The tar doesn’t totally ruin the roof functionally, but it does totally ruin the roof aesthetically, and it takes about 50 years to wear off. In the meantime, besides having to look at an ugly roof for the rest of your life, if the roof does need repaired (and it will that’s why it was coated it leaked!) the job of repairing it properly is much more difficult when the roof is all glued together with tar. If you run into a contractor offering to tar your slate roof, smile, speak gently, humor him, and get rid of him as quickly as possible. One common source of leaks on slate roofs is a hole in a slate. Holes often result from nails that weren’t nailed down far enough when the roof was installed, or which backed out of the roof as the sheathing boards dried. The nail heads then worked against the overlying slate eventually to a hole right through.
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