Screws explained: gauge, length and head types

Learn what screw gauge, length and head type mean so you always pick the right screw for the job.

SC-01Understand gauge (thickness)Gauge is the diameter of the screw shank, shown as a number like No.6, No.8 or No.10 - the higher the number, the thicker the screw. Thicker screws hold more weight but need a bigger pilot hole (a small guide hole drilled first) in hardwood or they can split the wood. For general jobs like fitting hinges or shelf brackets, No.6 to No.8 covers most things.

SC-02Understand length and how to pick itLength is measured from the tip to where the head sits flush, in mm. A simple rule: the screw should go at least two-thirds of its length into the solid material it's gripping, not just through the thing you're attaching. Beginners often grab whatever screw is in the box - too short and it won't hold, too long and it pokes through the other side.

SC-03Recognise head typesPozidriv has a cross with extra small lines between the arms; Phillips looks like a plain cross but is slightly rounded inside; Torx is a six-point star shape; slotted is just one straight line. They are not interchangeable - using a Phillips bit on a Pozidriv screw (or vice versa) makes the driver slip and chews up the screw head, called cam-out.

SC-04Match the screw type to the materialWood screws have a coarse thread and a smooth section near the head. Self-tapping screws cut their own thread into thin metal or plastic. Masonry screws have a special thread for brick or concrete and always need a pilot hole first. Using a wood screw in masonry, or a masonry screw in timber, simply won't grip properly.

SC-05Fit the driver bit correctly before you startPush the screwdriver bit firmly into the screw head and hold it dead straight, not at an angle. Apply steady downward pressure as you turn - most cam-out happens because people turn hard but push weakly. If the bit keeps slipping out, you likely have the wrong bit type or size for that screw.

SC-06Practice on a scrap offcut, not the real jobTake a spare piece of timber and try driving two or three different screws by hand or with a drill driver on a low torque setting. Feel how a correctly sized pilot hole lets the screw go in smoothly, while no pilot hole in hardwood makes it hard to turn or splits the wood. This is a low-stakes way to get a feel for resistance before you fix anything real.

6 each · Assorted wood screws

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1 each · Timber offcut

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SC-07Check you've got it rightA correctly chosen and driven screw sits flush with the surface, doesn't wobble, and turned in with steady, even resistance the whole way. If the screw head is chewed up, the wood has split, or the screw feels loose and spins without gripping, something was wrong: too big, too small, wrong head match, or no pilot hole. Undo it, check which of these it was, and try again with a correction rather than just forcing it in.

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