Lead has been used for hundreds of years due to the flexibility and durability of the material that enable installers to create complex waterproof seals at roof junctions. However, its high scrap value makes it an attractive target for thieves. Although this problem affects churches and schools more than private houses, it can still be an issue and some European countries have banned the use of it in construction to safeguard their properties.
Lead alternatives are available to avoid this potential problem but you should make sure you ask your roofer about the materials they are using to be on the safe side.