Any materials that can be thrown or used as weapons should be chained or locked up.
Building materials can be expensive, people do actually look for building works because they know they can usually help themselves to a few bricks , a bit of guttering and so on. It's in your interest to keep your materials safe.
Remember, hopefully your tradesperson will be responsible and look after a lot of the above. If he's not then it is up to you - it is your responsibility.
Always e-mail me for explanations of terms you do not understand. You should always know what is going on and I am happy to help and advise you. Please e-mail me at harry@buildadvice.com.