Frost that looks like a leak
Many owners call a roofer in February or March about a ceiling stain. It has not rained, the roof is covered in snow, and yet water is dripping.
The pattern is a classic. During deep cold, frost builds up under the decking and on the nail tips poking through. At the first thaw, it all melts within a few hours. The water runs along the rafters or drips onto the insulation.
The roof has no hole. The ceiling is letting air through, and the attic ventilation can no longer carry off the moisture.
Where the moist air comes from
Warm air rises and finds the smallest opening in the top-floor ceiling. The most common paths are these.
- the attic access hatch, with no weatherstripping or insulation
- recessed light fixtures
- the gaps around plumbing stacks and electrical wires
- a bathroom fan or range hood whose duct ends in the attic, or has come loose
What moisture does to the roof
A little frost in extreme cold is not alarming if it dries quickly. The problem starts when the wood stays wet from one season to the next.
- The decking turns black. Dark stains appear, mostly on the north side and near the bottom of the slopes.
- Mould sets in on the wood and on top of the insulation.
- Wet insulation packs down and insulates less well. The attic warms up even more.
- The decking goes soft. The panels sag between the rafters, and the roof looks wavy from outside.
Fixing the problem at the source
The order of the work matters. First keep moisture from getting in, then make sure it can get out. Work inside the attic is for a contractor.
- Bring the humidity in the house down to a reasonable level in winter. This is the only step that is up to you.
- The contractor seals the air leaks in the ceiling, from above, in the attic.
- The contractor adds weatherstripping and insulation to the access hatch.
- The contractor connects the fans to an outdoor exhaust, with an insulated, well secured duct.
- For our part, we check that air moves well from the soffits to the ridge, so the remaining moisture can escape.
Adding vents without sealing the ceiling can make things worse. An attic that draws harder pulls in more air from the house.
Frequently asked questions
How do you tell condensation from a roof leak?
Condensation differs from a roof leak in when and where the water appears. A leak follows rain and is concentrated under an entry point. Condensation appears after deep cold, without rain, as scattered drops or as frost under the decking.
Is frost in the attic normal?
A thin layer of frost on the nail tips in extreme cold happens in many houses. Thick frost covering the decking or the rafters signals too much moist air coming from the house. The ceiling leaks then need to be tracked down.
Is attic mould dangerous for the house?
Attic mould shows that the wood is staying damp. Wood that stays damp ends up rotting if nothing changes. For questions about the occupants' health, contact an indoor air quality specialist. In every case, fix the cause of the moisture first.