The flexible covering (3) is drawn or withdrawn over a roller (8) mounted on the roof ridge. Control of the covering is by pulleys (6), control wires (5) and a central drive (7). The flexible covering is guided in channels which contain a trapped edge of the covering. The covering is infinitely adjustable over the central gap formed between the sections of fixed roofing (1), thus allowing a variety of ventilation settings. USE/ADVANTAGE - Flexible roof covering. Particularly suitable for glasshouses, reduces building complexity and thus cost.