Pilot holes: what they are and when you need one

Learn to drill a pilot hole so screws grip properly without splitting wood or cracking walls.

PH-01Understand what a pilot hole doesA pilot hole is a small hole you drill before putting in a screw. It guides the screw in a straight line and stops the material splitting or cracking. Skip this step near the edge of a piece of wood and it will often split clean in half. In brick or plaster, a pilot hole also gives a wall plug somewhere proper to sit.

PH-02Pick a drill bit that's slightly narrower than the screwLook at the screw's core, the solid metal shaft under the thread, not the width across the threads. Your drill bit should match that core width, or be just a touch narrower. The beginner mistake is choosing a bit the same size as the whole screw. That makes the hole too big, so the screw spins without gripping anything.

PH-03Mark the spot before you drillMake a small dent with a pencil, or better, a bradawl (a pointed hand tool you twist into the surface). This stops the drill bit skidding sideways when you start it. On hard or shiny surfaces the bit will wander without this, and you'll end up drilling in the wrong place.

PH-04Check what's behind the wall before you drill into itIf you're drilling into a wall rather than a loose piece of wood, expect there to be a cable or pipe there and prove otherwise, don't assume it's clear. Use a cable and pipe detector to scan the area first. But a detector reduces risk, it doesn't remove it: it can miss cables that run deep, sit inside metal conduit, hide behind metal-backed plasterboard, or simply run somewhere unexpected, which happens a lot with older or amateur wiring. If you have any doubt at all, isolate the circuit at the consumer unit (the fuse box) before drilling, and turn off the water at the stopcock before cutting into anything that could carry a pipe.

PH-05Drill the hole straight and steadyHold the drill at the same angle you want the screw to end up at, usually 90 degrees to the surface. Start slowly to let the bit bite, then keep a steady speed without forcing it in. If you're pushing hard, the bit is probably blunt or the wrong size, not something to muscle through.

PH-06Stop at the right depthThe pilot hole should be a little shorter than the screw itself, so the screw's tip still has to cut its own path for the last few millimetres. Wrap a bit of tape round the drill bit at the depth you want as a visual stop. Drilling too deep is a common mistake and it means the screw won't grip properly once it's in.

PH-07Test the screw to check you got it rightDrive the screw in by hand or with a screwdriver bit. It should go in with steady, even resistance the whole way, not spin freely and not need heavy force. If you hear or see cracking, the hole is too small or too shallow. If the screw feels loose and wobbly once fully in, the hole was too big.

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