Have you ever stepped on a nail? This can be a very unpleasant experience for you, your children and even your pet. Ask your roofer what he will do to prevent nails from being left in your yard.
The good roofers always use a hand carried magnet to sweep over your grass and plant beds removing any nails or metal barbs. He may also drag a large magnet through your yard for the same reason. As the roofer proceeds with your roofing work it is a certainty that nails will wind up in your yard. It is the responsibility of your roofer to remove every one of them before they become a hazard.
Laying down of tarps and Visqueen will be a good first step measure but it alone will not be adequate to prevent all of the nails from finding their way to your lawn or planting beds. They can be a real nuisance if they are overlooked and cause harm or expensive damage regardless of where they are found.
Your roofer's ground observer should continually sweep with a large magnet the areas below the continued roofing work. Walk these areas every day with your roofer to be assured that adequate removal methods are being utilized.