Al Bundy wrote:

So why do they use these plastic cap nails for felt?

http://www.simplexnails.com/PlexCapDataSheet.html

I had never seen them used till I saw pro roofers doing roofs in the
south. I always saw and used just staples myself. You see the roofers in
the northeast with one of those swing staplers and it goes fast.

Those plastic cap ones, with the volume a roofer uses, must tap into
profits and probably take a lot longer to put in.

Once the shingles are on, the shingles and nails hold everything down
anyway.

Regional differences are curious. In New England furring the ceiling,
aka strapping, is standard operating procedure. It's rarely done in
other areas. That's an even bigger time/money sink and with similarly
small return on investment.

R