Do I have to replace all the flashings when I replace my old slate roof?

Usually, the answer is yes. In order to warranty an entire new roof, all flashings should be replaced. Some roofers may quote roof replacement without replacing any flashings at all, but this is only appropriate if existing flashings are just a few years old and were installed correctly. 

Otherwise, your brand-new roof may still leak in the old flashing areas. Further, it is less expensive to install new flashings when all old roofing coverings have already been removed for your roof replacement. Slates overlap flashings, so your new slates would have to be removed to replace flashing that leaks in the future. Finally, flashing replacements should NEVER involve installing new metal on top of old metal. Read your quote carefully. A proper flashing replacement includes removal of all old flashings before new flashings are installed.