What is a pilot hole and when do I need one?

It seems backwards that removing material makes a screw hold better, and the reason is worth understanding: a screw holds by its threads cutting into wood, and a pilot hole removes only the material the solid shank has to displace anyway.

Safety

Step by step

1. Understand what the screw is doing.The threads cut into the wood and grip. The solid core in the middle has to force wood apart to make room for itself, and that wedging force is what splits timber.
2. Size the pilot to the core, not the thread.The pilot should match the screw's shank diameter measured across the solid part, not across the threads. That leaves the threads full material to bite into while removing what the core would otherwise wedge apart.
3. Hold the screw against the bit to check.Hold the drill bit in front of the screw. You should see the threads either side of the bit and the core hidden behind it. That is the right size, without any arithmetic.
4. Always pilot in hardwood.Oak, ash, beech and similar. They split readily and are hard enough that a screw driven without a pilot will often shear off instead.
5. Always pilot near an edge or an end.Within a few centimetres of the end of a board is where wood splits most easily, because there is nothing beyond to resist the wedging. This is the most common split.
6. Always pilot into the edge of MDF.MDF has no grain and its edge splits or mushrooms almost every time without a pilot. The face is more forgiving; the edge is not.
7. Pilot for anything large.Coach screws and long structural screws generate substantial wedging force. A pilot is not optional for those, and a second larger clearance hole through the top piece is often needed as well.
8. Use a clearance hole in the top piece.For pulling two pieces together, drill the top piece large enough for the screw to pass through freely and pilot only the lower piece. That is what lets the screw clamp them tight rather than holding them apart.
9. Countersink where the head must sit flush.A countersunk head forced into the surface without a countersink lifts the fibres and splits the face. A countersink bit takes seconds and gives a clean flush finish.
10. Skip it for softwood and plasterboard fixings.Pine well away from an edge, or a modern self-drilling screw into softwood, usually needs nothing. Plasterboard fixings have their own instructions and generally do not want a pilot.
11. Match the depth to the screw.Drill slightly shallower than the screw's length so the tip still bites fresh material. A pilot drilled right through gives the threads nothing to hold at the end.
12. Lubricate a stubborn screw.A little wax or soap on the threads reduces the force needed considerably in hardwood, and reduces the chance of shearing the screw.

Tips

• Mark the drill depth with a piece of tape on the bit. It is quicker than a depth stop and accurate enough for almost everything.
• If a screw is difficult to drive, stop and drill the pilot larger. Forcing it either splits the wood or shears the screw, and neither is easily undone.
• Keep a small drill bit permanently in a second drill or a bit holder. Most people skip pilot holes because of the swapping, not the drilling.

Common mistakes

• Drilling the pilot the same size as the thread — There is nothing left for the threads to bite into, so the screw spins freely and holds almost nothing. Size it to the solid core.
• Screwing into the end of a board without a pilot — It is the most likely place for wood to split, because nothing beyond the screw resists the wedging force.

If it doesn't work

Questions people ask