A greenhouse has a roof including a plurality of substantially parallel crossbeams, at least two longitudinal sections which are mounted to the crossbeams and extend at a distance from each other in transverse direction of the crossbeams, a transparent foil which is mounted to the longitudinal sections at respective foil attachment locations thereof so as to form a cover between the longitudinal sections. The roof also includes a displacement mechanism for displacing one of the longitudinal sections with respect to the other between a non-foil-stretching position and a foil-stretching position. The distance between the respective foil attachment locations is larger in the foil-stretching position than in the non-foil-stretching position. The displacement mechanism can be operated and is lockable at an upper side of the roof.