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Ontario Building Code Attic Ventilation Requirements

Attic and roof ventilation in the 2024 Ontario Building Code: 1/300 of insulated ceiling area, 1/150 for low slopes, 25% high and low, baffles and hatches.
vent area to insulated ceiling area, the general rule
1/300
vent area to insulated ceiling area, the general rule
for slopes under 1 in 6 or roof joists
1/150
for slopes under 1 in 6 or roof joists
of vent area at the top, and at the bottom
25%
of vent area at the top, and at the bottom
air space above insulation in roof joist spaces
63 mm
air space above insulation in roof joist spaces

Last updated

Under the 2024 Ontario Building Code, an attic or roof space over an insulated ceiling must have an unobstructed vent area of at least 1/300 of the insulated ceiling area, rising to 1/150 where the roof slope is less than 1 in 6 or the roof is framed with roof joists (OBC Div. B 9.19.1.2.OBC Div. B 9.19.1.2. Vent Requirements(1) Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area. (2) Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area. (3)…). At least 25% of the vent area must be at the top of the space and at least 25% at the bottom, spread evenly on opposite sides of the building.

This page covers attic and roof space venting for houses and other Part 9 buildings. Your building department decides how the code applies, and some roof designs (for example unvented assemblies) need design or an approved alternative.

Attic ventilation rules at a glance

Item Rule in short Where to look
When venting is required insulation between a ceiling and the roof sheathing, unless shown unnecessary 9.19.1.1.
Vent area, general at least 1/300 of the insulated ceiling area 9.19.1.2.(1)
Vent area, low slope or roof joists at least 1/150 9.19.1.2.(2)
Vent types roof, eave, gable-end or a mix 9.19.1.2.(3)
Distribution uniform on opposite sides, 25% top, 25% bottom 9.19.1.2.(3)
Roof joist spaces interconnect with 38 × 38 mm purlins unless each space is vented 9.19.1.2.(4)
Air space above insulation, roof joist spaces at least 63 mm 9.19.1.3.(1)
Eave baffles at least 25 mm air space, 50 mm above insulation 9.19.1.3.(2)
Mansard or gambrel lower slope need not be vented 9.19.1.4.
Attic hatch size 550 × 900 mm, or 0.32 m² (500 mm min.) for one dwelling unit 9.19.2.1.(2)

This is our own summary of the articles, not a copy of the code.

Why attics are vented

Where insulation sits between a ceiling and the roof sheathing, 9.19.1.1.OBC Div. B 9.19.1.1. Required Venting(1) Except where it can be shown to be unnecessary, where insulation is installed between a ceiling and the underside of the roof sheathing, a space shall be provided between the insulation and the sheathing, and vents shall be installed to permit the movement of air from the space to the exterior. (See Note… requires a space between the insulation and the sheathing and vents that let air move from that space to the outside, unless it can be shown that venting is unnecessary. Venting carries away moisture that leaks up from the house, which otherwise condenses on cold sheathing in winter. It works together with the air barrier and vapour barrier in the ceiling: ventilation cannot fix a leaky ceiling, it only handles what gets through.

How much vent area

9.19.1.2.OBC Div. B 9.19.1.2. Vent Requirements(1) Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area. (2) Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area. (3)… works off the insulated ceiling area, not the roof area:

  • 1/300 for a typical pitched roof.
  • 1/150 where the slope is less than 1 in 6 (a low-slope roof), or where the roof is built with roof joists, such as a cathedral or flat roof with the ceiling fixed to the joists.

The figure is the unobstructed vent area, so use the net free area of the vent product, not its overall size. Vents must meet CAN3-A93-M, "Natural Airflow Ventilators for Buildings."

Our worked example

A bungalow has an insulated ceiling of 150 m² under a 5 in 12 gable roof.

  • Ratio: 1/300 (the slope is steeper than 1 in 6 and the roof uses trusses, not roof joists).
  • Required net vent area: 150 / 300 = 0.5 m².
  • At least 25% at the top (0.125 m², ridge or roof vents) and at least 25% at the bottom (0.125 m², soffit vents), spread on both sides.

If the same ceiling were under a roof framed with roof joists, the requirement would double to 150 / 150 = 1.0 m².

Where the vents go

Vents can be roof type, eave (soffit) type, gable-end type or a combination. They must be distributed uniformly on opposite sides of the building, with not less than 25% of the required openings at the top of the space and not less than 25% at the bottom (9.19.1.2.(3)OBC Div. B 9.19.1.2. Vent Requirements(1) Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area. (2) Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area. (3)…). This low-and-high split lets air enter at the eaves and leave near the ridge.

For roofs built with roof joists, each joist space must either be vented on its own or be connected to the next with purlins at least 38 × 38 mm laid across the top of the joists (9.19.1.2.(4)OBC Div. B 9.19.1.2. Vent Requirements(1) Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area. (2) Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area. (3)…).

Keeping the air path open

Insulation must not block the airflow (9.19.1.3.OBC Div. B 9.19.1.3. Clearances(1) Except as provided in Sentence (2), where venting is provided to a roof joist space, not less than 63 mm of space shall be provided between the top of the insulation and the underside of the roof sheathing. (2) Where venting is provided at the junction of sloped roofs and exterior walls and where preformed baffles…):

  • In vented roof joist spaces, leave at least 63 mm between the top of the insulation and the underside of the sheathing.
  • At the eaves, where preformed baffles hold the insulation back, the baffles must leave an air space of at least 25 mm with enough cross-section to meet the vent area, and must extend at least 50 mm above the top of the insulation.
  • Ceiling insulation must not restrict airflow through the roof vents or any part of the attic.

Blown-in insulation that buries soffit vents is the most common way a compliant design fails in the field.

Mansard and gambrel roofs

The lower, steep part of a mansard or gambrel roof need not be ventilated. The upper part follows the normal rules (9.19.1.4.OBC Div. B 9.19.1.4. Mansard or Gambrel Roof(1) The lower portion of a mansard or gambrel style roof need not be ventilated. (2) The upper portion of roofs described in Sentence (1) shall be ventilated in conformance with Articles 9.19.1.1. to 9.19.1.3.).

Attic access hatches

An attic or roof space needs an access hatch when it contains a fuel-fired appliance, or when it is large enough to enter. Read Clause (1)(a) of 9.19.2.1.OBC Div. B 9.19.2.1. Access(1) Every attic or roof space shall be provided with an access hatch where the attic or roof space (a) measures not less than (i) 10 m² in area, (ii) 1,000 mm in length or width, and (iii) 600 mm in height over at least the area described in Subclauses (i) and (ii), or (b) contains a fuel-fired appliance. (See Note… for the size thresholds, which combine area, length or width, and height. The hatch must be at least 550 mm by 900 mm, or, where it serves not more than one dwelling unit, at least 0.32 m² with no dimension less than 500 mm, and it must have a door or cover. Seal and insulate the cover so the hatch does not become the biggest air leak in the ceiling.

Common mistakes

  1. Using roof area instead of insulated ceiling area.
  2. Counting gross vent size instead of net free area.
  3. All soffit, no high vents, or all ridge vent with blocked soffits.
  4. Insulation stuffed into the eaves with no baffles.
  5. Applying 1/300 to a cathedral ceiling built with roof joists, which needs 1/150.

How to look it up yourself

In Division B, Part 9 of the 2024 Compendium, go to Section 9.19 "Roof Spaces". Search terms that appear in the code text:

  • "insulated ceiling area" (the ratio in 9.19.1.2.)
  • "Vent Requirements"
  • "preformed baffles" (9.19.1.3.)
  • "Mansard or Gambrel Roof"
  • "access hatch" (9.19.2.1.)

Or open 9.19.1.2. in the code viewer.

BCIN exam angle

Vent ratios and the 25% split are quick calculation questions on the House and Small Buildings exams. Section 9.19 is taught in the free House chapter on columns, crawl spaces, masonry, chimneys and fireplaces and the Small Buildings chapter on foundations, framing and structure. For the energy side of attics, see the SB-12 energy efficiency method in our Calculation Lab. Then try the free quiz on the House exam or Small Buildings exam page.

Based on the official 2024 Building Code Compendium on ontario.ca.

© King's Printer for Ontario, 2024. Reproduced with permission.

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Frequently asked questions

How much attic ventilation is required in Ontario?
The unobstructed vent area must be at least 1/300 of the insulated ceiling area. Where the roof slope is less than 1 in 6, or the roof is built with roof joists, it must be at least 1/150 (Division B, Article 9.19.1.2.).
How should attic vents be split between soffit and roof?
Vents must be spread uniformly on opposite sides of the building, with at least 25% of the required area at the top of the space and at least 25% at the bottom (Division B, Sentence 9.19.1.2.(3)).
How much space do I need between insulation and roof sheathing?
For vented roof joist spaces, at least 63 mm between the top of the insulation and the underside of the sheathing. Where preformed baffles are used at the eaves, they must leave at least 25 mm of air space and extend at least 50 mm above the insulation (Division B, Article 9.19.1.3.).
What size does an attic hatch have to be?
At least 550 mm by 900 mm, except that a hatch serving not more than one dwelling unit may be reduced to 0.32 m² with no dimension under 500 mm (Division B, Sentence 9.19.2.1.(2)).
Does a mansard roof need venting?
The lower, steep portion of a mansard or gambrel roof need not be ventilated, but the upper portion must be (Division B, Article 9.19.1.4.).