A roof ventilator includes a tubular member extending to an upper flange defining an upper sealing edge and providing a radially outwardly extending lip which prevents moisture from entering the ventilator. Three tabs are mounted at circumferentially spaced points around the tubular member and support a cover which extends over the tubular member and downwardly to a flange positioned below the upper sealing edge. An annular screening section extends between the tubular member and the cover to prevent foreign objects from entering the ventilator. A plate is normally engaged with the upper sealing edge under the action of gravity to seal the ventilator and includes slots which receive the tabs to guide the plate for vertical movement out of engagement with the sealing edge in response to pressure developed within the tubular member.

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