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    Home/Know your materials/Screws

    Screws

    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.

    Screw heads

    The design of the screw head indicates the type of screwdriver needed to turn it. The most common are listed below.

    Slot-head

    Instantly recognisable by the single slot across the diameter of the screw head.

    Phillips

    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.

    Posidriv

    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.

    Dual-purpose

    A screw that can be turned by two different types of screwdriver e.g. a slot head and a Phillips.

    Hexagonal-headed

    Less common is the hexagonal or hex screw that needs a hex screwdriver to be turned.

    Square-headed

    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.

    Screw profile

    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.

    Countersunk 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.

    Round head

    The domed head of a round head screw will sit proud of the surrounding surface.

    Pan head

    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.

    Masonry screw

    As the name suggests these screws are used for fixing to masonry without a wall plug. Some masonry screws have a blue coating.

    Drywall screw

    For fixing heavy plasterboard to a timber stud frame, drywall screws are coated with black phosphate.

    Decking screw

    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.

    Coach screw

    A heavy-duty fixing with a thread that reaches to the underside of the head.

    Chipboard screw

    A stainless steel screw usually with a countersink head and a deep thread used for fixing chipboard flooring in place.

    Self-tapping screw

    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.

    Mirror screw

    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.

    Dowel screw

    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.

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