One of the unique features of a slate roof is that a slate roof is easily repairable. A slate roof is made up of thousands of individual pieces of natural slate. The combination of individual pieces of slate and separate flashing components allow repairs to be made in localized areas without replacing the entire roof.
Individual pieces of slate or areas of damaged slate can be replaced since the slates are not connected in anyway to the slates around them. This unique feature of slate (and tile) roofs makes it important not to use roof cement or tar (not to mention caulk) to repair slate roofs in areas other than at the hips or ridges.
The photograph below shows a valley coated in roof cement. The adjacent slates were also coated. Normally, when a valley is leaking, the valley metal should be removed. To do this, the slates around the valley are removed and salvaged for re-installation. In this case, slate cannot be salvaged because now they are connected to all of the slate around them making a repair more costly.
Wherever possible, slates should be fastened to the roof using copper nails and hooks. Copper nails can be used when replacing a larger continuous area of slate. Hooks can be used when replacing individual slates in areas other than the ridge and starter course. For broken slate in the starter course, the slate in the second row can be pulled and the first row nailed and then the second row can be hooked. The first few rows of slate is an area vulnerable to leaking and ice damming. Creating additional holes in slates in this area using the nail and bib method is a bad idea. Nailing and bibbing is a repair that involves punching a hole in the slate that is going to be used as the replacement slate, nailing the slate in place between the keyways, and then placing a sheet metal "bib" over the nail.