Screw Lengths, Gauges & Tips
The type of screw is not the only thing you have to think about; you’ll also need to carefully consider the screw length, screw tip, and even the screw gauge. This guide will explore how these topics help to determine not only how easy your screws are to fix in place, but also how securely your materials are held together.
Intro
When it comes to deciding how long a screw should be, you can often find yourself playing a bit of a guessing game - however, the single most significant factor that will determine the strength of your fastening job is the screw’s length, so it's worth taking the time to do your research. Other factors, such as a screw’s tip or gauge, won’t make or break the job at hand and the differences are often nugatory, however, if you want to avoid pre-drilling, a specialised screw tip will have to be chosen (namely a self-drilling screw). This guide will tackle the questions regarding these screw features, such as how to measure a screw, what is a screw gauge and what applications different tip styles are built to handle.
If you need help dissecting what different threads, heads and gauges do, take a look through our other Screw guide pages:
Screw Anatomy
Before we take a look at the different types of screws, it's a good idea to get acquainted with their different parts first:
How to Measure a Screw
Measuring the length of your screw is a crucial step in ensuring that your materials are going to hold fast together; if your screws are too short they may not even reach your second material, or provide a weak, flimsy hold if they do. However, if your screws are too long, they risk not tightening down properly - or even worse: poking through to the other side.
The length of the screw is measured from the tip of the thread to where the head sits flat, though this can be a little tricky as you have to take into account what screw head you’re using as different style heads will rest differently on surfaces, as shown in the diagram below:
Deciding Screw Length
There are no strict rules set in place to govern the size of screw you should be using, but the typical rule of thumb is that the screw should go through at least half the thickness of the bottom material, e.g. ¾” into a 2x4. There are some other general tips that are helpful to follow:
- It should not be long enough to go right through the materials you’re fastening.
- It should not be thick enough to split the materials you’re fastening.
- If you need pulling force, use a longer screw with more thread.
- If you need lateral force, use a thicker screw.
Tip: When measuring the length of screws, the technical term for the overall measurement of the screw is called the effective length.
What is Screw Gauge
The gauge of a screw refers to the diameter of its shank (the unthreaded part) and can be anywhere from 1 to 16 - the larger the number, the larger the diameter of the screw will be.
The following metric to imperial conversions are widely accepted as the UK industry norm:
What number you select is dependent on whatever material you’re working with, but generally…
- Very thick or heavy materials: #12-14
- For general, average materials: #8
- For finer materials: #4
Tip: The imperial gauge range can be traced back to the industrial revolution and was used throughout the British Empire for many years!
How to Measure Screw Gauge
In the UK, the screw gauge - or ‘diameter’ of the shank is roughly half the size of the screw head, and as stated above, most of the time a #8 will suffice - the most important factor in choosing your screw is the length, not the gauge, so don’t worry about this too much. However, if you’d like to be more specific and measure your screw diameter, there are two main ways to do this:
Gauge Size Checker
The easiest way to measure screw gauge is to use a gauge size checker which is a simple piece of plastic with different sized holes so you can quickly identify the diameter of the shank
Micrometer Screw Gauge
You can also use a micrometre screw gauge, which is a more difficult tool to master but can provide you with highly accurate measurements of your screws as well as other small items. If you’d like to learn more about how to use a micrometer, take a look through Disher's How To Use A Micrometer Guide.
Screw Tip Types
The point or tip is the area of the screw that enters the material first. There is a large range of screw tips available, but they come in three main categories:
Thread-forming Tips
Thread-forming screws create their own thread in the material that is being drilled by pushing into it, rather than ‘drilling’ into it. Most of the time these types of screws will require a pilot hole, but others, such as a self-tapping screw, won’t always need one made. As these screws push through your chosen material, they form a very strong and secure fix and are able to be re-inserted time and time again with little ill-effect. However, as it pushes through the material rather than cutting through it, more pressure and stress is put on the screw.
There are 3 main types of thread-forming screws:
| AB POINT | B POINT | SUPERSHARP POINT |
|---|---|---|
|
|
|
| These points are ideal for common wood screws or for heavy sheet metal. They have a sharp point that is able to pierce the materials that the thread can catch | B tips are most frequently found in machine thread screws and are created for metal, plastic and plywood. They need a pilot hole to be drilled for them and are often most effective for shallow holes. | A sharper version of the AB point, these are also called a needle or gimlet point. They’re made to work with timber, MDF, plastic and light gauge metal. |
Thread-cutting Tips
Thread-cutting tips have a flute or slot that is designed to cut and remove some of the material while its being drilled in, rather than simply being pushed in - a little like a drill bit. Due to this, the strain on the screw is reduced and can be inserted into the material with much more ease. However, because of the material the screw is removing, its hold is ultimately weakened and its ability to be re-inserted is lessened dramatically:
| TYPE 17 POINT | |
|---|---|
|
Can be driven in without predrilling a pilot hole and reduces the chance of the material splitting. Commonly used in wood and decking screws |
Self-drilling Tips
Self-drilling tips have what is effectively a drill bit on the point of the screw, allowing them to drill their own pilot hole into the chosen material whilst the screw is being fixed. Sometimes these types of screws even have ‘wings’ which are designed to widen the hole so the thread doesn’t catch on it. These tips are created to hold two different kinds of materials together securely, however, they are more expensive than other types of screw tips and require a lot of force to be applied when being fixed in place.
| SELF-DRILLING SCREW | |
|---|---|
|
Can be driven in without predrilling a pilot hole and are specifically designed for joining wood to metal. |
