Hello, my name is Ben and it is my pleasure to assist you with your question today.
In situations such as this one, it would generally be considered that the storm that caused the damage was an act of God and as such you would not be responsible for any damage that occurred to a third party's property. However, if they can show that you had been negligent in the upkeep of the roof and that the tiles had either not been properly secured or maintained, then you may have some responsibility for the damage under the tort of negligence. However, if they were properly secured and the only reason they blew off was due to a 'freak' storm that would not have been expected to occur frequently then it is an act of God and the person whose car was damaged would have to either claim through their own car insurance or pay for the damage themselves.
Nevertheless it is important to check your buildings insurance to see exactly what is covered. Even if you give them the details of your insurance they will not be able to claim from it unless this type of damage was covered so they would simply be told they cannot claim and then that leaves them with the only option of suing you directly or claiming through their own insurance. If they were to sue you then they would have to prove negligence on your part before they can attribute any liability on you.
Please let me know if this has answered your original question or if you need me to clarify anything else for you in relation to this? Thanks