Tar & Gravel roof construction starts with layers of fiberglass felt paper and asphalt. This intermat is not a waterproofing layer until it is layered with asphalt and more fiberglasses felt paper. Over time the membrane surface wears down. Eventually the fiberglass felt will become exposed. It is generally accepted that when this occurs it is time to replace the roof. Without the fiberglass intermat holding the roof’s waterproofing construction together the membrane can no longer withstand thermal shock (expansion and contraction) and will crack, possibly allowing seepage.