In short
- Natural Resources Canada gives the starting ratio of 1 to 300: clear vent area to insulated ceiling area.
- The Ontario Building Code text raises it to 1 to 150 for roofs with a slope under 1 in 6 or built with roof joists.
- NRCan suggests half of the openings as continuous soffit vents and half at the ridge, gable or roof.
- Screens and baffles shrink the clear area, so the vent count has to go up.
- The company sets the price for vent work after it sees the attic.
What the roof space is doing
In winter an attic stays cold so that snow on the roof does not melt and refreeze at the eaves. In summer it lets heat out. Natural Resources Canada (NRCan) puts it this way: attic ventilation reduces summer heat build-up, which prolongs the life of roofing, and after air sealing it is the second line of defence against water vapour that reaches the attic. A cold, well-vented attic is less prone to ice dams at the eaves.
That makes ventilation part of any re-roof.
Intake low, exhaust high
Air has to enter at the bottom of the roof and leave near the top. The usual layout is a continuous soffit vent along the eaves, plus a ridge vent, gable vents or roof vents at the peak. NRCan says that ideally half the openings are continuous soffit vents and half are gable, ridge or roof vents. It prefers ridge vents where practical, provided they carry baffles that deflect wind and keep out rain and snow.
Inside, the air needs a path from soffit to ridge. CAA-Quebec, drawing on CMHC, describes baffles that keep an air gap between insulation and the roof deck, so that insulation does not smother the soffit opening.
The numbers in the code
NRCan gives 1 to 300 as the ratio for peaked roofs with accessible attics: unobstructed vent area against insulated ceiling area. It adds that the area must be increased when vents carry insect screens or rain baffles, because those cut the clear opening.
The Ontario Building Code text on roof space vents (Sentence 9.19.1.2, as reproduced on a code-reference site that shows the 2012 and 2017 editions) sets the same 1 to 300, and 1 to 150 where the slope is under 1 in 6 or the roof is built with roof joists. It lets the vents be roof type, eave type, gable-end type or a mix, spread uniformly on opposite sides of the building, with at least 25% of the openings at the top of the space and 25% at the bottom. That site says the current legal edition is the 2024 code, so the company should confirm the figure for your job.
Ventilation and ice dams
CAA-Quebec’s ice dam page, crediting CMHC, describes the sequence: heat escapes from the house and warms the roof, snow melts, water runs to the cold eaves and freezes, and the ice then holds back later meltwater that looks for a path inside. The page names poor attic and soffit ventilation as the main cause. The ice dams page walks through what to ask a roofer about it.
NRCan puts the order of work as stopping air leaks from the house into the attic, then insulation, then possibly better roof ventilation. Vents alone do not cure a leaky ceiling. Pot lights, the attic hatch, plumbing stacks and bathroom fan ducts are the leak points it names.
Two things to leave to the company
NRCan advises against electric attic exhaust fans, which can pull more air than the soffits supply and so draw house air upstairs. It also says exhaust fans from the bathroom or kitchen should discharge to the outside, and not directly below the soffit vents. A roofer checks both before it adds a vent.
Vent work is also work at height. Ontario requires workers on construction projects who use fall protection to hold approved working at heights training, valid for three years.
When to raise it
Raise ventilation when you collect quotes for roof replacement or asphalt shingle roofs. New shingles over blocked soffits repeat the old problems. For water already coming in see roof leak repair; soffit and fascia covers the boards that carry the intake vents.
Who you are dealing with
An independent Ottawa roofer receives the request, checks the attic and the roof and tells you what it would change. Ask it for the clear vent area in writing, before and after.
Questions to ask the company
- How many square feet of clear vent area does my attic need, and how many does it have now?
- Are the soffit vents open, or is insulation blocking them?
- Will you fit baffles at the eaves so the insulation leaves an air gap under the deck?
- Where will the exhaust vents go, and are they near the ridge?
- Does the bathroom fan vent outside, and is that outlet away from the soffit?
Have this ready for the call
- Where ice, icicles or frost show up in winter, and on which side
- Photos of the soffits from the ground, showing whether there are slots or vents
- Stains on top-floor ceilings or damp spots on attic wood, with the dates
- The type of vents you can see on the roof or gable: ridge strip, box vents, turbines
Questions people ask
Can I add more vents to cure an ice dam?
Vents are only one part of it. NRCan says the first line is stopping house air from leaking into the attic, then insulation, and only then possibly better roof ventilation. It also says not to increase ventilation automatically. A company should check the attic floor and the ceiling leaks before it adds openings.
Why do roofers talk about soffits when I asked about the roof?
Outside air has to enter low, at the soffit, and leave high, near the ridge. Without clear soffits the ridge vent has nothing to draw, and insulation pushed into the eaves can shut the intake. CAA-Quebec, citing CMHC, says ice at the roof edge is almost always caused by poor attic and soffit ventilation.
Do electric attic fans help?
NRCan advises against electric exhaust fans for attic ventilation. They can draw more air than the soffit vents supply and pull warm house air into the attic. A roofer will describe the passive layout it plans instead, soffit intake and vents near the ridge, and give you the vent area in writing.
Does a new roof need new vents?
A roof replacement is the moment to check the vents, because the crew has the soffits and ridge open. The vent area is measured against the insulated ceiling, so a house with added insulation may need a different count. One Ottawa roofer says shingles over a poorly ventilated attic restart the same problems.
What does the building code say about vents on a house?
The Ontario Building Code text on roof space venting sets 1 to 300 of the insulated ceiling area, 1 to 150 for low slopes and roofs built with roof joists, with vents spread on opposite sides and at least a quarter of the openings high and a quarter low. Confirm the current edition with the City.