Q & A: Why Predrill Screw Holes?

Why Predrill Screw Holes?

Q:

I think predrilling screw holes is a real drag, so I rarely do
it. Most screws seem to work fine without all that bother. Why
are you always advocating predrilling?

A:

Call us old fashioned,but when you’re working in solid
wood,we believe you’ll get the most effective and longest-lasting
fastening power from screws when you drill correctly
sized clearance and pilot holes.

First, drill a pilot hole through both boards.Pilot holes
guarantee that your screw won’t break off and your wood
won’t crack. For most hardwoods, the pilot hole should be
at least as large as the screw’s minor diameter. If the screw
has deep threads, or the wood is very hard, the pilot hole
should be another 1/64-in. larger than the minor diameter.
For softer woods, the pilot hole can be 1/64-in. smaller.

Skipping the pilot hole, or drilling one too small, can
create hairline cracks in solid wood and MDF. Visible
cracks are obviously bad, but other cracks that are too small
to see can eventually widen and cause the joint to fail.

Second, enlarge the pilot hole in the top board to make
a clearance hole. This larger hole allows the screw to pull
the two boards together. The clearance hole should be at
least as large as the major diameter of the screw. Skipping
the clearance-hole step often results in a small gap between
the two boards you’re joining together. Glue won’t
effectively bridge a gap larger than the thickness of a piece
of paper.