On roofs, tar-like materials shed water by forming dark water-resistant layers, but modern roofing products are often bitumen or asphalt rather than traditional wood tar.
The word tar is often used loosely in roofing. Many modern roof products called tar in everyday speech are bitumen, asphalt, mastic, or modified roofing compounds. Their behaviour can be very different from pine tar or traditional wood tar.
Roof exposure is severe. A roof coating faces direct sun, standing water, temperature swings, wind, and movement in the underlying structure. A dark tar-like layer can become very hot, soften, expand, and then stiffen again as temperatures fall.
Water-shedding depends on continuity. Small cracks, poor edges, blisters, loose laps, and bad drainage can defeat a coating. Tar-like materials can seal small areas, but they are not a cure for a roof that has structural movement or water trapped beneath the surface.
The behaviour on roofs is therefore product-specific. Traditional tar, coal-tar pitch, bitumen coatings, and modern roof mastics should not be treated as one material simply because they look black and sticky.