Abstract: A greenhouse structure is formed with a substantially flat and horizontal roof formed of a plurality of roof sections each pivoted about a pivot or hinge end to swing upwardly from a closed position to an open position to admit ambient air to flow into or from the interior of the structure to cool the same. The roof sections are lifted by a lift means which becomes part of the roof supporting structure, and, to this end, the lift means is associated with the vertical support columns. Preferrably, the lift means comprises elongated straight lift members each carried by and stabilized by an associated support column for traveling along vertical and rectilinear paths of travel. The preferred means for shifting the lift members and thereby the roof sections comprises a rotatable nut in threaded engagement with the lift member to shift the same vertically as the nut is turned by a worm gear means.