Why is my ceiling dripping in cold weather when it hasn't rained?

Greater Victoria's winters are mild enough that most attics never see sustained frost, which is exactly why the drip surprises people. A week of Arctic outflow in December or January, with clear nights and the temperature well below freezing, loads the sheathing with frost from every warm-air leak below. The first sunny morning melts it and the ceiling below the worst spot drips as if the roof had failed, in a week with no rain at all. Then the mild weather returns and the 'leak' vanishes until the next cold snap.

The housing most prone to it is the 1960s to 1980s stock in Saanich, Colwood and Esquimalt, where bathroom fans were added or replaced and simply left blowing into the attic, and where an extra layer of blown insulation buried the soffit vents. Heritage homes in Fernwood and James Bay have the opposite problem: leaky, uninsulated attics that ventilate so freely they rarely frost, but that lose heat instead. A new metal roof can also show condensation on its underside if the underlayment and venting were not specified for it.

On a cold morning, open the attic hatch and look with a flashlight without climbing onto the insulation. Frost glittering on the nails, a duct lying loose in the insulation, or a wet patch that does not line up with any roof penetration are all signs pointing at condensation. Look for daylight at the soffits; if you cannot see any, the vents may be blocked. Photograph what you find and stop there; a home inspector or roofer can do the rest.

• Frost or beads of water on nail tips
• Bath or dryer duct not connected to a roof or wall hood
• Insulation pushed into the soffit, no baffles
• No wet trail from a chimney, vent or skylight

The fixes are cheaper than a roof: connect every fan to an insulated duct that exits through a roof or wall hood; seal the attic hatch, plumbing stack and light fixtures with foam or gaskets; install baffles so soffit vents breathe; and add a ridge vent or roof vents if the attic has too few. CMHC and BC Housing guidance on coastal building envelopes treats this as an air-sealing and ventilation problem, and that is the order to fix it in. If a re-roof is coming, make sure the quote specifies ridge and soffit ventilation so the new roof does not inherit the frost.

A drip in dry cold weather can occasionally be a slow roof leak that stored water in insulation during the last storm and released it later, or snowmelt on the rare occasions Victoria has a snow load. If the drip lines up with a chimney, vent boot or skylight and the attic shows a wet trail from that point, treat it as a leak. A roofer's inspection with a written report ($150 to $400 in our model) is the way to settle a disputed case before any repair.