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Attic Ventilation in Ontario: The 1-in-300 Rule, Soffit and Ridge Done Right, and the Mistakes That Cook a Roof
Roofing
Attic Ventilation: Air In at the Bottom, Air Out at the Top, and Nothing in the Way
A shingle roof over a well-vented attic lasts its rated life. The same roof over an attic that reaches 60 C in July and holds frost in January lasts two-thirds of it, and no warranty covers the difference. Most attics in Ontario are under-vented, and most of those have the vents already installed and blocked. This is how much you need, where it goes, and the five things that go wrong.
How much
The Code and every shingle manufacturer agree: 1 square foot of net free vent area for every 300 square feet of attic floor (insulated ceiling area), with the vents split roughly half at the low side (soffit intake) and half at the high side (ridge or roof exhaust), and the intake never less than the exhaust. Net free area is the actual open area of the vent after the screen and louvres, which is printed on the product and is a lot less than the size of the hole. A 1,500 sq ft attic needs 5 sq ft of net free area: 2.5 at the soffits, 2.5 at the top. A ridge vent gives about 18 sq in. per foot, so 20 ft of ridge vent is 2.5 sq ft. Continuous soffit vent gives about 9 sq in. per foot; 40 ft of it, along both eaves, is 2.5 sq ft. Low-slope and low-ceiling attics (under about 2 in 12, or where the vents are not well separated) need 1 in 150.
How it works, and why the intake matters more
Warm air rises and leaves at the ridge; cold air is drawn in at the soffits to replace it. That flow carries out the summer heat that cooks the shingles from below and the winter moisture that frosts on the sheathing. If the top is open and the bottom is not, nothing flows; worse, the ridge vent draws its make-up air from the warmest place it can find, which is the house below through every gap in the ceiling, so you heat the attic with your furnace and make the ice dams worse. The soffits are the half nobody looks at and the half that decides whether the system works.
Check the soffits in ten minutes
- From outside, look up at the soffit (the underside of the eave). Perforated aluminum or vinyl, or vent strips, means there is intake, maybe. Solid plywood or boards with a few round plugs means there is very little.
- From the attic on a bright day, lights off: you should see daylight at the eaves along both sides. No daylight means the soffit vent is blocked, usually by insulation pushed into the eave, sometimes by the original builder’s solid blocking, sometimes because the vinyl soffit was installed over an old solid one with no holes behind the perforations. That last one is common and invisible from outside.
- Baffles (cardboard or foam chutes stapled between the rafters at the eave) hold the insulation back and keep a channel open. Every rafter bay should have one. If they are missing, that is the fix: $3 to $5 a baffle, plus the labour of crawling the eaves, $400 to $900 on a typical house.
Exhaust: ridge vent, box vents, turbines, powered fans
Ridge vent
A slot cut along the ridge, covered by a vent with an external baffle and a filter, capped with shingles. Even exhaust along the whole roof, invisible, nothing to break. $12 a foot installed with a roof. The right answer on any roof with a decent length of ridge. Must have soffit intake to feed it.
Box (static) vents
Square hoods near the ridge, one per 150 sq ft of attic or so. Cheap ($75 to $150 each installed), work fine, ugly in multiples, and they are holes in the roof with a flange to flash. Fine on a hip roof with little ridge.
Turbines (whirlybirds)
Spin in wind and pull more air than a static vent when it blows, nothing when it does not. Bearings wear and squeak. Not wrong, not necessary.
Powered attic fans
Mostly not worth it in Ontario. A powerful fan without matching intake pulls conditioned air out of the house through the ceiling, costs electricity, and the thermostat runs it on hot afternoons when the attic is going to be hot anyway. Passive done right beats powered done badly, every time. The exception is a complicated attic with no way to get passive flow.
Do not mix ridge vent with box vents or a gable vent on the same attic: the ridge vent will draw from the nearer opening instead of the soffits and short-circuit the flow. If a ridge vent goes in, the gable vents get closed and the old box vents removed.
The five mistakes
- Vents with no intake. The commonest. Three box vents and a ridge vent on a roof whose soffits are solid or stuffed with insulation. The attic is a sealed box with holes in the lid.
- A bathroom or dryer fan venting into the attic. Warm wet air from a shower dumped onto cold sheathing is frost in January and black mould by March. The fan goes outside through the roof or a wall with an insulated duct. Its own page.
- Insulation over the soffits. The insulation contractor blew another foot of cellulose in and buried the eaves. Baffles first, then insulation.
- Ventilating instead of air sealing. Vents cannot keep up with a ceiling full of gaps; the attic stays warm and damp. Seal, insulate, then vent. The ice dam page explains the order.
- Mixing exhaust types. Ridge vent plus gable vents is a ridge vent that breathes through the gables and leaves the soffits idle.
What it costs to fix
| Work | With a new roof | On its own |
|---|---|---|
| Ridge vent, 40 ft | $480 | $900 to $1,400 |
| Replace solid soffit with vented aluminum, both eaves of a bungalow | $1,200 to $2,500 | Same; a siding or eavestrough contractor |
| Baffles at every rafter bay and pull insulation back | $400 to $900 | $400 to $900 |
| Box vents, each | $75 to $150 | $200 to $350 |
| Bathroom fan ducted through the roof | $350 | $500 to $900 |
| Close gable vents when adding ridge vent | $100 to $300 | Same |
The time to do all of it is when the roof is off: the ridge is open, the eaves are visible, and the roofer is already up there. The calculator has the ridge vent, soffit and fan lines for that reason.
Questions people ask
How much attic ventilation do I need?
1 sq ft of net free vent area per 300 sq ft of attic floor, half at the soffits and half at the ridge or roof vents, intake never less than exhaust. A 1,500 sq ft attic needs about 2.5 sq ft of each.
Is a ridge vent better than roof vents?
Yes, on a roof with enough ridge: even flow along the whole length, nothing visible, nothing to fail. It only works with open soffits, and it should not be combined with gable or box vents.
How do I know if my soffit vents are blocked?
From the attic on a bright day with the lights off: daylight at the eaves means open. No daylight means blocked, usually by insulation or by a solid soffit behind a perforated cover.
Do I need a powered attic fan?
Almost never in Ontario. Passive soffit-and-ridge ventilation sized to 1 in 300 outperforms a fan that pulls air from the house, and it costs nothing to run.
Can poor attic ventilation cause mould?
Yes, with the air leaks and the bathroom fan doing most of the work: warm moist air from the house condenses on cold sheathing, and the black staining on the north side of the attic is the result. Seal, vent the fan outside, then ventilate.
Go and look for daylight
Attic, bright day, lights off. Daylight at both eaves and at the ridge is a roof that will last. Darkness at the eaves is a $600 fix and ten years of shingle life.
Sources and further reading
- Ontario’s Building Code: the official page, with the current regulation (O. Reg. 163/24). Roofing is Section 9.26, attic ventilation 9.19.
- O. Reg. 163/24, the Building Code regulation: the full text if you want to read 9.26 and 9.19 yourself.
- 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.
