Fire ventilation

side). The negative pressure on the leeward side is approximatelyhalf the pressure on the wind side. The pressure onroof surfaces exposed to wind depends on the angle of theroof. At angles of over 45° a positive pressure is created onthe wind side, and a negative pressure on the leeward side.Both the positive pressure and the negative pressure aregreatest at the respective bases of the roof, and diminishsuccessively up towards the ridge. At roof angles between30° and 45° there can, however, also be a negative pressureon the wind side closest to the ridge. At roof angles of lessthan 30° the entire roof is exposed to negative pressure. Thenegative pressure is highest on the wind side.Gable roofs can be exposed to negative pressure alongthe 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 aresult of the roof angle can in certain cases be utilised toDirection of windThe distribution ofpressure over a flat roof.There is normally anegative pressure overthe entire surface of theroof.roof angle < 30°roof angle 30–45°roof angle > 45°The distribution ofpressure over a gableroof. The pressure variesfrom negative pressureto positive pressure,depending on the angleof the roof and where onthe roof one is.35