Having removed the damaged slate and replaced it with a new sound slate, a single copper nail is installed through the slot above to fasten the slate.  Some mechanics prefer to use two nails, but this is usually not necessary as the surrounding slate will steady the replacement in place.  The holes are prepunched, to avoid breakage.  Look closely and you will see a second, unused hole above this one in the slot.  This is often wise in case your first nail should encounter a void in the deck.

Here we use a nailset to set the nail snug (not tight) to the slate being secured.  You may have to force the slot apart slightly to accomodate the head of the nail.  (Note: face nail two slates above is in a ridge slate, impossible to blind nail.)

Installing the copper "bib", cut four inches wide and a height equal to the exposure of the slate, plus two inches.  You may be able to slide the bib under the slate and over the nail head by hand, ash shown, or you may find it necessary to lift the slates in the upper course slightly.

Here, the bib gets a mechanical assist from a John Stortz and Son slate ripper, to position it just above the leading edge of the upper course and just below the head of the nail.

Once the bib oxidizes, it will become completely invisible.  It prevents frost from reaching the nail and holds it, and the slate, in place.  Can the bib slide out?  We have never seen it happen nor do we expect to.