Q: We need more strength from our nailed joints. Should we use a longer nail, fatter nail?

A: I am assuming that the strength you want is withdrawal and not side or lateral strength. I will use the term withdrawal resistance. Also, we are concerned mainly about the tip of the nail and the characteristics (density and grain angle) of the piece of wood that it is driven into. (Note: that staples are considered to be two nails; the discussion below applies, in general, to staples as well.)

Increasing strength  

First and foremost, understand that the density of the wood into which the fastener is driven is extremely important. If the wood member's density is 10 percent higher, the withdrawal is more than 30 percent greater. Also, if the head of the nail is not large enough in softer woods, the head will pull through the wood, even though the tip of the nail stays in place. With softer woods, head size may have to be increased.

The withdrawal resistance of a nail is a direct relation to diameter; double the diameter and double the resistance. It is also related to the depth of penetration into the second piece; that is, the second piece is the piece into which the tip is driven. Double the depth of penetration and the withdrawal resistance doubles. In addition, the withdrawal can be increased by using a coating on the nail to increase surface friction or using various surface roughness designs including burrs, flutes, spiral flutes and acid etching.

Decreasing strength  

The withdrawal strength drops substantially if the wood splits when the nail is driven. For this reason, we often prefer a nail with a dull tip that can tear the fibers as it enters the wood rather than wedge the fibers apart causing a split. In some cases where splitting is likely (near the end of a piece, with high-density wood, with over-dried wood, or with brittle wood), it may be necessary to predrill the hole in both pieces of wood being fastened to prevent splitting. The hole can be about 90 percent of a nail's diameter without losing strength.

The withdrawal strength is greatest when the nail is driven into side grain rather than along the grain (that is, driven into end grain). End grain withdrawal can be 50 percent lower than side grain. When the tip will be in end grain, driving the nail at a 30 degree angle (sometimes called toe-nailing) or when using two nails, driving them at different angles (called slant driving) can help develop more strength.

Finally, when using several nails, they should not be too close to each other, as they may split the wood and reduce the strength overall.

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