Clues from timing
- Only during rain: likely the roof, flashing, or parapet.
- Only when the cooler runs: likely the evaporative cooler, its float valve, overflow, or the curb seal under it.
- On hot, humid days with the AC running: possibly a clogged condensate line or pan.
- All the time, regardless of weather: possibly plumbing.
Clues from location
A stain directly beneath or near the cooler duct points toward the cooler or its curb. Stains along an exterior wall below a parapet suggest the parapet cap or a scupper. A stain near a bathroom or water heater closet could be plumbing.
Evaporative cooler basics
A swamp cooler holds water in a pan and pumps it over pads. If the float valve sticks, the pan can overflow. Over time, the pan can rust through and water can run down the curb and into the roof. Shutting off water to the cooler and seeing if the leak stops is a useful test.
Who to call
Cooler and AC issues usually go to an HVAC technician; roof, curb, and flashing damage goes to a roofer. Sometimes you need both. If water has soaked insulation or drywall, ask about drying to prevent mold.
If you think it is the roof, request a roofing estimate.