Slates should always be nailed near the top end, where the hole will be covered by the next slate up. Face nailing is when a roofer deviates from this simple, crucial rule and nails through the front or "face" of a slate.
Face nailing only solves your roofers problem, not yours. When a roofer repairs a roof this way, he is not only doing a bad job of repairing your roof but he is damaging it further! A nail through the exposed face of a slate, as with anything you put a hole in, is no longer waterproof; however, he is not only ruining one slate, but also the one below it, making a hole straight through your roof.
Another issue with face nailing is when new snowguards are installed. As demonstrated below, the snowguard installer might just put a face nail through to get the job don quickly, despite it being an incorrect and damaging "repair".
If you think your home may have issues with face nails, give us a call and we'll happily come out to inspect any potential damage and give you an estimate!