A roof vent suitable for a conservatory roof of the type comprising glazing bars supporting glazing panels therebetween, the roof vent comprises a surround mountable between a pair of glazing bars and a cover for the surround and hingedly connected thereto. In certain embodiments, the assembly comprises a harness coupling designed to slide freely within the vertical member of a safety ladder so long as the forces exerted on the coupling are small. 1 to 20 mm in an amount of 5 to 120 parts by weight on the basis of 100 parts by weight of a diene rubber, the non-metallic short fibers being oriented in the circumferential direction of the tire. The activation layer is formed on the gate insulation layer.