Plastic Cap Nails for Underlayment

Synthetic underlayment is engineered to be tough — but a bare nail head is a stress concentrator that can tear right through under wind load. The 1" plastic cap spreads the head load over ~50x more area, which prevents tear-through and keeps the underlayment in place during the gap between underlayment and shingle install.

Underlayment manufacturers (Owens Corning, GAF, Atlas, Polyglass) require plastic cap fasteners for warranty.

Two systems exist: cap nails (a coil-fed nail with the plastic cap pre-attached) and cap staples (a staple driven through a separate plastic cap from a cap-collated strip). Both work; both are code-approved. The difference is the gun:

Cap nailers (ProBilt PCN45, Bostitch N66BC-CT, Stinger CN100, Hitachi NV50AP3) fire the integrated cap nails. Cap staplers (Stinger CS150, ProBilt PCS150) fire the cap+staple combination. Don't try to load one fastener into the other gun.

1" nail for standard synthetic and 30-lb felt over 1/2"+ sheathing. 1-1/2" nail for house wrap or thicker underlayment. 2" nail for foam board, polyiso, or continuous insulation systems. When in doubt, longer is fine — too short voids the underlayment warranty.