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Roof Leak or Attic Condensation? The Stain That Only Appears in a Thaw, and How to Tell Which One You Have
Roofing
Leak or Condensation? Ask When It Happens, Not Where
A brown ring on the bedroom ceiling in February gets a roofer called, and the roofer finds a sound roof and a dry attic, because it has not rained in three weeks and the “leak” was the attic’s frost melting on the first warm afternoon. It is the most common misdiagnosis in Ontario housing, it costs a service call and sometimes a roof, and the two problems are told apart by one question: what was the weather doing when the stain appeared?
The timing test
| What you see | Likely a leak | Likely condensation |
|---|---|---|
| When the stain appears or grows | During or just after rain; after wind-driven rain from one direction; in a thaw when there is an ice dam at the eave | On mild days in January to March after a cold spell, with no rain; sometimes during a long cold snap as frost builds |
| Summer | Continues with every rain | Stops completely; the stain dries and stays |
| Where | Anywhere, usually downhill of a penetration, flashing or valley; often near an exterior wall if it is an ice dam | Away from walls, often in the middle of a room or a hallway, under the coldest part of the roof (north side), or directly under a bathroom fan’s duct |
| Shape | One spot that grows outward, or a line along a wall | Several spots, or a drip pattern that follows the nail lines, or a stain under each roof truss |
| Amount | Can be a lot, fast | Usually a drip or a dribble, briefly |
The attic test
Up with a flashlight, ideally on a cold day for condensation and during rain for a leak.
- Frost on the underside of the sheathing, or a sparkle of ice crystals on the nail points, in January: condensation. Those nails are the coldest things in the attic and they rust in rings; a thousand rusty nail rings on the sheathing is a decade of frost, not a leak.
- Uniform grey or black staining across a whole slope, worst on the north side and at the ridge: condensation, long-term.
- A single trail running down the sheathing from a specific point, darker at the top, with white mineral deposits: a leak. The top of the trail is the hole; the leak diagnosis page from here.
- Wet or matted insulation under the bathroom, with a flex duct lying nearby: the fan.
- Daylight at the eaves or around a vent: a leak path, and water follows light.
- Both, which happens: a condensation attic with a real leak at the chimney. Fix the roof leak now and plan the attic for spring.
Why the attic condenses
Warm house air carries water vapour. It gets into the attic through the ceiling: the attic hatch, pot lights, the tops of interior walls, plumbing and wiring penetrations, and especially a bathroom or dryer fan that was never ducted outside. In the attic it meets a roof deck that is at the outdoor temperature, and the vapour freezes onto it. A well-ventilated attic flushes some of it out; most Ontario attics are not well ventilated and the frost builds all winter. The fix is the same three things in the same order as ice dams, because it is the same problem seen from inside: seal the air leaks, insulate deeper, then ventilate. The ice dam page has the order and the costs; the ventilation page has the vents.
What each costs to fix
A leak
$250 to $500 for a vent boot, $800 to $2,000 for a chimney, $1,000 to $2,500 for a valley, after a $250 to $450 service call that finds it. Prices on the repair cost page.
Condensation
$350 to $900 to duct the fan outside, $800 to $2,500 for air sealing, $1,800 to $4,500 for insulation to R-60, $600 to $2,000 for ventilation. Done by an insulation or energy-retrofit contractor, not a roofer, and most of it qualifies for the energy rebate programs if an audit is done first.
And the ceiling itself: once the cause is fixed, a stain is primed with a stain-blocking primer and painted. If the drywall is soft or sagging, it is cut out and patched: $200 to $500.
Before calling anyone
Write down the dates the stain appeared and what the weather was. Take a photo of the stain with something for scale. Go into the attic and photograph what is above it. Then call the right trade with the photos: a roofer for a trail, an insulation contractor for frost. A roofer called for frost will, honestly, tell you so and charge the service call; less honestly, sell you a roof that will have the same frost under it next winter. The $99 review exists partly for the second case: send the quote and the attic photos and get told which problem you have.
Questions people ask
How do I know if my ceiling stain is from a roof leak or condensation?
Timing. A stain that appears or grows with rain is a leak. One that appears on mild winter days with no rain, and never in summer, is attic condensation. The attic confirms it: a trail on the sheathing is a leak; frost, rusty nail rings and uniform staining are condensation.
Why does my ceiling leak only in winter?
Either frost in the attic melting in a thaw (condensation) or an ice dam at the eave backing water under the shingles. Both are heat-loss problems fixed from the attic; the ice dam also needs the eave membrane on the next roof.
Why is there frost on the nails in my attic?
Moist house air reaching the cold roof: air leaks in the ceiling and, most often, a bathroom fan venting into the attic. The nails are the coldest surfaces and frost first. Seal, vent the fan outside, then ventilate the attic.
Can attic condensation damage the roof?
Yes. Years of frost and melt delaminate the sheathing and grow mould, and the deck has to be replaced at the next re-roof, at $120 a sheet. The $350 fan fix prevents a $3,000 deck bill.
Who do I call for attic condensation?
An insulation or energy-retrofit contractor, or a home energy auditor with a blower door and thermal camera to find the leaks first. Not a roofer; there is nothing on the roof to fix.
Rain or thaw?
Rain: roofer. Thaw with no rain: attic. Both: roofer now, attic in April. The weather on the day of the stain is the whole diagnosis.
Sources and further reading
- Keeping the Heat In, Chapter 5: Roofs and attics: Natural Resources Canada. The best free guide to attic insulation, air sealing and ventilation written for Canadian houses.
- Addressing moisture and mould in your home: Health Canada. What attic mould means for the house and when to call someone.
