The BC Building Code requires 1 sq ft of unobstructed vent area for every 300 sq ft of insulated ceiling in a house attic, with at least a quarter of it low (soffits) and a quarter high (ridge or roof vents), and 1 in 150 for low-slope roofs under 2:12 or roofs framed with joists. In our wet climate, ventilation is what keeps a shingle roof from cooking in summer and the attic from dripping in winter. Here is how it works, how the vent types compare and what it costs to fix during a re-roof.
Why attic ventilation matters so much here
Two reasons, one per season. In summer, a sealed attic under dark shingles runs hot, and heat from below ages the shingles faster than sun from above. In winter, warm, moist air from the house leaks into the attic through pot lights, the hatch and gaps around pipes; without airflow it condenses on the cold underside of the sheathing, drips, wets the insulation and grows mould. Roofers see frost on the nail tips in January attics across the Lower Mainland, and homeowners mistake it for a leak. Ventilation also keeps the roof deck cold in a cold snap, which limits ice dams at the eaves.
Ventilation doesn't work alone. The ceiling has to be air-sealed so the moisture doesn't get up there in the first place, and the insulation has to stay clear of the soffit vents so air can enter.
What the code requires
- Vent area: not less than 1/300 of the insulated ceiling area, free (unobstructed) area. A 1,500 sq ft ceiling needs 5 sq ft of net vent, split between intake and exhaust.
- Low-slope and joist roofs: 1/150.
- Distribution: vents uniformly on opposite sides of the building, with not less than 25% at the top of the space and not less than 25% at the bottom.
- Clearance: at least 63 mm between the top of the insulation and the sheathing, and baffles at the eaves keeping a 25 mm air space so the soffit vents aren't buried.
Manufacturers' vents are rated in square inches of net free area (NFA); a roofer or supplier converts your ceiling area into the number of vents needed.
Soffit, ridge, roof and gable vents compared
| Vent | Role | Pros | Cons |
|---|---|---|---|
| Soffit (eave) vents | Intake | Continuous strip or panels along the eave; invisible from the street | Blocked by insulation or paint; many older houses have solid soffits |
| Ridge vent | Exhaust | Continuous along the ridge, even airflow across the whole deck, nearly invisible | Needs enough ridge length; useless without soffit intake |
| Box (static) roof vents | Exhaust | Cheap ($100 to $300 each), fit any roof | Several needed, visible, uneven airflow; each one is a flashing to fail |
| Gable vents | Both, poorly | Already there on many houses | Cross-flow at the gable ends skips the middle of the attic; can short-circuit ridge vents |
| Turbine (whirlybird) | Exhaust | Moves air in wind | Noisy when worn, leaks when seized, out of fashion |
| Powered attic fans | Exhaust | Move a lot of air | Can pull conditioned air from the house if the ceiling leaks; mostly unnecessary here |
The combination most roofers install on a re-roof is continuous soffit intake plus a ridge vent, with box vents only on roofs that lack ridge length (hip roofs). If you have gable vents and add a ridge vent, the roofer may suggest closing the gables so air comes up from the soffits instead of across.
What ventilation costs
| Work | Typical price (Lower Mainland) |
|---|---|
| Box roof vents, installed | $100 to $300 each |
| Ridge vent | $350 for a short ridge to $3,000 for a full system |
| Soffit or intake vents | $250 to $2,000 |
| Ventilation upgrade during a re-roof | $300 to $2,000 |
| Ventilation retrofit on an existing roof | $1,500 to $4,000 (single source) |
Prices from Lower Mainland roofers' published 2025 and 2026 rates. Doing it during a re-roof is far cheaper, because the shingles are off anyway. The attic ventilation service page covers the options.
Signs your attic is under-ventilated
- Frost or drips on the underside of the sheathing in winter without rain
- Dark staining or mould on the sheathing, especially the north side
- Damp or compressed insulation
- Shingles aging fast on an otherwise sound roof: curling, granule loss by year 12
- An attic that's noticeably hotter than outside on a summer afternoon
- Ice dams at the eaves in cold snaps
- Soffits with no visible vents, or vents painted over
A roofer or home inspector can measure the existing vent area against the 1/300 rule in minutes.
Cathedral ceilings and low-slope roofs
Roofs without an attic (vaulted ceilings, flat and low-slope roofs framed with joists) need 1/150 and a continuous air channel between the insulation and the sheathing in each rafter bay, from soffit to ridge, or they need to be built as unvented assemblies with insulation against the deck. Retrofits are harder; talk to a roofer who does low-slope work and, for big changes, a building envelope consultant.
Ventilation and your warranty
Shingle manufacturers' warranties generally require adequate ventilation, and roofers know an under-ventilated attic will shorten the roof's life whatever the paperwork says. Any re-roof quote should state how intake and exhaust will be handled and how much net free area results. If a quote doesn't mention ventilation, ask; the quote comparison guide puts it on the checklist for that reason.
Common questions
How do I know if my attic is under-ventilated?
Look in winter: frost on the nail tips or sheathing, damp insulation, dark staining or mould on the plywood, and a musty smell. In summer, an attic far hotter than outside. Count the soffit vents too; many are blocked.
Can I add ventilation without a new roof?
Yes, but it's more expensive: one Lower Mainland roofer prices a retrofit at $1,500 to $4,000 versus $300 to $2,000 as part of a re-roof, because the vents have to be cut into an existing roof.