Why do my screws split the wood near the end of a board?

Timber splits along the grain because that is its weakest direction. A screw driven near an end concentrates a splitting force exactly where the timber can least resist it, and once a split starts it runs.

What you'll need

• drill bits for pilot and clearance holes
• countersink (optional)
• clamps (optional)

Step by step

1. Move the fixing away from the end.As a working rule, keep screws at least five screw diameters from the end of a board and two and a half from an edge. Where the design allows, moving the hole solves the problem outright.
2. Drill a pilot hole sized to the screw core.The core is the solid shaft between the threads. The pilot should match it, so the threads still bite but the shank is not wedging timber apart.
3. Drill a clearance hole in the top piece.Full shank diameter, so the thread cannot grip in the piece being pulled down. Without it the screw jacks the two pieces apart instead of drawing them together, and the extra force splits the top piece.
4. Go one size up on hardwood and old timber.Dense and dried-out timber splits far more readily. A slightly larger pilot costs little holding power and prevents the split.
5. Countersink so the head does not act as a wedge.An unseated head crushing its way into the surface adds a splitting force of its own.
6. Clamp across the split zone.A clamp or a sash cramp across the end of the board while you drive the screw physically prevents it opening.
7. Choose screws with a thin shank or a split point.Modern woodscrews with a smaller core, a serrated tip or a split point cut rather than wedge, and are markedly better near ends.
8. Avoid screwing into end grain where you can.End grain holds screws poorly and splits readily. Where the joint needs it, use a dowel cross-pinned through the end grain, or a proper joint.

Tips

• Hold the screw against the drill bit and sight along it: if you can see the threads either side of the bit but not the core, the pilot size is right.
• Blunting a nail's point stops it splitting timber; the equivalent for screws is a bigger pilot.
• A cramp across the end of a board while you drive the screw is a two-second insurance policy.

Common mistakes

• Pilot hole drilled through both pieces at the same small size — The thread grips the top piece as well as the bottom, so the screw pushes the pieces apart. The joint never closes and the added force splits the timber.
• Driving screws into the end grain of thin stock — End grain offers almost no resistance to splitting, and the screw acts directly along the weakest plane.

If it doesn't work

Questions people ask