Installation
Metal roof condensation and ventilation
Water dripping from a metal roof is usually not a leak. It is condensation, it comes from inside the house, and a new roof alone will not fix it.
By Charles · · 9 min read
If water is dripping from the underside of your roof on a cold morning and stops by lunchtime, you probably do not have a leak. You have condensation — and it is coming from inside the house.
This is one of the most misdiagnosed problems in Ontario roofing. People replace a perfectly sound roof covering, the dripping continues, and everyone is baffled.
The mechanism, briefly
Warm air holds more moisture than cold air. Your house generates a surprising amount of it — showers, cooking, laundry, houseplants, and simply breathing. A family of four puts several litres of water into the air every day.
That warm, moist air rises. If it reaches a surface cold enough, the moisture condenses out of it as liquid water.
The underside of a roof deck in an Ontario January is very cold indeed.
So the problem is not that water is getting in. It is that water is being produced inside and finding the coldest surface available.
Why people blame the metal
Two reasons, one fair and one not.
The unfair one: metal gets blamed because condensation is more visible on it. On a shingle roof, moisture condenses on the underside of the wood sheathing and is partly absorbed. On an uninsulated metal surface — a barn, a shop, a bare-purlin structure — it beads and drips visibly. Same physics, more obvious.
The fair one: an unvented metal roof over an unvented cavity genuinely can make things worse. If someone strapped over old shingles and sealed the soffits, they built a moisture trap. That is a workmanship failure, not a material one.
On a properly built residential assembly — solid deck, underlayment, ventilated attic or ventilated cavity — a standing seam roof does not cause condensation and is not more prone to it than asphalt.
The three things that actually fix it
In order of importance. Note that only the third is about the roof.
1. Air sealing the attic floor
This is the highest-value step and the one most often skipped.
Moist air does not diffuse gently through your ceiling. It pours through holes: pot light housings, bathroom and kitchen fan penetrations, plumbing stacks, the attic hatch, the gap around a chimney chase, wiring penetrations, the top plates of interior walls.
Seal those and you cut the moisture reaching the roof deck dramatically. It is unglamorous work — foam, caulk, gaskets, weatherstripping on the hatch — and it beats every other intervention.
A particular Ontario offender: bathroom fans vented into the attic rather than through the roof or wall. Astonishingly common in houses built before the 1990s. That is a machine pumping shower moisture directly at your roof deck. If yours does this, fix it before anything else.
2. Ventilation — intake and exhaust, both
Ventilation only works as a pair. Air must come in low at the soffits and leave high at the ridge.
The failure we see constantly: someone tops up attic insulation, blown cellulose or fibreglass drifts into the eaves, and the soffit intake is buried. Now there is ridge exhaust with nothing feeding it. The system does nothing.
Baffles — rigid channels stapled between the rafters at the eave — keep that path open. They cost very little and they are the difference between a ventilation system and a decoration.
Roughly: you want continuous intake at the soffit, continuous exhaust at the ridge, and a clear path between them. More intake than exhaust, not the reverse.
3. Insulation depth, and the roof assembly itself
Once air sealing and ventilation are right, insulation depth keeps the ceiling warm and the deck cold — which is what you want. A cold, dry, well-ventilated deck does not condense.
At the covering, our strapped assembly adds a ventilated cavity between the deck and the panel. Air moves behind the metal, keeping it closer to outdoor temperature. That helps both condensation and ice damming, which share a root cause.
The Ontario situation that produces this most
Season-extended cottages. This is far and away the most common version we are called about, particularly through the Kawarthas, Muskoka and along the Innisfil shoreline.
A building constructed for summer use — modest insulation, minimal vapour control, ventilation sized for an unheated shell — gets insulated, gets a woodstove or a heat pump, and is now occupied through January.
Nothing about that roof assembly was designed for a heated interior in a Canadian winter. It sweats. Then it ice dams. Then someone concludes the roof is failing.
Putting metal over that hides it. The deck keeps getting wet, just out of sight.
So we look at the cavity before we quote the covering. Sometimes the honest answer is that ventilation and air sealing need doing first, and that is the answer you should want to hear from anyone quoting your roof.
How to tell condensation from a leak
| Condensation | Leak | |
|---|---|---|
| Timing | Cold mornings, warms off by midday | During or just after rain |
| Location | Spread widely, often worst at the north side | Traceable to one point |
| Season | Winter | Any, but usually storms |
| Attic clues | Frost on nail tips, damp sheathing, mould bloom | A defined stain track |
| After a cold snap | Worse | Unchanged |
If it is worse after a cold night and better by afternoon, it is condensation. If it follows the weather, it is a leak.
What to look for yourself
Go into the attic on a cold morning, with a torch:
- Frost or beads on nail points poking through the sheathing — unambiguous condensation
- Dark staining on the underside of the deck, especially near the eaves
- Damp or matted insulation
- A musty smell
- Daylight at the soffits — if you cannot see light at the eave, your intake is blocked
- Bathroom fan ducts — follow them. They must exit the building, not terminate in the attic
Photograph anything you find. It is genuinely useful for whoever quotes the work.
The honest summary
Condensation is an air-sealing and ventilation problem that presents as a roofing problem. A new covering alone will not solve it — and any roofer who tells you otherwise either has not diagnosed it or is not telling you.
Fix the attic floor, open the intake, then install an assembly that does not care. In that order.
If you have unexplained moisture and want somebody to actually look at the cavity rather than just quote a roof, call 705-816-5282 or get in touch.