Attic Ventilation for Metal Roofs: Intake and Exhaust as One System
A metal panel is airtight by design, so every moisture and temperature problem a roof has gets solved underneath it. Here is how intake and exhaust work together, and how attics fail.
Metal panels do not breathe, and they are not meant to. Everything a roof assembly does about moisture and heat happens underneath the panel — in the vent space, in the attic, and at the ceiling plane below it. That is why attic ventilation is worth understanding before a metal roof goes on rather than after. Once the panels are up and the ridge cap is closed, changing the ventilation strategy means opening the roof again.
What Attic Ventilation Is Actually For
Attic ventilation has three jobs, and in Southern Ontario the winter job decides whether an assembly survives. First, it carries away water vapour that leaks up from the house before it can condense or frost on cold sheathing. Second, in summer it moderates the temperature of the deck. Third, through the winter it keeps the underside of the deck close to outdoor temperature so snow melts at roughly the same rate everywhere instead of melting high and refreezing at the cold eave — the ice damming story, which is a heat and air control problem long before it is a roofing problem.
Ventilation Is Two Halves of One System
A vented attic needs low openings and high openings. Intake happens near the eave — perforated soffit, vented drip edge, or fascia vents. Exhaust happens near the top, usually a ridge vent or off-ridge vents. Neither half works alone. Exhaust with no matching intake pulls make-up air from wherever it is easiest to get, which on most houses is the heated space below.
What Actually Drives the Air
Nothing pushes air through an attic except pressure differences, and there are only two of consequence on a house.
- ✓Stack effect: warm attic air is less dense than the cold air outside, so it rises and pushes out the highest openings while cold air is drawn in low. The bigger the indoor-outdoor temperature difference, the stronger the effect — which is why ventilation works hardest in January, exactly when moisture removal matters most.
- ✓Wind: air moving over a roof creates positive pressure on the windward slope and negative pressure over the ridge line. A ridge vent with an external baffle is built to exploit that suction and to keep wind-driven snow and rain from being pushed back into the slot.
- ✓Neither driver is strong. Attic ventilation moves air at very low pressure, which is why blocked intake, undersized openings, or a partly filled vent slot make such a large difference.
Net Free Area: The Only Number That Counts
Vent products are rated by net free area, or NFA — the actual open area left after insect screen, louvres, baffles, and perforation patterns take their share. A perforated aluminum soffit panel might look like a continuous opening and deliver only a small fraction of that in free area. Ridge vents are rated in net free area per linear measure of ridge. Box vents are rated per unit. Measuring the length of your soffit or counting the vents on your roof tells you very little; adding up published NFA tells you something real.
The 1:300 and 1:150 Convention
The rule of thumb you will hear from most contractors is one unit of net free vent area for every 300 units of insulated ceiling area, provided the venting is split roughly evenly between high and low openings and a vapour retarder is present at the ceiling — and one to 150 where those conditions are not met. It is a widely specified convention, not a law of physics, and not a substitute for what your local building department enforces. Confirm the requirement for your project with your municipality.
A ratio describes an area of opening, not a rate of airflow. Two attics with identical net free area can perform completely differently if one has its soffit intake buried under blown insulation and the other does not. Always check the path, not just the arithmetic.
What Changes When the Roof Is Metal
The panel itself is impermeable, so all ventilation happens through purpose-built openings and through the assembly beneath. Under standing seam, exhaust is normally handled at the ridge: the deck is slotted, a vent material sits over the slot, profile closures seal the panel ribs, and a raised ridge cap covers the assembly while leaving the air path open. Off-ridge vents are possible but each one becomes a flashed penetration through a continuous panel, which is why we would rather solve exhaust at the ridge where the geometry allows it. On exposed-fastener panels, every rib is either an opening that needs a closure or a deliberate part of the vent path, and getting that wrong is a common source of wind-driven snow in an attic.
How Attics Fail Around Here
The same handful of problems turn up on Southern Ontario houses year after year, and almost all of them are visible from inside the attic on a cold morning.
- ✓Soffit intake blocked by blown insulation that drifted into the eave, with no baffles to hold it back.
- ✓A ridge vent installed correctly on the outside while the continuous slot in the deck was never cut, or was cut too narrow.
- ✓Bathroom and kitchen exhaust fans terminating in the attic instead of through the roof or wall, dumping litres of water vapour into the coldest space in the house.
- ✓Gable vents left open after a ridge vent is added, so the ridge draws from the gables instead of from the eaves and the lower attic never sees airflow.
- ✓Recessed lights, an unsealed attic hatch, plumbing stacks, and top plates that leak warm humid air upward faster than the vents can remove it.
What We Look At Before a Metal Re-Roof
A re-roof is the one moment when the ridge, the eave, and the deck are all accessible at once, so it is the natural time to correct ventilation. That means adding up the published net free area of what is actually installed rather than what is visible, looking in the attic for the evidence — frost staining, rusted nail points, matted insulation at the eaves — and confirming the soffit is genuinely open behind the perforated panel. The ridge detail has to be settled before panels are ordered, because it affects panel length and the cap assembly.
Frequently Asked Questions
Does a metal roof need more ventilation than asphalt shingles?
The requirement is driven by the insulated ceiling area below, not by the roofing material, so the target does not change when you switch to metal. What changes is how the openings get built. Metal exhaust and intake details are fabricated rather than cut into a shingle field, so they need to be planned before the panels are made.
Can a metal roof go on over an attic that is under-vented?
Physically, yes — the panel does not care. But an under-vented attic keeps accumulating moisture under the new roof, and the deck under a long-lived panel is the last thing you want to be dealing with in ten years. A re-roof is the practical moment to open the soffits, install baffles, and build a proper ridge, because everything is already accessible.
Will adding more roof vents fix a damp attic?
Usually not, and often it makes things worse. If intake is limited, extra exhaust openings just increase the suction on the ceiling plane and pull more warm, humid indoor air into the attic. The order of operations is: seal the ceiling plane, open and protect the intake, then size the exhaust to match.
My attic has spray foam on the underside of the roof deck. Do I still need soffit vents?
No — that is an unvented, or hot roof, assembly and it follows a different set of rules. Venting a foamed roof deck defeats the design. What matters there is that the foam is continuous, thick enough for the climate, and airtight at every edge and penetration.
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