A slate roof is a specialized roof and very expensive. It is important to confirm with the contractor you want to use that they specialize in slate roofs before you allow them on your roof.
A slate roof is also very expensive to maintain and must be inspected by a slate specialist on a yearly basis to ensure the roof stays in order for years to come. Broken slate tiles should be replaced and valleys & flashings should be kept clean.
Slate tiles give your roof a natural look and no waterproofing products should be used on a slate roof. The correct way to waterproof a slate roof is to repair it and main it. When repairing a slate roof it is important to inspect the underlay, if this is damaged the roof will leak. To repair the leaking roof the underlay must be replaced and the cracked and broken tiles must be replaced.