Screws

Screws are a fairly modern invention, only occurring in widespread availability within the last 100 years. Since their introduction within the construction industry, manufacturing methods have made them more affordable and easier to obtain. In the early days of production, they were extremely laborious to make, being reserved only for the most essential of tasks. In modern times, screws come in a variety of sizes and materials for use in different applications and have a few advantages over the use of nails. Namely, they are much more resistant to draw out than nails and can often be purchased in longer lengths than would be practical for nails of a similar length. They can also be installed in tighter spaces due to the use of a driver in contrast to a hammer, and they can be easier to use than nails for newcomers to the trade that are unfamiliar with using a hammer.  

Most varieties of nails are hardened, and as such are very easily snapped when bent. This makes regular screws a poor choice for use in structural applications where shear load is applied. In these scenarios, more expensive structural screws must be used. Here we shall take a look at the different types of screws and their intended purposes, as well as their limitations.  

Anatomy of a screw

As with nails, screws come in a variety of shapes and sizes intended for different applications, though they all follow a similar construction. The main length of the screw is also known as the shank, similar to nails. The primary characteristic of screws are the threads that run around the shank. The threads of screws are machined metal extrusion that surround the shank. These threads spiral from the point of the screw up the length of the shank. The sharp point of the screw is started into the material with a driver, and the threads begin to wind into the material. As the screw is driven into the wood, the threads engage with the material, increasing the screws hold within the material. Once the screw is fully driven into a material the threads provide incredible resistance to being drawn out.  

The longer and wider in diameter the screw, the more friction is created when the screw is being driven. Long screws like this often require the use of an impact driver, or for an appropriate sided pilot hole to be drilled beforehand. The diameter of the hole that should match the diameter of the shank of the screw. This ensures that the threads that surround the shank still have enough material to engage in. A screw's diameter is measured by the internal shank, excluding the threads that surround it. 

Most modern construction wood screws have a notch cut into the tip parallel to the length of the shank. This cut out is known as a flute and enables the screw to be more effectively driven into a material without requiring a pilot hole. The flute allows the saw dust created by the spiralling action of the screw to exit the hole as the screw is being driven in. The saw dust is carried out of the hole by the threads as they are driven in. Most woodworking screws will have a single thread - 1 continuous machined thread that spirals from the bottom to the top. More specialist screws for use in brittle materials may have 2 threads that wind around each other. These types of screws are known as double or twin threaded screws. The two different threads wind into the material separately from each other, increasing the area of contact between the material and the threads. This increases the resistance to draw out. These dual threads are also a lower angle than single threaded screws and are less aggressive when being driven into the material. This reduces the risk of splitting soft or brittle materials. The opposite end of the screw that receives the driver bit is known as the head. Most modern construction wood screws have serrations on the underside of the head that help to self-countersink the screw. Other forms of screws that don't have these serrations may need to have a hole countersunk in the material for the head to receive into. This is especially true in harder materials. There are many different types of head shapes and drive types which we will look at now.  

Most common head types

As mentioned, there are many different shapes of heads and drive types that screws may possess. Each type is intended for a different application and are often associated with certain activities. This being said, there are only a handful of head shapes and bit drive types that are used commonly in carpentry. The screws that an electrician or a heating specialist may use will vary differently from those that a carpenter will use, due to the different requirements of tasks carried out. Modern carpentry related screws mostly feature a small selection of drive types and head shapes, though there are a variety that were used in the past. These screws can still be found in older houses when carrying out renovation work and vary in shape from those used in modern times. This is due to advances in modern technology. Here we will look at the different head shapes of common construction and carpentry screws.  

Bugle head

Bugle heads are designed for use with plasterboard/drywall. The top of the head is flat, and the underside is countersunk. The concave curve of the countersunk head allows for maximum hold on the plasterboard as it is driven in. These types of screws are typically found with a PH2 drive. 

Wafer head

The wafer head is also intended for use with plasterboard/drywall. The wide flat head of the screw allows for greater support on the face of the sheets. They have a low smooth profile that sits slightly proud of the surface of the board. Different sizes are available though they are typically shorter than other types of drywall screws.

Pan head

Pan head screws are a non-application specific type of non-countersinking head. These heads are a very common choice for fixings that are included in kits or furniture. The production cost of this type of head is typically low. The head sits proud of the surface and is fairly wide. The top of the head is flat and comes in a variety of different drive types. The heads are almost a standard for general use, low cost, low torque applications. 

Flat countersunk head

This type of head is the most commonly used for construction wood screws. The profile of the underside of the head allows the screw to sit neatly in a pre-countersunk hole. Some varieties of this screw feature notches on the underside of the head that can countersink their own hole into softer materials. They are cost effective to make and come in a variety of different drive types. Screws of this type are sold in a variety of different sizes in bulk and make up the majority of the types of screws that we use. 

Round head

The round head is very explanatory in description of its appearance. The head is a round dome shape and comes mostly in a PH2 or slotted drive type. Whilst manufacturers still produce these types of screws they are most commonly found in older houses. They are not for structural applications and serve mainly to hang items in a house or assemble premade furniture. The head is more aesthetic than other head types and is a good choice for a finish screw. The traditional type of screw that is associated with a round head is often only half threaded on its shank.  

Fillister head

The fillister head is a round and slightly domed head that works well for most woodworking applications. Whilst they are not really used in structural applications, they are a good choice for use in interior finishes. The depth of the head is intended to be sunk below the surface of a material in a counterbored hole, with the slight curve of the screw protruding past the surface of the material. This is an aesthetic finish. The majority of the fillister heads available are slotted, though they are not so commonly used in modern times.

Hex washer head

These heads are the perfect style for larger structural screws. The hex drive type has a very positive engagement with the socket bit and is very resistant to slipping when being driven. The inbuilt washer on the head provides a large area for the force of the screw to be distributed over the material. These heads come on a variety of different sized structural coach screws.  

Hex head

These heads are hexagonal in shape and are intended to be driven with a socket or a wrench/spanner. The head shape allows for a very positive engagement with the driving tool, allowing for a lot of torque to be applied to the screw. These are typically associated with bolts, machine screws, or structural screws.  

Oval countersunk head

This type of head is countersink compatible with a slightly domed top. The screw is not self-countersinking and a countersunk hole must be pre bored before the screw is driven. The slight dome on the top protrudes past the top surface of the timber and is an aesthetic choice of screw for these applications.  

Most common drive types

Many tool brands sell small kits that contain a very decent selection of bit types for use with drivers. These kits are very handy to have as they cover most of the scenarios that may be encountered on site. With this being said though, there are only a small handful of drive types that are commonly used in construction. In the below descriptions, we will look at some of the most common types of drive types on screws. In the descriptions, some drive types will be referred to as positive registration and others not. By this, it is meant that a bit that positively receives into the head of a screw is very unlikely to slip out of the head of the screw, such as a Torx bit. The opposing end of this spectrum would be a bit that is very likely to slip out of the screw head, like a flat bit. With that in mind, these drive bits are: 

PH2 (Philips drive)

This type of drive was introduced in the 1950’s and is amongst the most commonly used drive types. The shape is a standard cross that deepens in the head of the screw towards the centre of the cross. The drawback of this type of head is that as high torque is applied to the screw, the bit tends to slip out of the detent and begin to strip the head. This type of drive is often associated with standard non application specific screws, as well as drywall screws. This is also the most common drive type found on cheap hardware, either sold cheap from a merchant, or supplied with self-assembly kits. This is due to the low cost of production.  

PZ2/3 (Pozidrive)

This type of drive is very similar to the PH2 drive, with a few more added grooves in between the lines of the original cross. This adds more friction between the driver and the head of the screw, allowing for better grip and more torque to be applied. Due to the added grooves, the PZ2 bits won't fit correctly in a PH2 slot, as they are too big. This type of drive is most commonly associated with general purpose wood screws. The PZ3 is just a larger variety than the PZ2 of Pozidrive bit type, for use with larger structural timber screws.   

Hex head

This type of drive head is intended for use with a socket or wrench/spanner. The head is hexagonal in shape and is mostly used on screws with much higher intended torques. The most common screw of this drive type used in carpentry and construction is the structural coach screws. 

Torx drive

The Torx drive is a unique hexagonal star shape that is cut into the head of a screw. An appropriately sized Torx bit allows for an impeccable registration of the bit in the head of the screw. This means that the bit never slips out of the head and also provides enough friction so that the screw can be placed over the end of the bit without falling out. This is very useful for the installation of the screw. This drive type is mostly seen in specialist wood screws or decking/cladding screws. The downside to this type of drive is that the smaller sized Torx bits can often deform when used in high torque applications.   

Flat/slotted

The slotted head and flat bit are the oldest type of drive that is still in use today. This method of screw manufacture is incredibly easy and was used in the very first manufactured screws hundreds of years ago. The flat head is not commonly used in construction in modern times due to its poor efficiency during mass installation. This type of drive can commonly be found in older properties though. The disadvantage to this drive type is that the registration of the bit in the head is very poor and often slips out of the slot sideways, particularly when operated fast with a power tool.  

Hexagonal drive

The hexagonal drive screw head is a standard hexagonal shaped slot that is cut into the head of the screw. The implementation of this type of screw is often intended for use with Allen keys and is mostly found in kits or premade furniture. This is a very positive type of drive registration. 

Square drive (Robertson)

Square drive screws have a perfectly square slot cut in them for use with a square bit. This type of drive was invented as an early form of positive drive registration, improving on the weaknesses of the Phillips drive. This type of screw is still in use in modern construction, though is not often used in the UK, with North America being a more suitable market.  

Common construction screws

Now that we understand the different components and drive types that makeup the selection of construction screws, its time to look specifically at the most common types that are utilised on site. Here we will look at a handful of the most widely used screws, with explanations of their construction and intended uses.  

Concrete screws

Concrete screws are a relatively modern invention, intended for fixing directly into masonry without the requirement of a plastic plug. The shanks of these screws are thicker than common construction screws, around 7.5mm in contrast to the typical 5mm. The heads of these screws are a flat countersunk style with some form of positive registration drive type for maximum torque. Lengths of up to around 200mm are available off the shelf from most merchants. The screws are made of zinc coated, hardened steel for external corrosive resistance and maximum shear resistance respectively.

The intended use for these screws is to fix materials like timber directly to masonry. For the screw to operate correctly, a pilot hole must be drilled into the masonry at the specific diameter stated by the manufacturer. For 7.5mm shanks screws the pilot hole diameter is recommended at 6.5mm. The hardened screw is then driven into the hole through the material, with the threads engaging in the masonry. These screws are suitable for engaging into brick, block, and concrete. Certain types of softer, older material like sand and lime masonry or crumbly breeze blocks are often too soft for the aggressive threads of the screw to engage in. 

Due to the hardened nature of these screws, they are very resistant to stripping and can be used and reused for temporary fixings during the course of the construction process. Overall, these screws are the perfect modern solution to fixing timber and other materials directly to masonry in structural applications.

Another proprietary adaptation of the concrete screw is the “Wall Dog”. These screws are a full stainless-steel construction and feature a more aggressive thread than the standard style of masonry screws. The heads of these screws are a round head countersunk style. The diameter of the shank is thinner than that of a regular masonry screw and mostly come in smaller lengths as well. The intended purpose of these screws is for fixing or mounting lightweight components into masonry without plugs. The examples of equipment that can be fixed with these screws are conduit and utility boxes. They are aimed at being implemented in finish mounting work.

Plasterboard/Drywall screws

Drywall screws are applications specific, finding proper use only during the installation of plasterboard or drywall. They are made of a soft carbon steel and are not sheer resistant. They come either in a brushed steel or black phosphate finish – appropriate qualities for implementation with plasterboard and wet gypsum products. They are extremely susceptible to corrosion and must not be used in external scenarios. They feature a bugle or wafer head with a Phillips drive for maximum load distribution across the face of the plasterboard. For regular tacking or over-boarding, 35mm or 50mm screws are suitable, though sizes up to 100mm are readily available from the merchants. They are thinner in their shank diameter than regular construction screws at around 4mm. These screws can be purchased on strips for use with a screw gun, or loose in a box for single application. These are known as collated and uncollated respectively. They are unsuitable for use in structural applications due to their poor sheer resistance.  

Coach screws

Coach screws are a heavy duty fixing intended for use in structural applications. They are constructed from hardened steel and finished with some variety of corrosive resistant coating. The screws are very resistant to draw out as well as shearing forces. Due to their hardened construction and thicker shank, they very rarely strip or break and can be used and reused numerous times. As such they are perfect for strong but temporary pinning or tacking applications. Coach screws can be used to hold timber components in place whilst installing alternative desired fixings. The heads of coach screws have some sort of positive registration, typically a hex drive with a washer head. This means high torque can be applied to the screw with the wide head capable of distributing much force across the surface of a material. The standard size for these hex drive screws is a 7.5mm shank with an 8mm hex drive head on them. Many sizes all the way up to a staggering 400mm in length are available on order from construction suppliers. Due to the wide head, they are great for pulling materials together. The tip of the screw has a flute cut in it that enables the screw to be effectively driven without a pilot hole being bored. Whilst these screws can be on the more expensive side of timber fixings, their reusability and reliability render them very cost effective. Whilst the average size of the screws for construction applications are 8mm drives, sizes on the M scale exist from M6 up to M12 (which we'll talk about in the bolts and threads section) - the latter of which are intended for extremely heavy-duty structural applications.  

Wood screws

Standard wood screws make up the bulk of the screws that are used within construction, specifically carpentry. These screws are very simple in their construction and utility and are used for most of the light duty tasks that are carried out. They come in a variety of shank thicknesses and lengths for use in different applications. The typical shank thickness for these screws is 5mm, often featuring a PZ2 drive countersunk flat head. Larger wood screws for heavier duty applications are available with a 6mm shank and a PZ3 drive. Smaller varieties at around 4mm are suitable for light duty fixing applications. These smaller screws might feature a Phillips drive if they are cheaper screws included in kits (See the cheaper screws section below). Lengths up to around 150mm are commonly available off the shelf in most merchants. The screws mostly come in either a zinc coating or a brushed steel finish. The general construction wood screws typically come with a self-tapping flute cut into the point and a flat countersunk head with self-countersinking grooves cut into the underside of the head. General application wood screws mostly feature a single thread, with more application specific varieties featuring a double thread for additional resistance to draw out. Whilst most screws of this type will be fully threaded, ½ or 3⁄4 threaded options are available. The intended application for these screws is to draw one material into another. When driving a screw of this type through a thin material or sheet good into a timber substrate, the threads will fully engage in the timber below, but not into the sheet good. The sheet is then pulled into the substrate as the head is wound into the sheet. This eliminates the chances of gaps between the two surfaces. These are especially useful for fixing loose floorboards. Holes must be predrilled in the ends of boards in order to ensure that splitting does not occur when driving screws. Whilst most wood screws are made from steel or iron, certain materials require the use of specific screw materials. For example, due to the corrosive Tannic acid that is present in Oak, iron or steel screws are unsuitable for use. Special brass screws must be used when hanging doors or other such tasks in these instances. 

Cheap screws

Many kits and pre-built furniture that require assembly come with cheap and poorly made hardware. The culprits we’re talking about here are the small wood screws that are provided in kits for fixing items to walls or attaching other hardware to components. These screws are made of incredibly soft metal that hasn't been hardened. Oftentimes kits will include screws that have been cast in a mould - a huge mistake for an item that will have torque and shear forces applied to it. These screws often strip when trying to drive them, and frequently the heads will twist off even if a hole is predrilled. My recommendation for installation of any components that come out of kits would be to replace the screws in the kits with the same size of proper screw. 

Lost head screws

Lost head screws are a modern type of screw that are generally used for applications where the head of a regular screw on display would be unsightly. These applications include installing cladding, internal finished timber components, and decking. They are thinner than most screws in their shank diameter and have a small head that leaves a fine hole in the material. The head is self-countersinking and can be easily driven below the surface of a material. The thread of the screw is typically ½ or ¾ of the way up the shank. This enables the small head to still effectively pull one material into another. The drive type of these screws is mostly some sort of positive registration, such as a Torx. The screws come with a corrosive resistant finish that is perfect for use in exterior applications. The sizes of these screws typically do not exceed around 70mm, as they are not used in structural framing applications. 

Truss screws

This very recent development is a modern solution for tying structural components together, in place of more traditional and labour-intensive hangers. The intended applications for this type of screw are for fixing through plates into rafters or trusses, as well as through plates into studs. As it currently stands these screws are patented, proprietary, and manufactured through one company. These screws are long (around 150mm) with a full thread, and a shank diameter of around 4mm. The heads are similar in shape to lost head screws, with a positive registration drive type, a Torx T30 to be specific. The screws are able to be installed at any stage during the construction process with a fair allowance on the angle of installation for added accessibility. These screws manufactured by this company are coated for corrosion resistance and are bright orange in colour. This allows for easy identification during inspection. The testament and experience of many contractors find that whilst these fixings are very effective and quicker to install than truss clips or hurricane ties, they are so new in entering the market that inspectors are not yet familiar with them.  

Deck screws

Deck screws are screws specifically designed and sold for the installation of decking boards. The screws can be shaped like a standard wood screw or like a lost head screw - with the choice depending on the desired finish of the deck. These screws are coated for exterior use. There are many companies that sell specialist proprietary systems alongside their screws, such as jigs or plugs that cover the head of the screws.  

Self-tapping screws

Self-tapping screws are a modern solution for fixing into steel and other metal materials. The screws are hardened steel in construction with a built-in drill bit on the end for boring a hole into the metal. Once the hole is drilled the hardened threads of the screw engage into the metal and tap a hole all the way through. Once the threads are tapped the rest of the screw is driven into the hole and engages itself in the thread. The head type is typically a flat countersunk style for the longer screws, and a flat wafer head for smaller screws. The drive type is some sort of positive registration, such as a hex head. Often the longer screws come with a rubber washer that more effectively distributes the load of the head over a material. The longer screws can reach up to around 120mm in lengths, all the way down to small screws that are around 15mm in length. Specialist sizes can be purchased from certain suppliers.