Abstract: A greenhouse roof structure for the usual rectangular in plan greenhouse comprises at least two rectangular roof panels each having a lower edge hinged to the structure for movement about a respective horizontal hinge axis. The roof panels are movable about their hinge axes between a fully closed position in which their upper edges meet to form a roof peak, and a fully open position in which they extend generally vertically at least approximately parallel to one another for the greenhouse space to be upwardly fully open to the ambient atmosphere, as is needed for example to "harden" new plants before they are exposed to the harsher ambient conditions outside the greenhouse. Preferably trusses extend transversely between the two outer side walls, support members are carried by the trusses, and transversely extending upwardly convex arch members are each connected at its ends to respective support members to connect them against the forces, such as wind forces, applied by the roof panels.