Screws are a common fixing and come in a wide a variety of styles and sizes. Some screws will be for general use while others are designed for a specific purpose. Always use the most appropriate screw for the job and select the correct screwdriver. This not only makes the job easier it will prevent damage to the screw head.
Generally, three factors govern the description of screws: the length, the gauge and the type of head.
The length of a screw refers to how much of it is embedded into the material. Therefore the length of a countersink screw is measured from its point to the top of the head, while a round head screw is measured from the point to the top of the shank.
A screw’s gauge refers to the diameter of the shank with most screws being available in gauges from 2 (2mm or 1/8 in) to 14 (6.5mm or 9/16 in), and as we can see, the smaller the gauge, the narrower the screw.
The design of the screw head indicates the type of screwdriver needed to turn it. The most common are listed below.
Instantly recognisable by the single slot across the diameter of the screw head.
Designed for use with a screwdriver bearing the same name, the screw head will have two bisecting slots that get deeper towards the centre of the head.
Often confused with a Phillips screw, the Posidriv screw also has two bisecting slots in the head, but it can be identified by the small, narrow indents that combined with the slots form a star shape.
A screw that can be turned by two different types of screwdriver e.g. a slot head and a Phillips.
Less common is the hexagonal or hex screw that needs a hex screwdriver to be turned.
Another screw that may be unfamiliar and like the hexagonal-headed screw, the square-headed screw requires a special screwdriver or allen key.
Clutch-head security screw
Designed to allow you to fix them in position using an ordinary slot-headed screwdriver but almost impossible to remove.
Another aspect of the screw head that relates to its description is its profile. The three types of screw head profile are countersunk head, round head and pan head.
Screws designed so that the head will sit just below the surface of the timber. To accommodate the screw head the opening of the hole needs to be enlarged with a countersink drill bit.
The domed head of a round head screw will sit proud of the surrounding surface.
The head of a pan head screw will sit slightly above the surrounding surface and resembles a flattened round head.
Types of screw
Let us now look at a selection of screws that you may come across.
Traditional wood screw
Used widely in construction and DIY the traditional wood screw has a tapered shank.
Modern wood screw
Recognisable by its short, untapered shank, the modern wood screw is considered less likely to split timber.
As the name suggests these screws are used for fixing to masonry without a wall plug. Some masonry screws have a blue coating.
For fixing heavy plasterboard to a timber stud frame, drywall screws are coated with black phosphate.
A long screw, usually with a countersink head, for fixing down timber decking. The screw will have a sharp thread and short shank to make it easier to drive into the timber.
MDF (medium density fibreboard) screw
This screw has a sharp point with a twin-thread that makes the initial penetration easier. Further up the screw the twin-thread reduces to a single, coarser thread that tightly grips the wooden fibres.
A heavy-duty fixing with a thread that reaches to the underside of the head.
A stainless steel screw usually with a countersink head and a deep thread used for fixing chipboard flooring in place.
Self-tapping screws cut their own thread as they are driven in. Most commonly used when making a fixing into a metal or plastic material.
The mirror screw is used when a more aesthetic finish is required (wall mirrors) or when access is important (bath panel). The head of the screw is covered by a decorative dome, which can be removed if the screw needs to be removed.
A very distinctive screw because it has a point at both ends and no head. Used to join together two long lengths of timber where the fixing needs to be concealed e.g. curtain poles.