The fibrated has organic fibers in it to reduce crackling, but find it better to use non fibrated over smooth surfaced roofs.
Coal tar has such a low melting point that it will flow into and seal smaller holes and/or alligatoring that may occur as the surface of the roof slowly dries out. This flow characteristic is why you would see some old roofing construction with drains that sit above the roof an inch or so. They used to do this to intentionally pond water to keep the pitch cool, so I read. I have never installed coal tar, but have torn off literally tons of it.
Are you planning in coating your coal tar roof? If so, why?
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