Screws - The Ultimate Guide : Helping you select the right size screw for your needs.

Screws are one of the most common, versatile and widely used fasteners not only in the UK but in the world, it doesn't matter if you are a trade professional joiner, roofer or electrician, or just a home DIY enthusiast everyone has come across wood screws and used them at some point in your lifetime.

We are going to explain the basics of screws, how to measure them and take you through the process of ensuring you select the right size screw for your application. Don't forget to check out the FAQ's at the end where we hope to provide answers to some of the most frequent questions.

We also have some handy conversion charts to check out.

Understanding the correct screw size for the type and thickness of material you are screwing into.

First thing you need to understand when selecting the correct screw size is not the screw itself, a screw is a fastener providing support to the main event, the material it is being used with is the first and most important step.

Thickness - Choosing the correct length and diameter of screw.

A general rule of thumb for selecting the correct size length and diameter of the screw you require all depends on the thickness. These are the 2 rules that most professionals stick to;

Length

Screwing into a wall is fairly straight forward, aim to have at least 2/3 of your screw length into the wall.

Example: Fixing a 15mm shelf thick shelf to the wall. 

15mm x 3 = 45mm Screw length

1/3 of the screw is in the shelf (15mm)

2/3 of the screw is in the wall (30mm)

Offering a reliable and secure fixing for years to come.

Alternatively;

When joining two pieces of wood together, you ideally want the screw length to be 3x the length of the top piece and no more than 2/3 of the total thickness of both pieces.

Example: Attaching a 25mm plank of wood to a 100mm post.

Top piece thickness = 25mm

Total thickness = 25mm + 100mm = 125mm

Rule 1 - No more than 2/3 of the total thickness = 125mm x 0.66 = Max 83mm

Rule 2 - Approximately 3x the top piece = 25mm x 3 = Approx. 75mm

As 75mm is under the max length of 83mm, this would be the perfect length screw to choose.

Diameter

Ideally you want to stick to the diameter of the screw being no more than 2/3 of the thickness of the material. Good rule of thumbs are 8 Gauge wood screws are good general purpose screw sizes for fixing boards and panels etc. As you get to heavier materials such as posts 12 to 14 Gauge becomes more appropriate. Click here to see our handy gauge to metric conversion chart

Which Head and Drive Type Screw Do I Need ?

There are numerous different types of screw heads and drive types but these can become more complex and are generally due to the niche requirement of most of the types. Here we like to keep things simple, so we are sticking to the most common types and explaining what benefits each screw type has over the others.

Drive Types

Slotted - Slotted screws are still likely the most common type you will see and feature a single straight line across the head of the screw. They are driven by straight end screwdrivers or driver bits and are useful when little torque/effort is required, generally into softer woods.

Slotted screws are however difficult to line up and stabilise when first screwing and subsequently are not the best application when speed is a necessity, which is generally most trades nowadays, which leads us onto our next type. 

Pozi/Phillips - Crosshead screws which encompasses both slotted and phillips types are the new modern go too screw for most applications and are quickly overcoming the slotted screws as the #1 most common type.

They generally come in all the same types as a slotted screw but offer much more ease of use with there 4 point drive design offering ease and stability when driving either with a screwdriver or more often than not a driver bit on a drill/impact gun. If professionals were to be asked, what's the best type of screw to use? Pozi drive or Phillips head would 100% be the most common answer. 

Tamper Resistant/Torx Security - Torx security screws, often referred to as tamper resistant screws are a great alternative to crosshead screws when security is important, they have a star shaped head design and often also have a raised pinpoint in the centre, they are still common screws in design but you require a more specialist tool to remove them, a torx key. 

Tx screws are often used on security applications to restrict access, gates, doors, locks and handles. They are also widely used within the automotive industry. The only issue with torx screws is they aren't nearly as durable or strong as pozi/phillips screws, they are harder to drive in and cannot be used where high torque is required.

Combi Head - A combo head screw is a great versatile option as it incorporates both a slotted head and a offers the user the option of either using a slotted drive or pozi drive. As capable as a pozi drive screw but can often be slightly more expensive due to the extra manufacturing requirement.

 

 

Head Types

Countersunk - The most common head types for standard applications, used by professionals and home diy enthusiast for decades. Often used on timber for a flush or recessed finish.

Double Countersunk - Extremely similar to the countersunk standard version above but basically mk 2 version featuring an upgraded design that gives additional strength to the head which allows more torque to be applied and is usually easier to drive and countersink.

Bugle (Plasterboard screw) - The #1 head type for securing plasterboard panels, used by professionals and home diy enthusiast for decades. Featuring a curved design underneath the head to prevent against ripping or tearing of the plasterboard. Countersunk for a recess to allow a smooth plastered finish to be applied.

Raised Head Pan Countersunk - This screw head is often used for hinges as it offers an aesthetically pleasing design with its rounded head design. It also features a countersunk under head for a strong secure fixing.

Pan Head - Round head with a flat under surface. A brilliant secure where a secure and reliable fixings is required, allows extreme clamping capability and is often used in metal applications or along with a washer in wood applications to spread the surface area load.

Wafer/Flat Head - Similar to a pan head but with a wider and shallower  head design, often used where a large clamping area or an unobtrusive head is required.

What are the different screw points and which do I need ?

Here we explain simply the various different types of screw points and why you would choose one over the other, offering you a simple guide to choose the correct screw type for your application.

AB Point - Designed with a sharp shallow point that quickly changes to the full diameter thread, most appropriately used for heavy sheet material applications, most commonly with metal.

Super Sharp Needle Point - A very common screw point that offers superb penetrative capability into the most common applications including; timber, mdf, plastic and light sheet metal. It's best feature is it can be used without the requirement for pre-drilling.

B Point - Used in shallow pre-drilled holes, doesn't include a sharp point but provides a strong fixing.

Self Drilling - For use in metal applications without the prior need to pre-drill, featuring a twisted shank end point that allows easy insertion.

Type 17 Point - Often referred to as a slash point screw, these screws are most commonly used for fixing timber without the need for pre-drilling. Featuring a slotted end shank design, this forms a cutting edge which reduces  the chance greatly of splitting timber. It can also be used for thin sheet metal with it's penetrative capabilities.

C2 Slash - Very similar to the type 17 slash point screw above but with an enhanced twin cut design that greater reduces the chance of timber splitting, a much more reliable choice but also often slightly more expensive.

Last but not least, we move onto screw thread types ...

What are the different screw threads and which is best to use ?

Course Thread - Course thread screws are very common and are generally used for all timber applications and steel applications when the thickness is less than 2.5mm.

Fine Thread - Fine thread screws, also often referred to as spaced threads are only used in metal/steel applications. They are usually used with thicknesses of 2.5mm and greater.

Twin Thread - Twin thread screws is used generally in timber applications and can be used for both softwood and hardwood. They feature a two threaded design running along the body of the screw and can be driven or removed extremely efficiently and much faster than single thread screws. An extremely common application especially as woodscrews.

Hi-Lo Thread - Hi-lo threads are used in softer or shallower materials but can be used in wood, plastics or metals. These screws have an  extremely strong holding power and are great when a reliable secure fixing is required.

Self Tapping - Used in metal applications, this thread allows insertion without the need for pre-drilling, a go too screw for trade professionals. Used commonly for new build offices and warehousing.

Serrated - Used generally on metal applications but can be used on other materials. This thread design helps reduce the torque required to drive the screw into the material with its serrated edges.

We hope you find this information on all the different screw variations helpful and it helps you decide on which screw is the correct one for your needs.

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FAQ's 

What type screw do I need for hardwood ? 

Hardwoods such as Oak, Walnut, Cherry and Ash usually require a finer thread as it provides better penetration to ease through tough, dense material without splitting the wood.

What type screw do I need for softwood ?  

For softwoods such as Cedars, Firs and Pines, coarser gauge screws are usually more suitable as they have deep and aggressive threads that grip into the soft fibers of the wood easily to ensure solid holding power. 

What is a self tapping screw ?

A self tapping screw is a type of screw that allows insertion without the need for pre-drilling, a go too screw for trade professionals. 

What is a coarse thread screw ?

A course thread screw is generally used for all timber applications and steel applications when the thickness is less than 2.5mm.

What is a fine thread screw ?

Fine thread screws are only used in metal/steel applications. They are usually used with thicknesses of 2.5mm and greater.

What is the difference between countersunk and double countersunk ?

Countersunk and double countersunk screws are extremely similar and essentially the same screw, double countersunk I like to think is the upgraded modern version, a countersunk 2.0, the double countersunk screw features an upgraded design that gives additional strength to the head which allows more torque to be applied and is usually easier to drive and countersink.