A roof vent (8) comprises a conduit having an open entry end adapted for being arranged in sealing communication with an aperture in a roof surface, and an exit end spaced from said entry end, shield means (31) adapted for preventing entry of rain into said conduit, a valve assembly for controlling flow of air or gas, said valve assembly comprising a valve member (13, 22) adapted for opening to passage of air or gas when the pressure at the entry end exceeds the pressure at the exit end and adapted for closing on other conditions to prevent back-flow of air or gas, and thermostat means (27) adapted for opening said valve member to bidirectional passage of air or gas when the temperature at said thermostat means exceeds a predetermined level.