Which roof vent areas matter most?

By Sam Gocher
Wednesday 10th May, 20232 minute to read

In this blog we're going to take you through the most important roof vent areas.

Geometric Area

First of all, we’re starting with Geometric Area. To put it simply, this is basically just the hole in the roof. This is also before any products get involved, no curbs etc.

Coefficient of Discharge

Next is the Coefficient of Discharge. This is primarily something that us as manufacturers care about because we have tests taken out on our smoke vents to ensure their efficiency. This is information which is then published in our declaration of performance.

Aerodynamic Free Area

Now we move onto the Aerodynamic Free Area. This is the Geometric Area in the roof, multiplied by the coefficient of discharge, the efficiency of the vent. But why does Aerodynamic Free Area matter? Well, it’s the most scientifically accurate way of measuring the performance of a smoke vent.

Free Area

You also need to be aware of Free Area. Free area is a way of describing how well air can travel through a vent for the purposes of natural ventilation. Basically it’s a non-scientific way of calculating the efficiency of the vent.

Approved document B states: "The total unobstructed cross-sectional area, measured in the plane where the area is at a minimum and at right angles to direction of airflow." So we take the vent, measure it up, and take away the area taken up by the motor, and blades if the vent has them, and this gives us the free area.

While free area is relatively non-scientific in that it doesn’t take things into account such as cross-winds and splayed curbs etc, it’s the number which matters in relation to building regulations, more specifically Approved Document B.

If you have questions about any of our products, our friendly team would love to chat with you! Contact Us or visit our YouTube channel where you'll find plenty of helpful videos.

Roof Vent Areas explained in 2023

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