while stainless-steel screws create a reaction when used in Colorbond metals, causing accelerated rusting of the metal sheet. If in doubt whether your screw is compatible, please don't hesitate to ask the Bunnings Workshop community. We're here to help.

 

Here are some common types of screws you will encounter:

 

Plain steel

Indoor use only. Not often seen today, these screws were used indoors for hinges and cabinetry and would typically be painted over to protect them. They corrode very easily.

 

Zinc plate

A zince plate screwA zince plate screwIndoor use only. Zinc provides a shiny silver finish but little corrosion resistance. Zinc plate screws are often used in flat-pack type products. Avoid use in humid areas such as bathrooms and laundries.

 

Gold passivated (gold zinc)

Indoor use only. A special treatment gives the zinc a gold finish, making it a little more corrosion-resistant. Gold passivated screws are more suitable for cabinets in bathrooms and laundries than plain zinc screws.

 

Galvanised

Indoor and outdoor use. Galvanised screws are silver-grey, often with a slightly rough textured appearance. High level of corrosion resistance. Can be used anywhere outdoors and ideal for wet areas indoors. May be unsuitable for use in some treated Pine.

 

Coated

Indoor and outdoor use. Climacoat is the most common of these treatments. The coatings give the screws a grey or dark brown colour and better corrosion resistance. Most coated screws are ideal for treated Pine as they resist chemical corrosion. The coating also makes them easier to drive.

 

Stainless steel

A stainless steel screwA stainless steel screwIndoor and outdoor use. Stainless steel screws have a shiny silver finish. Maximum corrosion resistance, even in marine environments. Ideal for use in decks near salt-water swimming pools. Take care when driving as they can snap if overdriven. Don’t use them with Colorbond-type metals as they will cause rapid corrosion.

 

Coloured finish

Use varies with product. There is a range of painted and powder-coated screws for various situations, including coloured head gold passivated screws. Some are simply painted-over zinc intended for indoor cabinetry. Others are powder-coated over a galvanised finish to match Colorbond metal sheeting products for outdoor use. Ensure that your screws suit your application (ie, indoors or outdoors), and drive them carefully to protect the coloured finish.

 

Bronze

Specialised uses, most often boating or traditional cabinetry. Highly corrosion resistant. Rarely used in the home unless needed around swimming pools or in furniture restoration. Examples include bronze hinge screws

 

Bronze and brass plated

Indoor use only. Low corrosion resistance. Generally only used for a vintage look, for example when securing exposed hinges in a rustic-look kitchen, or where the colour matches the timber.

 

Thread types

The thread of a screw is the part that does the work, both in the short term and the long term. It gets the screw into the material to be secured and then holds it there. The thread is on the part of the screw called the shank. There are many types of threads, some of them for special purposes.

 

Coarse timber thread and fine metal thread screwsCoarse timber thread and fine metal thread screws

Here are the main types of threads:

 

Coarse thread

Coarse thread is the typical open thread on general-purpose screws for use in soft timbers such as pine, particle boards and chipboards.

 

Fine and twin thread

Fine and twin thread is more tightly spaced for better hold in higher-density timbers such as hardwoods. Some are also designed for metal fixing.

 

Metal thread

Metal thread is often described as a self-tapping thread as it will cut a thread into the material it is fixing to. Used most often in metal but also used in dense timber and plastics.

 

Split threads

In split threads, a section of the shank is bare, with thread only on the parts where holding strength is needed. Most often seen in roofing-type screws.

 

G stands for gauge

Many a D.I.Y. enthusiast has assumed that the G in a screw description is a millimetre width measurement. The G actually stands for gauge and is a different scale. For example, a 4G x 55mm screw is 2.9mm wide by 55mm long.

 

It is important to understand screw gauges if you are trying to match existing holes or you are planning to do any pre-drilling.

 

Pre-drilling

There are three main types of drilling you may need to do before inserting a screw.

 

Pilot hole

A pilot hole is a hole smaller than the screw that will allow for easy insertion of the screw while still ensuring the screw will be firmly embedded. This may be in hardwoods or metal or even laminated particle boards where it can be difficult to get a screw started because of the smooth, slippery surface. A pilot hole will also help to keep the screw from sticking as it is driven in.

 

Check your screw pack for pilot-hole information, but as a rough guide:

 

  • 4G = 1.5mm pilot
  • 5G and 6G = 2.2mm pilot
  • 7G and 8G = 3mm pilot
  • 10G = 3.5mm pilot
  • 12G = 4mm pilot
  • 14G = 4.5mm pilot.

Clearance hole

A clearance hole is the same size as the screw (or even slightly larger) and may be used in conjunction with a pilot hole. A clearance hole is used for screws where the material closest to the screw head will be held securely by the screw fixing to a second material. For example, fixing decking very close to the end of a board you may drill a clearance hole through the decking board and a pilot hole into the joist.


Here’s a rough clearance hole guide:

 

  • 4G = 3mm
  • 5G = 3.5mm
  • 6G and 7G = 4mm
  • 8G = 4.5mm
  • 10G = 5mm
  • 12G = 6mm
  • 14G = 6.5mm.

Counter sinking

Counter sinking is where you drill a cone shape into your timber or other material so that that the screw head can sit flush with or slightly lower than the surface. You can use traditional countersinking drill bits but there are also very useful bits that are a countersinking and pilot-drilling bit in one unit.

 

Screw heads

Just as there are different threads, screws also have different types of heads for different applications. Here are the ones you’re most likely to encounter:

 

Countersunk

A plain countersunk head screwA plain countersunk head screwSometimes also called flat screws for their flush-sitting top face, countersunk screws are general-purpose conical-head screws. Some will have ribs beneath the head to make the head self-embedding, but for a neat finish countersink first. If using for hinges or similar situations ensure you use plain heads, not ribbed ones.

 

Bugle head

Similar to a countersunk head, but the cone of a bugle head screw tapers into the shank of the screw. There is often no thread on the part of the shank closest to the head. This is to reduce damage to material such as plasterboard. Larger versions called bugle battens are used for heavy-duty construction and landscaping work.

 

Button head

Button head are sometimes called stitching screws. On the drive face they are raised towards the centre. They have a flat edge like a flange or washer, and a flat under-surface. The width of the head makes them ideal for joining or “stitching” materials together.

 

Pan head

Pan head screws are also known as round-head. These have a conical face with a chamfered edge and they are flat underneath. Used when fixing materials that can’t be countersunk, such as metal or acrylic sheets to another surface.

 

Wafer head

Wafer head screws have a small, thin, flat head with a low profile. Often used with sheet metals but has many uses where a low-profile head is needed.

 

Hex head

A hex head screwA hex head screwHex head screws have a nut-like head. These screws are most often used in heavy-duty construction work, most often in metal. They’re commonly used in roofing but smaller versions are used for securing aluminium fence rails and fittings. You’ll often see these with pre-fitted rubber washer seals for uses such as cladding and roofing.

 

Drive types

The drive type is one of the most important aspects of the screw.

 

The traditional type (not often used in D.I.Y.  today) is the slotted drive, which is driven with a typical flat-bladed screwdriver. This type of drive is easy to overdrive and “cam out” – a process by which the screwdriver slips or twists out of the screw head. Camming out can strip the screw head, making it impossible to work with. The slotted drive has been superseded by a range of other head types.

 

Here are the most common screw head types:

 

Phillips or cross-head drive

First appearing nearly 100 years ago, Phillips drive was designed to replace the slotted head as it is less likely to cam out. It’s the most common head today but is also being superseded.

 

Square drive

A square drive screwA square drive screwAlso known as the Robertson screw, the square drive is increasingly replacing Phillips-drive heads. These screws have a slightly tapered square socket in the head. They are highly resistant to camming out and have the advantage of the screw sitting securely on the bit, making it easier and safer to secure or remove screws with one hand.

 

Hex or internal-hex drive

Not to be confused with a hex-head screw, the hex-drive screw has a hexagonal socket in its head. These are most often seen on larger screws, such as bugle batten screws. It has a very secure grab on the bit and is less likely to cam out.

 

Hex head

A hex head screw has a hexagonal nut-type head and requires a socket bit to drive in. This is a very robust head, most often used in heavy-duty applications such as roofing. The bit for driving a hex head screw is often called a nut-setter.

 

Point types

Different points on screws serve different purposes and are designed to make screwing easy. It is important, however, that you select a screw with the right tip for your project and the materials you are screwing to and through.

 

Here are the different point types:

 

Needle point

A needle point screwA needle point screwWith a very sharp tip, needle point screws are designed to easily pierce materials such as plasterboard and thin steel.

 

Drill point

The tips of drill point screws are hardened and shaped to function like a drill bit. This allows them to self-drill into metal without pre-drilling.

 

Type 17

Type 17 screws look much like a needle point screw but one side of the tip is fluted. This helps the screw embed faster and clears debris that might otherwise clog the screw hole.

 

Self-tapping

Self-tapping screws create their own thread in the metal they are being drilled through to create a firm grab. Generally not self-drilling; they usually need a pilot hole to get started.

 

Wing tip

Wing tips are most often found on self-drilling screws with fine metal threads for use when fixing timber to metal. The wings are designed to clear debris from timber, which can otherwise cause the fine thread to clog and bind on the timber. The wings break off as soon as the screw hits metal.

 

Screw length and embedment

Getting the right length of screw is critical. Part of the way to determine the length is to understand how much embedment you require – how much of the screw will actually be doing the securing. This will vary with the material you are securing and fixing to. For example, when fixing to sheet metal an embedment of just 1mm or 2mm might be enough because the screw will have cut a thread into the metal, fixing firmly. When fixing the same load to timber the embedment may need to be 20mm or more.

 

Thankfully, getting the selection right is made easier by the packaging of the screws. On the back of the pack you should find a panel of embedment and fixing information. This is your guide for the materials the screw is suitable for and how it needs to be fixed.

 

Screws are also measured in different ways depending on their head type. For screws with flat head undersides (button head, pan head, hex head, wafer head) the length of the screw is from the underside of the head to the tip. For screws with conical head undersides (bugle head, countersunk head) the length of the screw is measured from the top face of the head.

 

Tips for using screws with your drill-driver

 

Most screwdriving today tends to be done by drill driver as it is fast and efficient. There are, however, a few things to remember:

 

  • Make sure your driver bit is the right type and size and not worn or damaged.

 

  • Set your drill driver to screw mode if it has a selection switch.

 

  • Adjust the clutch on your drill-driverAdjust the clutch on your drill-driverAdjust the clutch (the numbered ring around the chuck) to make sure you do not over-drive your screws. If possible use a piece of scrap material to get the setting right.

 

 

  • Make sure the bit is firmly seated in the drive before starting to avoid cam-out. Otherwise the screw head may be damaged, making the screw difficult to embed or remove.

 

  • Avoid using drill drivers with stainless-steel screws unless holes have been pre-drilled. Check the screw box as they may recommend hand-tightening only.

 

  • If using an impact driver, make sure the drive bit and the screws are suitable for use with impact tools.

 

  • For larger screws (such as bugle batten and landscape screws) use both hands on the drill.

 

  • If your drill has a handle, fit it for driving these larger screws.

 

  • Always wear safety glasses when driving screws. If screwing into metal or using an impact driver, also wear ear protection.

 

Need a hand with choosing the right screws for your project? Don't hesitate to hit the Start a discussion button and ask our friendly community for assistance. 

 

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