If properly constructed and cared for, a slate roof will remain a waterproof covering for your home for over a hundred years. Slate roofs do not require any underlayment; as long as the slate and flashing are in good condition and installed properly, the roof will not leak. To prevent leaks or to stop a slate roof from leaking you will need to determine whether your roof needs to be repaired or replaced, then act accordingly.
Check for broken or missing slates. If slates are broken, missing or faulty, have the slates removed and replaced. Try to use slates of similar size, type, shape and colour if possible. It only takes one missing tile to cause a roof leak.
Check the flashing. The metal flashing on a slate roof will wear out before the slate, and this can be a source of leaks. Older flashings such as copper or terne coated steel (sometimes called tin) need to be painted regularly to avoid corrosion. If the flashings have failed and are leaking, they will need to be replaced. This can be done without replacing the entire roof.
Know what type of slate is on the roof. Some types of slate wear out faster than others. Try to identify the type of slate by sight. If this is not possible, have the slate identified by a professional, contact a local slate quarry or a slate roofing contractor. A link to the Slate Roofing Contractors Association of North America with a listing of contractor members is listed in the Resource section.
Replace a failing roof. Hire a reputable contractor with slate experience to repair or replace your slate roof or flashing. Check references and make sure all details about the roof repair or replacement are put in the contract, including the type of slate being used. Although slate roofs are durable, a slate roof is only leakproof if it is installed correctly.