Why does my roofer talk about intake and exhaust balance in Alberta?

Balanced attic ventilation is one of the most important factors in how long a Calgary roof lasts, and it matters more here than in most Canadian cities because of the freeze-thaw cycle.

What intake and exhaust actually do

Your attic needs a continuous flow of outside air: cool, dry air enters low at the soffits (intake), travels across the attic floor, picks up heat and moisture, and exits high at the ridge or through roof vents (exhaust). When those two sides are balanced, the attic stays close to outside temperature in winter and sheds heat in summer.

The National Building Code (2023 Alberta Edition, in force since May 2024) sets the minimum: total vent area must equal at least 1/300 of the insulated ceiling area, with at least 25% of that area at the top and at least 25% at the bottom. On low-slope roofs or roof-joist construction the ratio tightens to 1/150.

Why Calgary specifically punishes an unbalanced attic: the city averages roughly 128 freeze-thaw cycles a year. Heat escaping through a poorly ventilated attic warms the roof deck, melts snow, and the meltwater refreezes at the cold eave as an ice dam. Water backs up under shingles, soaks the deck, and eventually shows up as stains on your ceiling or frost on the rafters in winter. Chinooks make this worse: a warm afternoon followed by a hard overnight freeze is a weekly event from November through March.

An overheated attic in summer also degrades shingles faster, which matters when you are trying to get 25 to 30 years out of an architectural shingle in hail country.

What goes wrong in practice: blocked soffit vents are the most common problem, usually from insulation pushed against the eave during an attic top-up. Too many exhaust vents without matching intake, or mixing vent types at the same height, can actually short-circuit airflow and draw outside air in through the upper vents instead of exhausting it. A roofer who spots this during a re-roof is doing you a favour by raising it.

What to check from inside: go into the attic on a cold morning and look for frost on the sheathing, daylight through the soffit baffles, or staining around the eave area. From outside, make sure soffit panels have open perforations and are not painted over or clogged with debris.

When a re-roof is the right time to fix it: tearing off old shingles gives a roofer access to the deck and the eave line. That is the moment to add or clear soffit baffles, replace a ridge vent, or correct a vent layout, before the new shingles go on. Doing it after costs more and may void the shingle manufacturer's warranty if ventilation does not meet their spec.

Ask any roofer quoting your job to note the existing vent area, what they plan to change, and why. A written scope should list the ridge vent length or the number and size of roof vents, plus any soffit work, so you can compare quotes on the same basis.