The Measurement of the Free Area Vent

Figure 1

The total unobstructed cross sectional area, measured in plane where the area is at a minimum and at right angles to the direction of air flow (as shown in the diagram). Generally 1.0m2 geometric free area is required for head of stair and 1.5m2 for end of corridor vents however each project will have its own design. Aerodynamic free area calculation is also allowed under Approved Document B.

The two images show how Approved Document B describes how you measure free area, but they do not illustrate how this is interpreted for a window.The image shows a window interpretation of

Approved Document B Diagram C7 as a bottom hung or side hung smoke vent. There are documents in existence produced by the Smoke Control Association that seek to give clarity on how this is measured which typically results in a double stacked bottom hung open out or side hung solution, however the ultimate regulation is ADB. Free area calculations should be submitted for approval to an approved Inspector to be assessed for ADB compliance.

At early stage of projects when Smoke ventilation is calculated, please contact us for guidance and a solution that meets the building regulations

Figure 2

Smoke  Free Area Calculator

Based on Figure 2

1.58 x 1.21 = 1.92 M2 Free Area

Figure 3

Figure 3  is the same window as that in figure 1 however the brickwork onsite was required to be altered in order to achieve the correct free area as here demonstrated and the opening orientation

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