Fire ventilation

side). The negative pressure on the leeward side is approximately
half the pressure on the wind side. The pressure on
roof surfaces exposed to wind depends on the angle of the
roof. At angles of over 45° a positive pressure is created on
the wind side, and a negative pressure on the leeward side.
Both the positive pressure and the negative pressure are
greatest at the respective bases of the roof, and diminish
successively up towards the ridge. At roof angles between
30° and 45° there can, however, also be a negative pressure
on the wind side closest to the ridge. At roof angles of less
than 30° the entire roof is exposed to negative pressure. The
negative pressure is highest on the wind side.
Gable roofs can be exposed to negative pressure along
the entire surface, if the wind blows parallel to the ridge,
regardless of the angle of the roof.
The differences in the pressure conditions that arise as a
result of the roof angle can in certain cases be utilised to
Direction of wind
roof angle < 30°
roof angle 30–45°
roof angle > 45°
The distribution of
pressure over a fl at roof.
There is normally a
negative pressure over
the entire surface of the
roof.
The distribution of
pressure over a gable
roof. The pressure varies
from negative pressure
to positive pressure,
depending on the angle
of the roof and where on
the roof one is.
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