The short version
- Roof leaks follow rain; condensation follows temperature and humidity
- Frost and rusty nail tips point to condensation
- Plumbing and HVAC can look like roof leaks
- The cause decides the right contractor
Roof leaks
A roof leak usually appears during or soon after rain, often more with wind. Stains have defined edges and may grow with each storm. In the attic, water is found along the path from an opening down to the ceiling.
The roof above the stain usually shows a defect.
Condensation
Condensation forms when warm, moist air meets a cold surface. In an attic it shows up as damp decking, frost in winter, rusty nail tips and mold, spread across wide areas rather than tracking a single path. It is caused by poor ventilation, air leaks from the house or lack of insulation.
It does not need rain.
Other sources
Plumbing lines, bathroom fans that vent into the attic, HVAC condensate lines and leaking ducts can all cause stains. The timing follows water use or cooling, not weather.
Moisture meters and thermal imaging help separate them.
Why it matters
Fixing a roof does not stop condensation, and sealing a plumbing leak does not help a failed flashing. Getting the cause right avoids paying twice.
An inspector can tell you which problem you have and who should fix it.
General information, not engineering, legal, insurance or safety advice. Roofs, policies and local requirements vary, and every roof should be judged on its own condition.
