Attic and moisture
Roof Ventilation in Calgary
Roof ventilation moves outside air through the attic so the underside of the sheathing stays close to outdoor temperature. A cold attic with moving air does not accumulate frost. A cold attic with still air does. That is the whole mechanism, and in Calgary it is the difference between a dry ceiling and a ceiling that drips every chinook.
- 24/7 emergency line
- Licence BL272564
- WCB 9892020
- $5M liability
- Free written estimate
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Tell us what the roof is doing
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City of Calgary licence
BL272564
WCB Alberta account
9892020
General liability
$5,000,000
Written estimate
Free, itemised
Call 403-700-7542 for a free written estimate.
01
What the code requires
The National Building Code sets three requirements for roof space venting.
Vent area. Unobstructed vent area of not less than 1/300 of the insulated ceiling area. Where the roof slope is less than 1 in 6, or the roof is a joist roof, that rises to 1/150.
Distribution. Openings must be distributed uniformly on opposite sides of the building, with not less than 25 percent of the required area at the top of the space and not less than 25 percent at the bottom.
Clearance. Not less than 63 mm of space between the top of the insulation and the underside of the roof sheathing. Where preformed baffles are installed at the roof and wall junction, the minimum air space may be reduced to 25 mm, provided the baffles extend vertically not less than 50 mm above the top of the insulation.
Unobstructed is the operative word in the first requirement. A 100 square centimetre vent behind a soffit panel packed with blown insulation has an unobstructed area of zero.
02
Intake is the half that gets missed
Ventilation is a circuit. Air enters low at the soffits, rises across the underside of the sheathing, and exits high at the ridge or through upper vents. Break either end and the circuit stops.
Contractors add exhaust because exhaust is visible from the ground and easy to sell. Intake is under the soffit where nobody looks. The result is a very common Calgary attic: six box vents on the back slope, and soffits that have been closed off by insulation for a decade.
An attic with exhaust and no intake does not sit neutral. The exhaust vents pull air from the easiest available source, which is the heated house below through every gap in the ceiling. That imports warm humid air into the attic, which is exactly the condition that produces attic rain.
Stop guessing
Get roof ventilation priced in writing
We look at the roof, photograph what we find, and hand you a scope with a number on it. If a repair is the right answer, we say so rather than quoting a replacement.
Estimates are free and written. Not a range, not a verbal number.
03
Vent types
Soffit vents. Continuous perforated soffit or individual vents. The intake side. Requires baffles at the eaves to stay open.
Ridge vent. Continuous exhaust along the ridge. Even distribution, low profile, and it works with the natural stack effect.
Box vents. Static vents set into the field of the roof. Simple and cheap. Effective only if there are enough of them and the intake is open.
Turbine vents. Wind-driven rotating exhaust. Higher flow when the wind blows, moving parts that wear.
Gable vents. In the end walls. They cross-ventilate rather than exhausting through the peak, and they can short-circuit a ridge vent.
Powered attic ventilators. Fans. In a cold climate with an imperfectly sealed ceiling they aggressively pull conditioned air out of the house. We do not recommend them for Calgary attics without a verified air-sealed ceiling plane.
04
Do not mix exhaust types on one attic
A ridge vent and box vents on the same attic space will short-circuit. The ridge, being the highest opening, pulls air in through the box vents just below it rather than from the soffits. The bottom two thirds of the attic stops ventilating while the vent count on paper looks generous.
One exhaust type per attic space, sized to the intake.
05
What a ventilation correction involves
- Measure insulated ceiling area and calculate required vent area at 1/300, or 1/150 where the slope is below 1 in 6.
- Inspect the soffits from inside the attic and confirm actual open intake, not nominal.
- Install baffles at every eave bay to hold the 25 mm to 63 mm clearance.
- Clear packed insulation out of the soffit space.
- Correct or add upper vents so that at least 25 percent of the required area sits at the top and at least 25 percent at the bottom.
- Confirm every bathroom, kitchen and dryer exhaust discharges to the exterior, not into the attic.
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Work on Calgary roofs
Every photograph on this site is taken on a job we did. No stock libraries.
Checkable, not claimed
Credentials you can verify before we set a ladder up
Marku Roofing Repair Ltd. holds City of Calgary business licence BL272564, WCB Alberta account 9892020, and $5,000,000 in commercial general liability through Sandbox Mutual Insurance.
No awards, no rankings, no "best in Calgary". A licence number, a WCB account and a policy limit are things you can check yourself.
More about the companyAnswers
Frequently asked questions
Still unclear? A five minute phone call usually settles it faster than reading.
Ask us directlyCan an attic be over-ventilated?
In a cold climate the practical failure is under-ventilation combined with air leakage. Excess exhaust without matching intake is the harmful case, not high airflow in a balanced system.
Should soffit vents be blocked in winter?
No. Winter is when the attic most needs to stay cold and dry.
Do I need ventilation on a cathedral ceiling?
Vented cathedral assemblies need a continuous air channel from eave to ridge in every bay. Unvented cathedral assemblies are a different system that relies on continuous insulation and an air barrier instead.
How do I check my own soffits?
Go into the attic on a bright day with the lights off and look along the eaves. If you can see daylight at the bays, the intake is open. If you cannot, it is blocked.
Free written estimate
Book it before the next chinook
Freeze-thaw turns a small opening into a large one faster here than almost anywhere else in Canada. Get it looked at while it is still a repair.
Serving Calgary, Airdrie, Cochrane, Okotoks and Chestermere.