How does roof ventilation work?

A properly ventilated roof moves air in a continuous, balanced flow — intake at the bottom, exhaust at the top. It sounds simple, and the principle is. The execution is where it either works or doesn’t.

Intake happens at the lower edge of the roof, typically through soffit vents where the overhang allows outside air to enter the attic. On roofs where soffit venting isn’t available or isn’t sufficient, we install on-roof intake vents — a product from Lomanco that sits in the lower third of the roof on the roof plane and draws air in without compromising the roof surface. Lomanco is a company that does one thing — ventilation — and does it exceptionally well. Their products carry lifetime guarantees, and we stock them in our office because most local suppliers don’t carry them.

Exhaust happens at the upper portion of the roof — through ridge vents, box vents, or turbines — allowing the hot, moist air that has accumulated in the attic to escape.

Balance is everything. Ventilation is calculated based on your attic’s square footage, and intake and exhaust must be matched to work correctly. We follow the 1:300 rule as our standard — one square foot of net free ventilation area for every 300 square feet of footprint space, split evenly between intake and exhaust. On certain low-slope applications where on-roof intake isn’t possible, we apply the 1:150 rule instead.

One critical thing to understand — you cannot mix exhaust types on the same continuous attic space. Installing both ridge vents and box vents in the same air space, for example, causes the system to short circuit. Instead of drawing air from the intake at the eaves, the ridge vent pulls air through the box vents — creating a loop that bypasses the attic entirely. The attic goes unventilated and you have the appearance of a ventilated roof without any of the benefit.

The exception is when a roof has separate, distinct air spaces — for example, a home with an addition or a complex roofline where portions of the attic are physically separated from each other. In that case, each air space is its own ventilation system and can be addressed with different exhaust solutions appropriate to each section. What you can never do is mix exhaust types within the same connected air space. It’s a common mistake and an expensive one.

One thing worth knowing about how we approach ventilation differently — most roofing contractors in our area simply replace whatever ventilation exists on the roof without ever asking whether it was right to begin with. We don’t. Our attic inspection tells us what’s actually happening before we spec a single vent. When we’re in an attic and can see daylight coming through the eave intake, we know air is moving. When we can feel the coolness of outside air flowing in, we know the system is working. When we can’t — we know it isn’t, regardless of what’s on the roof above us. That’s the difference between replacing a roof and actually improving it.