Shingle construction starts with a fiberglass intermat. During production the fiberglass intermat is saturated with asphalt and dried repeatedly until the desired thickness is achieved. This intermat is not a waterproofing layer but is necessary to keep the shingle from cracking and separating. Over time the shingles surface wears down. Eventually the intermat will become exposed. It is generally accepted that when this occurs it is time to replace the roof. Without the fiberglass intermat holding the shingle construction together the shingle cannot longer withstand thermal shock (expansion and contraction) and will crack, possibly allowing seepage. You may want to consider replacing the roof at this time.
This layer of randomly laid fiberglass not only strengthens your shingles, but also acts as a fire barrier. Fiberglass shingles typically obtain a class “A” fire rating as the fiberglass mat resists fire better than organic/paper mats. This is why it is important to have your roof inspected on a regular basis.
All weather conditions combined with time will have an impact on your shingle roof. Hail storms causing the loss of the granule cover will over time expose your fiberglass layer. Poorly ventilated attics will over time bake your shingles, breaking down the components, causing cracks and fissures in the surface of your shingles. This breakdown will allow the elements in through your roof.