How deep does a nail need to go to puncture a tire?
An average of 1/2 inch. It varies considerably with the tire casing design, tread depth and where the nail is trying to go through the tire. DO NOT PULL THE NAIL!
Are nails or screws worse for tires?
A nail does the least damage to your tire because it’s straight and smooth. A screw is more dangerous because of its threads. And a rock, or other jagged object, is the worst because of its irregular nature. So if you do have a puncture, you have the best chance of repair if it was caused by a nail.