What causes attic rain?
Short answer. Attic rain happens when warm, moist air from the house leaks into a cold attic and freezes as frost on the underside of the roof deck and on nail tips during a cold snap; when the sun or milder weather warms the roof, the frost melts at once and drips onto the insulation and ceiling. The root causes are too much moisture getting into the attic and too little ventilation to carry it out.
Where the moisture comes from
- Bath fans, kitchen hoods or dryer vents that end in the attic instead of outdoors.
- Air leaks through recessed lights, attic hatches, wall top plates and plumbing chases.
- High indoor humidity from humidifiers, unvented gas logs, a full house or a newly built home that is still drying out.
- Leaky duct joints in the attic pushing humid indoor air out.
Why it shows up suddenly
Frost builds over several cold nights, especially under clear skies when the roof radiates heat and drops below air temperature. When temperatures rebound, it melts in a day. Homeowners see drips or stains and assume a roof leak, but the shingles are fine.
Telling it from a leak
- Water appears after a cold spell with no rain.
- Wetness is spread widely across the deck instead of at one spot.
- Nail tips are wet or rusty all over the attic.
Fixes
- Route every exhaust fan outdoors.
- Air seal the attic floor.
- Restore intake with baffles and clear soffits, and confirm exhaust is adequate.
- Keep indoor humidity moderate in winter.
In Georgia it is most common in the mountain counties and during hard cold snaps elsewhere. Read roof ventilation basics and common causes of roof leaks.