Not all roof leaks begin as damage to a shingle. Sometimes the cause of the leak is actually inside of the home. Over time, damage to the interior roof creates an opening to the outside, and a leak is a result.
Damage can be severe, in part because it is not limited to the shingles on the roof. The condensation typically collects on the attic ceiling, which is also the bottom decking layer of the roof. The plywood decking absorbs the water, and mold and moisture rot can begin to soften and degrade the wood.
The wet decking transfers moisture through the roof so that now the condensation is also collecting beneath the roofing felt and shingles. These are not surfaces designed for constant moisture exposure, so eventually, they begin to degrade from the consistent damp conditions. The shingles may begin to curl at the edges, or the granule layer may begin to flake off.
The cause of this condensation is usually poor ventilation. Warm, moist air in a home rises up into the attic space. Ideally, cool dry air should be drawn in through soffit vents in the eaves, where it will then push out this moist air through the ridgeline vent.
If there aren't enough vents on the roof to provide the proper airflow, moisture may collect in the attic instead of venting out properly. Complicated roof shapes or attics that are cut up by various internal walls may also have pockets that aren't vented properly, which leads to moisture buildup in the affected areas.
Ventilation and condensation issues can be repaired. The first task is to improve rooftop ventilation. In some cases, this only means cleaning out or replacing existing vents so they work better. In other cases, this may require putting in more vents along roof ridges, in the soffits, or even on the roof plane between the peak and eaves.
Your roofer will then make repairs to the damaged areas. Damage that occurs from the inside out will usually require the replacement of the affected decking as well as the shingles above. Depending on the extent, you may only need to replace part of the roof. If condensation persists after ventilation and roof repairs, then your roofer may recommend installing an attic fan to further aid in air movement.
Contact a residential roofer if you have concerns about condensation buildup in your attic.