Ventilation is the least-talked-about part of a roofing system — and one of the most consequential. Poor ventilation causes mold, ice dams, premature shingle failure, and voided warranties.
Nobody talks about roof ventilation when they're shopping for a new roof. It doesn't show up in before-and-after photos. It's not visible from the street. Contractors rarely lead with it in their pitch. But ask any experienced roofer what separates a 15-year roof from a 25-year roof — and ventilation is almost always part of the answer.
In Ontario's climate, attic ventilation is especially critical. Our winters create the moisture conditions that, without proper airflow, lead directly to mold and ice dams. Our summers push attic temperatures high enough to literally cook shingles from below.
Your attic exists in a separate thermal zone from your living space. In a well-ventilated attic:
Both require the same thing: a continuous flow of outside air through the attic space.
A properly ventilated attic uses a simple principle: cool air enters low, warm air exits high.
Ontario's Building Code follows a standard ventilation ratio: 1 square foot of net free ventilation area for every 150 square feet of attic floor space, split roughly 50/50 between intake and exhaust.
In practice, this means a 1,200 sq ft attic needs approximately 8 sq ft of net free ventilation area — 4 sq ft from soffit vents and 4 sq ft from ridge or roof vents. Properly installed soffit vents and a continuous ridge vent on a typical GTA home will generally meet this requirement.
This is the most common — and most damaging — ventilation problem we find. When insulation is blown into an attic without first installing baffles at the eaves, it piles up and blocks the soffit vents completely. The exhaust vents are open but there's no intake. The attic can't breathe properly. This is frequently the primary cause of attic mold and ice dams in GTA homes that "had insulation done."
Installing a ridge vent AND box vents on the same roof creates a ventilation short-circuit. Air exits through the closest opening (the box vents on the upper slope) without ever drawing fresh air from the soffits. The ridge vent effectively becomes another intake rather than an exhaust. The two types should not be used together.
This is a building code violation that occurs surprisingly often in older homes. Exhaust fans that duct warm, humid air directly into the attic space — instead of through to outside — are a primary driver of attic mold. Every bathroom and kitchen exhaust in your home should exit through the roof or a side wall, not terminate in the attic.
Ridge vents or powered attic fans without adequate soffit intake create negative pressure in the attic. Instead of pulling air through the attic, they pull conditioned air from the living space — bringing moisture with it and raising your heating and cooling bills.
Ventilation assessment is a standard part of every roofing inspection and installation we do. When we're on your roof and in your attic, we check:
A roof replacement is the ideal time to correct ventilation issues. Ridge vent installation, soffit vent clearing, and vent replacement are all part of the work happening on your roof anyway. If your existing ventilation is inadequate, adding a ridge vent or correcting the intake during a reroof costs a fraction of what it would cost as a standalone project.
When we come for a quote, we'll assess your attic ventilation at no charge — it takes 15 minutes and can save thousands in premature repairs. Serving Thornhill, Aurora, Richmond Hill, Vaughan, Markham, and the GTA.
Book a Free Inspection 416-889-0629