Barbed underlayment nails have barbs on the ring shank that dig into the wood, providing increased holding power. Annular ring underlayment nails have a series of rings around the shank, which also improves holding power but is less aggressive than barbs.

The appropriate nail size for underlayment depends on the thickness of the underlayment and the subfloor. Typically, 1-1/4 inch underlayment nails are used for 1/2 inch underlayment, and 1-3/4 inch underlayment nails are used for 5/8 inch underlayment.

Underlayment nails should be spaced approximately 6 inches apart along the edges and 12 inches apart in the field. This spacing ensures that the underlayment is securely fastened to the subfloor.

Underlayment nails typically have a ring or disc-shaped head. The ring or disc head provides additional holding power by distributing the nail's force over a larger area.